cve/published/2025/CVE-2025-37931.mbox

From bippy-1.2.0 Mon Sep 17 00:00:00 2001
From: Greg Kroah-Hartman <gregkh@kernel.org>
To: <linux-cve-announce@vger.kernel.org>
Reply-to: <cve@kernel.org>, <linux-kernel@vger.kernel.org>
Subject: CVE-2025-37931: btrfs: adjust subpage bit start based on sectorsize

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

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

btrfs: adjust subpage bit start based on sectorsize

When running machines with 64k page size and a 16k nodesize we started
seeing tree log corruption in production.  This turned out to be because
we were not writing out dirty blocks sometimes, so this in fact affects
all metadata writes.

When writing out a subpage EB we scan the subpage bitmap for a dirty
range.  If the range isn't dirty we do

	bit_start++;

to move onto the next bit.  The problem is the bitmap is based on the
number of sectors that an EB has.  So in this case, we have a 64k
pagesize, 16k nodesize, but a 4k sectorsize.  This means our bitmap is 4
bits for every node.  With a 64k page size we end up with 4 nodes per
page.

To make this easier this is how everything looks

[0         16k       32k       48k     ] logical address
[0         4         8         12      ] radix tree offset
[               64k page               ] folio
[ 16k eb ][ 16k eb ][ 16k eb ][ 16k eb ] extent buffers
[ | | | |  | | | |   | | | |   | | | | ] bitmap

Now we use all of our addressing based on fs_info->sectorsize_bits, so
as you can see the above our 16k eb->start turns into radix entry 4.

When we find a dirty range for our eb, we correctly do bit_start +=
sectors_per_node, because if we start at bit 0, the next bit for the
next eb is 4, to correspond to eb->start 16k.

However if our range is clean, we will do bit_start++, which will now
put us offset from our radix tree entries.

In our case, assume that the first time we check the bitmap the block is
not dirty, we increment bit_start so now it == 1, and then we loop
around and check again.  This time it is dirty, and we go to find that
start using the following equation

	start = folio_start + bit_start * fs_info->sectorsize;

so in the case above, eb->start 0 is now dirty, and we calculate start
as

	0 + 1 * fs_info->sectorsize = 4096
	4096 >> 12 = 1

Now we're looking up the radix tree for 1, and we won't find an eb.
What's worse is now we're using bit_start == 1, so we do bit_start +=
sectors_per_node, which is now 5.  If that eb is dirty we will run into
the same thing, we will look at an offset that is not populated in the
radix tree, and now we're skipping the writeout of dirty extent buffers.

The best fix for this is to not use sectorsize_bits to address nodes,
but that's a larger change.  Since this is a fs corruption problem fix
it simply by always using sectors_per_node to increment the start bit.

The Linux kernel CVE team has assigned CVE-2025-37931 to this issue.


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

	Issue introduced in 5.13 with commit c4aec299fa8f73f0fd10bc556f936f0da50e3e83 and fixed in 6.12.28 with commit b80db09b614cb7edec5bada1bc7c7b0eb3b453ea
	Issue introduced in 5.13 with commit c4aec299fa8f73f0fd10bc556f936f0da50e3e83 and fixed in 6.14.6 with commit 396f4002710030ea1cfd4c789ebaf0a6969ab34f
	Issue introduced in 5.13 with commit c4aec299fa8f73f0fd10bc556f936f0da50e3e83 and fixed in 6.15 with commit e08e49d986f82c30f42ad0ed43ebbede1e1e3739

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-2025-37931
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/b80db09b614cb7edec5bada1bc7c7b0eb3b453ea
	https://git.kernel.org/stable/c/396f4002710030ea1cfd4c789ebaf0a6969ab34f
	https://git.kernel.org/stable/c/e08e49d986f82c30f42ad0ed43ebbede1e1e3739