CVE-2026-89600: fanotify: fix use-after-free of file range info

CVE-2026-89600: fanotify: fix use-after-free of file range info

[Greg Kroah-Hartman]

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

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

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

fanotify: fix use-after-free of file range info

fsnotify_pre_content() builds its file_range on the triggering task's
stack. fanotify_alloc_perm_event() saves a pointer to range.pos in the
heap-allocated permission event so copy_range_info_to_user() can report
the offset later.

The event reader can set the event state to FAN_EVENT_REPORTED and then
sleep while preparing the file descriptor. If a signal interrupts the
triggering task at that point, fanotify_get_response() changes the state
to FAN_EVENT_CANCELED and returns. This unwinds the file_range stack
frame while the reader still owns the event. The reader then dereferences
pevent->ppos and copies the stale stack value to userspace.

KASAN reported:

  BUG: KASAN: use-after-free in fanotify_read+0x293e/0x2970
  Read of size 8 at addr ffff88811434fc50 by task fanotify_inotif/95
  Call Trace:
   fanotify_read+0x293e/0x2970
   vfs_read+0x177/0xa20
   ksys_read+0xf7/0x1c0
   do_syscall_64+0xf9/0x540
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Store the range position directly in the permission event and use
FANOTIFY_NO_RANGE when range information is unavailable. The event remains
alive until the reader finishes, so the reported offset no longer depends
on the triggering task's stack.

The Linux kernel CVE team has assigned CVE-2026-89600 to this issue.


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

	Issue introduced in 6.14 with commit 870499bc1d4dc04cba1f63dd5e7bc02b983e2458 and fixed in 6.18.50 with commit 66aa9a9e6481bfa512ee5b6380eaaec455a954d2
	Issue introduced in 6.14 with commit 870499bc1d4dc04cba1f63dd5e7bc02b983e2458 and fixed in 7.2.4 with commit b8f73b163b646bc56d4dce9c697a578e333f4894
	Issue introduced in 6.14 with commit 870499bc1d4dc04cba1f63dd5e7bc02b983e2458 and fixed in 7.3-rc1 with commit d7f1cf5be33ef0175a4e8ed8687aeb98fb00a851

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-2026-89600
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.c
	fs/notify/fanotify/fanotify.h
	fs/notify/fanotify/fanotify_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/66aa9a9e6481bfa512ee5b6380eaaec455a954d2
	https://git.kernel.org/stable/c/b8f73b163b646bc56d4dce9c697a578e333f4894
	https://git.kernel.org/stable/c/d7f1cf5be33ef0175a4e8ed8687aeb98fb00a851