linux-cve-announce - CVE-2024-35798: btrfs: fix race in read_extent_buffer

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Message-ID: <2024051737-CVE-2024-35798-8d90@gregkh>
Date: Fri, 17 May 2024 15:23:38 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-35798: btrfs: fix race in read_extent_buffer_pages()

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix race in read_extent_buffer_pages()

There are reports from tree-checker that detects corrupted nodes,
without any obvious pattern so possibly an overwrite in memory.
After some debugging it turns out there's a race when reading an extent
buffer the uptodate status can be missed.

To prevent concurrent reads for the same extent buffer,
read_extent_buffer_pages() performs these checks:

    /* (1) */
    if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
        return 0;

    /* (2) */
    if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
        goto done;

At this point, it seems safe to start the actual read operation. Once
that completes, end_bbio_meta_read() does

    /* (3) */
    set_extent_buffer_uptodate(eb);

    /* (4) */
    clear_bit(EXTENT_BUFFER_READING, &eb->bflags);

Normally, this is enough to ensure only one read happens, and all other
callers wait for it to finish before returning.  Unfortunately, there is
a racey interleaving:

    Thread A | Thread B | Thread C
    ---------+----------+---------
       (1)   |          |
             |    (1)   |
       (2)   |          |
       (3)   |          |
       (4)   |          |
             |    (2)   |
             |          |    (1)

When this happens, thread B kicks of an unnecessary read. Worse, thread
C will see UPTODATE set and return immediately, while the read from
thread B is still in progress.  This race could result in tree-checker
errors like this as the extent buffer is concurrently modified:

    BTRFS critical (device dm-0): corrupted node, root=256
    block=8550954455682405139 owner mismatch, have 11858205567642294356
    expect [256, 18446744073709551360]

Fix it by testing UPTODATE again after setting the READING bit, and if
it's been set, skip the unnecessary read.

[ minor update of changelog ]

The Linux kernel CVE team has assigned CVE-2024-35798 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 6.5 with commit d7172f52e993 and fixed in 6.6.24 with commit 0427c8ef8bbb
	Issue introduced in 6.5 with commit d7172f52e993 and fixed in 6.7.12 with commit 3a25878a3378
	Issue introduced in 6.5 with commit d7172f52e993 and fixed in 6.8.3 with commit 2885d54af2c2
	Issue introduced in 6.5 with commit d7172f52e993 and fixed in 6.9 with commit ef1e68236b91

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-35798
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	fs/btrfs/extent_io.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/0427c8ef8bbb7f304de42ef51d69c960e165e052
	https://git.kernel.org/stable/c/3a25878a3378adce5d846300c9570f15aa7f7a80
	https://git.kernel.org/stable/c/2885d54af2c2e1d910e20d5c8045bae40e02fbc1
	https://git.kernel.org/stable/c/ef1e68236b9153c27cb7cf29ead0c532870d4215