CVE-2026-89595: fsnotify: Fix stale object mask after concurrent mark updates

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

fsnotify: Fix stale object mask after concurrent mark updates

When a mark gets a new event bit, fanotify and inotify may avoid
recalculating the object mask if the cached aggregate already contains that
bit. This is racy with a recalculation triggered by a concurrent update to
another mark on the same connector.

The concurrent scan can read the mark before the new bit is added, while
the updater reads the old aggregate before that scan publishes its result.
The updater then skips recalculation and the scan publishes a mask without
the bit, leaving the object mask stale after both updates complete.

This can be reproduced with two fanotify groups watching the same inode:
one thread removes FAN_MODIFY from one existing mark while another thread
adds FAN_MODIFY to the other mark. After both fanotify_mark() calls return,
writes can fail to produce FAN_MODIFY for the group whose mark now contains
the bit. This was reproduced on an unmodified v6.12.95 kernel. The
equivalent inotify interleaving loses IN_MODIFY events.

For normal fanotify additions, recalculate whenever the raw mark mask
changes. The normal mask is not cleared asynchronously, so an unchanged
addition cannot introduce missing interest. Always recalculate ignore-mask
updates because FS_MODIFY handling may clear the ignore mask without taking
mark->lock, making snapshot comparisons unreliable.

Always recalculate after updating an existing inotify watch. Its replace
path temporarily sets mark->mask to zero, so a concurrent scan can observe
zero even when the old and final masks are equal. Assigning the replacement
mask directly would avoid the transient zero, but existing-watch updates
are infrequent, so unconditional recalculation is simpler.

The Linux kernel CVE team has assigned CVE-2026-89595 to this issue.


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

	Issue introduced in 2.6.31 with commit 63c882a05416e18de6fb59f7dd6da48f3bbe8273 and fixed in 6.18.51 with commit 947400af98b9e63841929d079a2c3ea0a8ba227b
	Issue introduced in 2.6.31 with commit 63c882a05416e18de6fb59f7dd6da48f3bbe8273 and fixed in 7.2.4 with commit ac8d8b599d466a129122bded77db3d0f8b96b461
	Issue introduced in 2.6.31 with commit 63c882a05416e18de6fb59f7dd6da48f3bbe8273 and fixed in 7.3-rc1 with commit e422777fdd4746de1109575c51e65038d4c5c1be

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-2026-89595
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/notify/fanotify/fanotify_user.c
	fs/notify/inotify/inotify_user.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/947400af98b9e63841929d079a2c3ea0a8ba227b
	https://git.kernel.org/stable/c/ac8d8b599d466a129122bded77db3d0f8b96b461
	https://git.kernel.org/stable/c/e422777fdd4746de1109575c51e65038d4c5c1be