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89 lines
3.6 KiB
89 lines
3.6 KiB
8 months ago
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From 4451e5b61ca07771ceef3e012223779e7a0c7701 Mon Sep 17 00:00:00 2001
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From: Eric Blake <eblake@redhat.com>
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Date: Mon, 30 Oct 2023 12:50:53 -0500
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Subject: [PATCH] generator: Fix assertion in ext-mode BLOCK_STATUS,
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CVE-2023-5871
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Another round of fuzz testing revealed that when a server negotiates
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extended headers and replies with a 64-bit flag value where the client
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used the 32-bit API command, we were correctly flagging the server's
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response as being an EOVERFLOW condition, but then immediately failing
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in an assertion failure instead of reporting it to the application.
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The following one-byte change to qemu.git at commit fd9a38fd43 allows
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the creation of an intentionally malicious server:
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| diff --git i/nbd/server.c w/nbd/server.c
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| index 859c163d19f..32e1e771a95 100644
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| --- i/nbd/server.c
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| +++ w/nbd/server.c
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| @@ -2178,7 +2178,7 @@ static void nbd_extent_array_convert_to_be(NBDExtentArray *ea)
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| for (i = 0; i < ea->count; i++) {
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| ea->extents[i].length = cpu_to_be64(ea->extents[i].length);
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| - ea->extents[i].flags = cpu_to_be64(ea->extents[i].flags);
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| + ea->extents[i].flags = ~cpu_to_be64(ea->extents[i].flags);
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| }
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| }
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and can then be detected with the following command line:
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$ nbdsh -c - <<\EOF
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> def f(a,b,c,d):
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> pass
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>
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> h.connect_systemd_socket_activation(["/path/to/bad/qemu-nbd",
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> "-r", "-f", "raw", "TODO"])
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> h.block_staus(h.get_size(), 0, f)
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> EOF
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nbdsh: generator/states-reply-chunk.c:626: enter_STATE_REPLY_CHUNK_REPLY_RECV_BS_ENTRIES: Assertion `(len | flags) <= UINT32_MAX' failed.
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Aborted (core dumped)
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whereas a fixed libnbd will give:
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nbdsh: command line script failed: nbd_block_status: block-status: command failed: Value too large for defined data type
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We can either relax the assertion (by changing to 'assert ((len |
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flags) <= UINT32_MAX || cmd->error)'), or intentionally truncate flags
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to make the existing assertion reliable. This patch goes with the
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latter approach.
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Sadly, this crash is possible in all existing 1.18.x stable releases,
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if they were built with assertions enabled (most distros do this by
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default), meaning a malicious server has an easy way to cause a Denial
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of Service attack by triggering the assertion failure in vulnerable
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clients, so we have assigned this CVE-2023-5871. Mitigating factors:
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the crash only happens for a server that sends a 64-bit status block
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reply (no known production servers do so; qemu 8.2 will be the first
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known server to support extended headers, but it is not yet released);
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and as usual, a client can use TLS to guarantee it is connecting only
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to a known-safe server. If libnbd is compiled without assertions,
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there is no crash or other mistaken behavior; and when assertions are
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enabled, the attacker cannot accomplish anything more than a denial of
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service.
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Reported-by: Richard W.M. Jones <rjones@redhat.com>
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Fixes: 20dadb0e10 ("generator: Prepare for extent64 callback", v1.17.4)
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Signed-off-by: Eric Blake <eblake@redhat.com>
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(cherry picked from commit 177308adb17e81fce7c0f2b2fcf655c5c0b6a4d6)
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Signed-off-by: Eric Blake <eblake@redhat.com>
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---
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generator/states-reply-chunk.c | 1 +
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1 file changed, 1 insertion(+)
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diff --git a/generator/states-reply-chunk.c b/generator/states-reply-chunk.c
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index 5a31c19..8ab7e8b 100644
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--- a/generator/states-reply-chunk.c
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+++ b/generator/states-reply-chunk.c
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@@ -600,6 +600,7 @@ STATE_MACHINE {
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break; /* Skip this and later extents; we already made progress */
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/* Expose this extent as an error; we made no progress */
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cmd->error = cmd->error ? : EOVERFLOW;
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+ flags = (uint32_t)flags;
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}
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}
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--
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2.39.3
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