linux-cve-announce - CVE-2023-54121: btrfs: fix incorrect splitting in btrfs_drop_extent_map

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Message-ID: <2025122416-CVE-2023-54121-691a@gregkh>
Date: Wed, 24 Dec 2025 14:07:18 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2023-54121: btrfs: fix incorrect splitting in btrfs_drop_extent_map_range

From: Greg Kroah-Hartman <gregkh@...nel.org>

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

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

btrfs: fix incorrect splitting in btrfs_drop_extent_map_range

In production we were seeing a variety of WARN_ON()'s in the extent_map
code, specifically in btrfs_drop_extent_map_range() when we have to call
add_extent_mapping() for our second split.

Consider the following extent map layout

	PINNED
	[0 16K)  [32K, 48K)

and then we call btrfs_drop_extent_map_range for [0, 36K), with
skip_pinned == true.  The initial loop will have

	start = 0
	end = 36K
	len = 36K

we will find the [0, 16k) extent, but since we are pinned we will skip
it, which has this code

	start = em_end;
	if (end != (u64)-1)
		len = start + len - em_end;

em_end here is 16K, so now the values are

	start = 16K
	len = 16K + 36K - 16K = 36K

len should instead be 20K.  This is a problem when we find the next
extent at [32K, 48K), we need to split this extent to leave [36K, 48k),
however the code for the split looks like this

	split->start = start + len;
	split->len = em_end - (start + len);

In this case we have

	em_end = 48K
	split->start = 16K + 36K       // this should be 16K + 20K
	split->len = 48K - (16K + 36K) // this overflows as 16K + 36K is 52K

and now we have an invalid extent_map in the tree that potentially
overlaps other entries in the extent map.  Even in the non-overlapping
case we will have split->start set improperly, which will cause problems
with any block related calculations.

We don't actually need len in this loop, we can simply use end as our
end point, and only adjust start up when we find a pinned extent we need
to skip.

Adjust the logic to do this, which keeps us from inserting an invalid
extent map.

We only skip_pinned in the relocation case, so this is relatively rare,
except in the case where you are running relocation a lot, which can
happen with auto relocation on.

The Linux kernel CVE team has assigned CVE-2023-54121 to this issue.

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

	Issue introduced in 2.6.33 with commit 55ef68990029fcd8d04d42fc184aa7fb18cf309e and fixed in 6.1.47 with commit 9f68e2105dd96cf0fafffffafb2337fbd0fbae1f
	Issue introduced in 2.6.33 with commit 55ef68990029fcd8d04d42fc184aa7fb18cf309e and fixed in 6.4.12 with commit b43a4c99d878cf5e59040e45c96bb0a8358bfb3b
	Issue introduced in 2.6.33 with commit 55ef68990029fcd8d04d42fc184aa7fb18cf309e and fixed in 6.5 with commit c962098ca4af146f2625ed64399926a098752c9c
	Issue introduced in 2.6.32.19 with commit c87afd35a28b2661a2626a1b28e9fd69adcad9f9

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-2023-54121
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_map.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/9f68e2105dd96cf0fafffffafb2337fbd0fbae1f
	https://git.kernel.org/stable/c/b43a4c99d878cf5e59040e45c96bb0a8358bfb3b
	https://git.kernel.org/stable/c/c962098ca4af146f2625ed64399926a098752c9c