CVE-2026-98446: xfs: don't let hidden_space go negative in xfs_metafile_resv_init

CVE-2026-98446: xfs: don't let hidden_space go negative in xfs_metafile_resv_init

[Greg Kroah-Hartman]

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

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

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

xfs: don't let hidden_space go negative in xfs_metafile_resv_init

LOLLM points out that if the amount of fdblocks that we can reserve for
a metadata btree file goes below the space already used by that file,
then the hidden_space subtraction can underflow, causing
xfs_dec_fdblocks to subtract a huge amount of space.  We never want the
target to be less than the used sapce, so fix the logic that adjusts
dblocks_avail downwards.

Also fix an error in the adjacent comment.

The Linux kernel CVE team has assigned CVE-2026-98446 to this issue.


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

	Issue introduced in 6.15 with commit 1df8d75030b787a9fae270b59b93eef809dd2011 and fixed in 6.18.55 with commit 930a9c1c321efdaa3e7f8caecb0479cee3baf342
	Issue introduced in 6.15 with commit 1df8d75030b787a9fae270b59b93eef809dd2011 and fixed in 7.2.9 with commit f0d8352a39f4a2e329ae45b71bb0d49e2ca0a6dc
	Issue introduced in 6.15 with commit 1df8d75030b787a9fae270b59b93eef809dd2011 and fixed in 7.3-rc5 with commit 476582d754cdc5110f806001417fea6c77824c13

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-2026-98446
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/xfs/libxfs/xfs_metafile.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/930a9c1c321efdaa3e7f8caecb0479cee3baf342
	https://git.kernel.org/stable/c/f0d8352a39f4a2e329ae45b71bb0d49e2ca0a6dc
	https://git.kernel.org/stable/c/476582d754cdc5110f806001417fea6c77824c13