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Fix assertion in ext-mode BLOCK_STATUS (CVE-2023-5871)

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Richard W.M. Jones 2023-10-31 14:22:21 +00:00
parent 0858d4ff45
commit 267af1b608
2 changed files with 93 additions and 1 deletions
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86
0001-generator-Fix-assertion-in-ext-mode-BLOCK_STATUS-CVE.patch Normal file
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@ -0,0 +1,86 @@
From 177308adb17e81fce7c0f2b2fcf655c5c0b6a4d6 Mon Sep 17 00:00:00 2001
From: Eric Blake <eblake@redhat.com>
Date: Mon, 30 Oct 2023 12:50:53 -0500
Subject: [PATCH] generator: Fix assertion in ext-mode BLOCK_STATUS,
CVE-2023-5871
Another round of fuzz testing revealed that when a server negotiates
extended headers and replies with a 64-bit flag value where the client
used the 32-bit API command, we were correctly flagging the server's
response as being an EOVERFLOW condition, but then immediately failing
in an assertion failure instead of reporting it to the application.
The following one-byte change to qemu.git at commit fd9a38fd43 allows
the creation of an intentionally malicious server:
| diff --git i/nbd/server.c w/nbd/server.c
| index 859c163d19f..32e1e771a95 100644
| --- i/nbd/server.c
| +++ w/nbd/server.c
| @@ -2178,7 +2178,7 @@ static void nbd_extent_array_convert_to_be(NBDExtentArray *ea)
|
| for (i = 0; i < ea->count; i++) {
| ea->extents[i].length = cpu_to_be64(ea->extents[i].length);
| - ea->extents[i].flags = cpu_to_be64(ea->extents[i].flags);
| + ea->extents[i].flags = ~cpu_to_be64(ea->extents[i].flags);
| }
| }
and can then be detected with the following command line:
$ nbdsh -c - <<\EOF
> def f(a,b,c,d):
> pass
>
> h.connect_systemd_socket_activation(["/path/to/bad/qemu-nbd",
> "-r", "-f", "raw", "TODO"])
> h.block_staus(h.get_size(), 0, f)
> EOF
nbdsh: generator/states-reply-chunk.c:626: enter_STATE_REPLY_CHUNK_REPLY_RECV_BS_ENTRIES: Assertion `(len | flags) <= UINT32_MAX' failed.
Aborted (core dumped)
whereas a fixed libnbd will give:
nbdsh: command line script failed: nbd_block_status: block-status: command failed: Value too large for defined data type
We can either relax the assertion (by changing to 'assert ((len |
flags) <= UINT32_MAX || cmd->error)'), or intentionally truncate flags
to make the existing assertion reliable. This patch goes with the
latter approach.
Sadly, this crash is possible in all existing 1.18.x stable releases,
if they were built with assertions enabled (most distros do this by
default), meaning a malicious server has an easy way to cause a Denial
of Service attack by triggering the assertion failure in vulnerable
clients, so we have assigned this CVE-2023-5871. Mitigating factors:
the crash only happens for a server that sends a 64-bit status block
reply (no known production servers do so; qemu 8.2 will be the first
known server to support extended headers, but it is not yet released);
and as usual, a client can use TLS to guarantee it is connecting only
to a known-safe server. If libnbd is compiled without assertions,
there is no crash or other mistaken behavior; and when assertions are
enabled, the attacker cannot accomplish anything more than a denial of
service.
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Fixes: 20dadb0e10 ("generator: Prepare for extent64 callback", v1.17.4)
Signed-off-by: Eric Blake <eblake@redhat.com>
---
generator/states-reply-chunk.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/generator/states-reply-chunk.c b/generator/states-reply-chunk.c
index 5a31c19..8ab7e8b 100644
--- a/generator/states-reply-chunk.c
+++ b/generator/states-reply-chunk.c
@@ -600,6 +600,7 @@ STATE_MACHINE {
break; /* Skip this and later extents; we already made progress */
/* Expose this extent as an error; we made no progress */
cmd->error = cmd->error ? : EOVERFLOW;
+ flags = (uint32_t)flags;
}
}
--
2.41.0

8
libnbd.spec
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@ -9,7 +9,7 @@
Name: libnbd
Version: 1.19.1
Release: 1%{?dist}
Release: 2%{?dist}
Summary: NBD client library in userspace
License: LGPL-2.0-or-later AND BSD-3-Clause
@ -25,6 +25,9 @@ Source2: libguestfs.keyring
# Maintainer script which helps with handling patches.
Source3: copy-patches.sh
# CVE-2023-5871
Patch: 0001-generator-Fix-assertion-in-ext-mode-BLOCK_STATUS-CVE.patch
%if 0%{patches_touch_autotools}
BuildRequires: autoconf, automake, libtool
%endif
@ -375,6 +378,9 @@ make %{?_smp_mflags} check || {
%changelog
* Tue Oct 31 2023 Richard W.M. Jones <rjones@redhat.com> - 1.19.1-2
- Fix assertion in ext-mode BLOCK_STATUS (CVE-2023-5871)
* Mon Oct 23 2023 Richard W.M. Jones <rjones@redhat.com> - 1.19.1-1
- New upstream development version 1.19.1