Thomas Moschny
6 years ago
parent
6b5998804d
commit
7ed695dafd
@ -0,0 +1,401 @@
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From 113a0557d7651385d30e181a23c8e68e696ad67f Mon Sep 17 00:00:00 2001
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From: Christian Brauner <christian.brauner@ubuntu.com>
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Date: Sat, 26 Jan 2019 01:19:29 +0100
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Subject: [PATCH] CVE-2019-5736 (runC): rexec callers as memfd
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Adam Iwaniuk and Borys Popławski discovered that an attacker can compromise the
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runC host binary from inside a privileged runC container. As a result, this
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could be exploited to gain root access on the host. runC is used as the default
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runtime for containers with Docker, containerd, Podman, and CRI-O.
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The attack can be made when attaching to a running container or when starting a
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container running a specially crafted image. For example, when runC attaches
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to a container the attacker can trick it into executing itself. This could be
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done by replacing the target binary inside the container with a custom binary
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pointing back at the runC binary itself. As an example, if the target binary
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was /bin/bash, this could be replaced with an executable script specifying the
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interpreter path #!/proc/self/exe (/proc/self/exec is a symbolic link created
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by the kernel for every process which points to the binary that was executed
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for that process). As such when /bin/bash is executed inside the container,
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instead the target of /proc/self/exe will be executed - which will point to the
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runc binary on the host. The attacker can then proceed to write to the target
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of /proc/self/exe to try and overwrite the runC binary on the host. However in
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general, this will not succeed as the kernel will not permit it to be
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overwritten whilst runC is executing. To overcome this, the attacker can
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instead open a file descriptor to /proc/self/exe using the O_PATH flag and then
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proceed to reopen the binary as O_WRONLY through /proc/self/fd/<nr> and try to
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write to it in a busy loop from a separate process. Ultimately it will succeed
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when the runC binary exits. After this the runC binary is compromised and can
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be used to attack other containers or the host itself.
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This attack is only possible with privileged containers since it requires root
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privilege on the host to overwrite the runC binary. Unprivileged containers
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with a non-identity ID mapping do not have the permission to write to the host
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binary and therefore are unaffected by this attack.
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LXC is also impacted in a similar manner by this vulnerability, however as the
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LXC project considers privileged containers to be unsafe no CVE has been
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assigned for this issue for LXC. Quoting from the
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https://linuxcontainers.org/lxc/security/ project's Security information page:
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"As privileged containers are considered unsafe, we typically will not consider
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new container escape exploits to be security issues worthy of a CVE and quick
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fix. We will however try to mitigate those issues so that accidental damage to
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the host is prevented."
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To prevent this attack, LXC has been patched to create a temporary copy of the
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calling binary itself when it starts or attaches to containers. To do this LXC
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creates an anonymous, in-memory file using the memfd_create() system call and
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copies itself into the temporary in-memory file, which is then sealed to
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prevent further modifications. LXC then executes this sealed, in-memory file
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instead of the original on-disk binary. Any compromising write operations from
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a privileged container to the host LXC binary will then write to the temporary
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in-memory binary and not to the host binary on-disk, preserving the integrity
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of the host LXC binary. Also as the temporary, in-memory LXC binary is sealed,
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writes to this will also fail.
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Note: memfd_create() was added to the Linux kernel in the 3.17 release.
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Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
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Co-Developed-by: Aleksa Sarai <asarai@suse.de>
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Acked-by: Serge Hallyn <serge@hallyn.com>
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---
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configure.ac | 12 +++
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src/lxc/Makefile.am | 4 +
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src/lxc/file_utils.c | 41 ++++++++-
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src/lxc/file_utils.h | 1 +
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src/lxc/rexec.c | 181 +++++++++++++++++++++++++++++++++++++
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src/lxc/syscall_wrappers.h | 14 +++
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6 files changed, 252 insertions(+), 1 deletion(-)
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create mode 100644 src/lxc/rexec.c
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diff --git a/configure.ac b/configure.ac
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index 950c8dde..631e6070 100644
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--- a/configure.ac
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+++ b/configure.ac
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@@ -706,6 +706,17 @@ AC_ARG_ENABLE([thread-safety],
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[], [enable_thread_safety=yes])
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AM_CONDITIONAL([ENFORCE_THREAD_SAFETY], [test "x$enable_thread_safety" = "xyes"])
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+AC_ARG_ENABLE([memfd-rexec],
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+ [AC_HELP_STRING([--enable-memfd-rexec], [enforce liblxc as a memfd to protect against certain symlink attacks [default=yes]])],
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+ [], [enable_memfd_rexec=yes])
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+AM_CONDITIONAL([ENFORCE_MEMFD_REXEC], [test "x$enable_memfd_rexec" = "xyes"])
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+if test "x$enable_memfd_rexec" = "xyes"; then
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+ AC_DEFINE([ENFORCE_MEMFD_REXEC], 1, [Rexec liblxc as memfd])
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+ AC_MSG_RESULT([yes])
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+else
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+ AC_MSG_RESULT([no])
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+fi
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+
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# Files requiring some variable expansion
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AC_CONFIG_FILES([
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Makefile
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@@ -934,6 +945,7 @@ Security features:
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- Linux capabilities: $enable_capabilities
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- seccomp: $enable_seccomp
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- SELinux: $enable_selinux
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+ - memfd rexec: $enable_memfd_rexec
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PAM:
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- PAM module: $enable_pam
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diff --git a/src/lxc/Makefile.am b/src/lxc/Makefile.am
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index 08e2fab6..8bbfdaf8 100644
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--- a/src/lxc/Makefile.am
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+++ b/src/lxc/Makefile.am
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@@ -173,6 +173,10 @@ if !HAVE_STRLCAT
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liblxc_la_SOURCES += ../include/strlcat.c ../include/strlcat.h
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endif
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+if ENFORCE_MEMFD_REXEC
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+liblxc_la_SOURCES += rexec.c
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+endif
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+
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AM_CFLAGS = -DLXCROOTFSMOUNT=\"$(LXCROOTFSMOUNT)\" \
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-DLXCPATH=\"$(LXCPATH)\" \
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-DLXC_GLOBAL_CONF=\"$(LXC_GLOBAL_CONF)\" \
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diff --git a/src/lxc/file_utils.c b/src/lxc/file_utils.c
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index f89aa638..930fd738 100644
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--- a/src/lxc/file_utils.c
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+++ b/src/lxc/file_utils.c
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@@ -31,7 +31,7 @@
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#include "config.h"
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#include "file_utils.h"
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#include "macro.h"
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-#include "string.h"
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+#include "string_utils.h"
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int lxc_write_to_file(const char *filename, const void *buf, size_t count,
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bool add_newline, mode_t mode)
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@@ -327,3 +327,42 @@ again:
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return ret;
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}
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+
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+char *file_to_buf(char *path, size_t *length)
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+{
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+ int fd;
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+ char buf[PATH_MAX];
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+ char *copy = NULL;
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+
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+ if (!length)
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+ return NULL;
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+
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+ fd = open(path, O_RDONLY | O_CLOEXEC);
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+ if (fd < 0)
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+ return NULL;
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+
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+ *length = 0;
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+ for (;;) {
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+ int n;
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+ char *old = copy;
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+
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+ n = lxc_read_nointr(fd, buf, sizeof(buf));
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+ if (n < 0)
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+ goto on_error;
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+ if (!n)
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+ break;
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+
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+ copy = must_realloc(old, (*length + n) * sizeof(*old));
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+ memcpy(copy + *length, buf, n);
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+ *length += n;
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+ }
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+
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+ close(fd);
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+ return copy;
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+
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+on_error:
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+ close(fd);
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+ free(copy);
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+
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+ return NULL;
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+}
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diff --git a/src/lxc/file_utils.h b/src/lxc/file_utils.h
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index 6361557a..518a61af 100644
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--- a/src/lxc/file_utils.h
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+++ b/src/lxc/file_utils.h
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@@ -55,5 +55,6 @@ extern bool is_fs_type(const struct statfs *fs, fs_type_magic magic_val);
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extern FILE *fopen_cloexec(const char *path, const char *mode);
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extern ssize_t lxc_sendfile_nointr(int out_fd, int in_fd, off_t *offset,
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size_t count);
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+extern char *file_to_buf(char *path, size_t *length);
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#endif /* __LXC_FILE_UTILS_H */
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diff --git a/src/lxc/rexec.c b/src/lxc/rexec.c
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new file mode 100644
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index 00000000..396bd617
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--- /dev/null
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+++ b/src/lxc/rexec.c
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@@ -0,0 +1,181 @@
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+/* liblxcapi
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+ *
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+ * Copyright © 2019 Christian Brauner <christian.brauner@ubuntu.com>.
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+ * Copyright © 2019 Canonical Ltd.
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+ *
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+ * This program is free software; you can redistribute it and/or modify
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+ * it under the terms of the GNU General Public License version 2, as
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+ * published by the Free Software Foundation.
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+ *
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+ * This program is distributed in the hope that it will be useful,
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+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
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+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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+ * GNU General Public License for more details.
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+ *
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+ * You should have received a copy of the GNU General Public License along
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+ * with this program; if not, write to the Free Software Foundation, Inc.,
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+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
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+ */
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+
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+#ifndef _GNU_SOURCE
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+#define _GNU_SOURCE 1
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+#endif
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+#include <errno.h>
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+#include <stdio.h>
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+#include <stdlib.h>
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+#include <string.h>
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+
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+#include "config.h"
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+#include "file_utils.h"
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+#include "raw_syscalls.h"
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+#include "string_utils.h"
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+#include "syscall_wrappers.h"
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+
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+#define LXC_MEMFD_REXEC_SEALS \
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+ (F_SEAL_SEAL | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_WRITE)
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+
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+static int push_vargs(char *data, int data_length, char ***output)
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+{
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|
+ int num = 0;
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+ char *cur = data;
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+
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|
+ if (!data || *output)
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+ return -1;
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+
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+ *output = must_realloc(NULL, sizeof(**output));
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+
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+ while (cur < data + data_length) {
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+ num++;
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+ *output = must_realloc(*output, (num + 1) * sizeof(**output));
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+
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+ (*output)[num - 1] = cur;
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+ cur += strlen(cur) + 1;
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+ }
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+ (*output)[num] = NULL;
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+ return num;
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+}
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+
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+static int parse_exec_params(char ***argv, char ***envp)
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+{
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+ int ret;
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+ char *cmdline = NULL, *env = NULL;
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+ size_t cmdline_size, env_size;
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+
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+ cmdline = file_to_buf("/proc/self/cmdline", &cmdline_size);
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|
+ if (!cmdline)
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+ goto on_error;
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+
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+ env = file_to_buf("/proc/self/environ", &env_size);
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+ if (!env)
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+ goto on_error;
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+
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+ ret = push_vargs(cmdline, cmdline_size, argv);
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+ if (ret <= 0)
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+ goto on_error;
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+
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+ ret = push_vargs(env, env_size, envp);
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|
+ if (ret <= 0)
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+ goto on_error;
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+
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+ return 0;
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+
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+on_error:
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+ free(env);
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+ free(cmdline);
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+
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+ return -1;
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+}
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+
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+static int is_memfd(void)
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+{
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+ int fd, saved_errno, seals;
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+
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+ fd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
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|
+ if (fd < 0)
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+ return -ENOTRECOVERABLE;
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+
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|
+ seals = fcntl(fd, F_GET_SEALS);
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|
+ saved_errno = errno;
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|
+ close(fd);
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|
+ errno = saved_errno;
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|
+ if (seals < 0)
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|
+ return -EINVAL;
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|
+
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|
+ return seals == LXC_MEMFD_REXEC_SEALS;
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|
+}
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+
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|
+static void lxc_rexec_as_memfd(char **argv, char **envp, const char *memfd_name)
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|
+{
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+ int saved_errno;
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|
+ ssize_t bytes_sent;
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|
+ int fd = -1, memfd = -1;
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|
+
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|
+ memfd = memfd_create(memfd_name, MFD_ALLOW_SEALING | MFD_CLOEXEC);
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|
+ if (memfd < 0)
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|
+ return;
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|
+
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|
+ fd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
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|
+ if (fd < 0)
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|
+ goto on_error;
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|
+
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|
+ /* sendfile() handles up to 2GB. */
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|
+ bytes_sent = lxc_sendfile_nointr(memfd, fd, NULL, LXC_SENDFILE_MAX);
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+ saved_errno = errno;
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|
+ close(fd);
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|
+ errno = saved_errno;
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|
+ if (bytes_sent < 0)
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+ goto on_error;
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|
+
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|
+ if (fcntl(memfd, F_ADD_SEALS, LXC_MEMFD_REXEC_SEALS))
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+ goto on_error;
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|
+
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+ fexecve(memfd, argv, envp);
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+
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+on_error:
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+ saved_errno = errno;
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|
+ close(memfd);
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|
+ errno = saved_errno;
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|
+}
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|
+
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|
+static int lxc_rexec(const char *memfd_name)
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|
+{
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|
+ int ret;
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|
+ char **argv = NULL, **envp = NULL;
|
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|
+
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|
+ ret = is_memfd();
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|
+ if (ret < 0 && ret == -ENOTRECOVERABLE) {
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|
+ fprintf(stderr,
|
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|
+ "%s - Failed to determine whether this is a memfd\n",
|
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|
+ strerror(errno));
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|
+ return -1;
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|
+ } else if (ret > 0) {
|
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|
+ return 0;
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|
+ }
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|
+
|
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|
+ ret = parse_exec_params(&argv, &envp);
|
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|
+ if (ret < 0) {
|
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|
+ fprintf(stderr,
|
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|
+ "%s - Failed to parse command line parameters\n",
|
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|
+ strerror(errno));
|
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|
+ return -1;
|
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|
+ }
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|
+
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||||||
|
+ lxc_rexec_as_memfd(argv, envp, memfd_name);
|
||||||
|
+ fprintf(stderr, "%s - Failed to rexec as memfd\n", strerror(errno));
|
||||||
|
+ return -1;
|
||||||
|
+}
|
||||||
|
+
|
||||||
|
+/**
|
||||||
|
+ * This function will copy any binary that calls liblxc into a memory file and
|
||||||
|
+ * will use the memfd to rexecute the binary. This is done to prevent attacks
|
||||||
|
+ * through the /proc/self/exe symlink to corrupt the host binary when host and
|
||||||
|
+ * container are in the same user namespace or have set up an identity id
|
||||||
|
+ * mapping: CVE-2019-5736.
|
||||||
|
+ */
|
||||||
|
+__attribute__((constructor)) static void liblxc_rexec(void)
|
||||||
|
+{
|
||||||
|
+ if (lxc_rexec("liblxc")) {
|
||||||
|
+ fprintf(stderr, "Failed to re-execute liblxc via memory file descriptor\n");
|
||||||
|
+ _exit(EXIT_FAILURE);
|
||||||
|
+ }
|
||||||
|
+}
|
||||||
|
diff --git a/src/lxc/syscall_wrappers.h b/src/lxc/syscall_wrappers.h
|
||||||
|
index 42d94db2..dca4d157 100644
|
||||||
|
--- a/src/lxc/syscall_wrappers.h
|
||||||
|
+++ b/src/lxc/syscall_wrappers.h
|
||||||
|
@@ -58,6 +58,20 @@ static inline long __keyctl(int cmd, unsigned long arg2, unsigned long arg3,
|
||||||
|
#define keyctl __keyctl
|
||||||
|
#endif
|
||||||
|
|
||||||
|
+#ifndef F_LINUX_SPECIFIC_BASE
|
||||||
|
+#define F_LINUX_SPECIFIC_BASE 1024
|
||||||
|
+#endif
|
||||||
|
+#ifndef F_ADD_SEALS
|
||||||
|
+#define F_ADD_SEALS (F_LINUX_SPECIFIC_BASE + 9)
|
||||||
|
+#define F_GET_SEALS (F_LINUX_SPECIFIC_BASE + 10)
|
||||||
|
+#endif
|
||||||
|
+#ifndef F_SEAL_SEAL
|
||||||
|
+#define F_SEAL_SEAL 0x0001
|
||||||
|
+#define F_SEAL_SHRINK 0x0002
|
||||||
|
+#define F_SEAL_GROW 0x0004
|
||||||
|
+#define F_SEAL_WRITE 0x0008
|
||||||
|
+#endif
|
||||||
|
+
|
||||||
|
#ifndef HAVE_MEMFD_CREATE
|
||||||
|
static inline int memfd_create(const char *name, unsigned int flags) {
|
||||||
|
#ifndef __NR_memfd_create
|
||||||
|
--
|
||||||
|
2.20.1
|
||||||
|
|
||||||
Loading…
Reference in new issue