linux-cve-announce - CVE-2025-39881: kernfs: Fix UAF in polling when open file is released

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Message-ID: <2025092301-CVE-2025-39881-a4e1@gregkh>
Date: Tue, 23 Sep 2025 08:01:12 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-39881: kernfs: Fix UAF in polling when open file is released

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

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

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

kernfs: Fix UAF in polling when open file is released

A use-after-free (UAF) vulnerability was identified in the PSI (Pressure
Stall Information) monitoring mechanism:

BUG: KASAN: slab-use-after-free in psi_trigger_poll+0x3c/0x140
Read of size 8 at addr ffff3de3d50bd308 by task systemd/1

psi_trigger_poll+0x3c/0x140
cgroup_pressure_poll+0x70/0xa0
cgroup_file_poll+0x8c/0x100
kernfs_fop_poll+0x11c/0x1c0
ep_item_poll.isra.0+0x188/0x2c0

Allocated by task 1:
cgroup_file_open+0x88/0x388
kernfs_fop_open+0x73c/0xaf0
do_dentry_open+0x5fc/0x1200
vfs_open+0xa0/0x3f0
do_open+0x7e8/0xd08
path_openat+0x2fc/0x6b0
do_filp_open+0x174/0x368

Freed by task 8462:
cgroup_file_release+0x130/0x1f8
kernfs_drain_open_files+0x17c/0x440
kernfs_drain+0x2dc/0x360
kernfs_show+0x1b8/0x288
cgroup_file_show+0x150/0x268
cgroup_pressure_write+0x1dc/0x340
cgroup_file_write+0x274/0x548

Reproduction Steps:
1. Open test/cpu.pressure and establish epoll monitoring
2. Disable monitoring: echo 0 > test/cgroup.pressure
3. Re-enable monitoring: echo 1 > test/cgroup.pressure

The race condition occurs because:
1. When cgroup.pressure is disabled (echo 0 > cgroup.pressure), it:
   - Releases PSI triggers via cgroup_file_release()
   - Frees of->priv through kernfs_drain_open_files()
2. While epoll still holds reference to the file and continues polling
3. Re-enabling (echo 1 > cgroup.pressure) accesses freed of->priv

epolling			disable/enable cgroup.pressure
fd=open(cpu.pressure)
while(1)
...
epoll_wait
kernfs_fop_poll
kernfs_get_active = true	echo 0 > cgroup.pressure
...				cgroup_file_show
				kernfs_show
				// inactive kn
				kernfs_drain_open_files
				cft->release(of);
				kfree(ctx);
				...
kernfs_get_active = false
				echo 1 > cgroup.pressure
				kernfs_show
				kernfs_activate_one(kn);
kernfs_fop_poll
kernfs_get_active = true
cgroup_file_poll
psi_trigger_poll
// UAF
...
end: close(fd)

To address this issue, introduce kernfs_get_active_of() for kernfs open
files to obtain active references. This function will fail if the open file
has been released. Replace kernfs_get_active() with kernfs_get_active_of()
to prevent further operations on released file descriptors.

The Linux kernel CVE team has assigned CVE-2025-39881 to this issue.


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

	Issue introduced in 6.1 with commit 34f26a15611afb03c33df6819359d36f5b382589 and fixed in 6.1.153 with commit 34d9cafd469c69ad85e6a36b4303c78382cf5c79
	Issue introduced in 6.1 with commit 34f26a15611afb03c33df6819359d36f5b382589 and fixed in 6.6.107 with commit 854baafc00c433cccbe0ab4231b77aeb9b637b77
	Issue introduced in 6.1 with commit 34f26a15611afb03c33df6819359d36f5b382589 and fixed in 6.12.48 with commit 7e64474aba78d240f7804f48f2d454dcca78b15f
	Issue introduced in 6.1 with commit 34f26a15611afb03c33df6819359d36f5b382589 and fixed in 6.16.8 with commit ac5cda4fae8818cf1963317bb699f7f2f85b60af
	Issue introduced in 6.1 with commit 34f26a15611afb03c33df6819359d36f5b382589 and fixed in 6.17-rc6 with commit 3c9ba2777d6c86025e1ba4186dc5cd930e40ec5f

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-2025-39881
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/kernfs/file.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/34d9cafd469c69ad85e6a36b4303c78382cf5c79
	https://git.kernel.org/stable/c/854baafc00c433cccbe0ab4231b77aeb9b637b77
	https://git.kernel.org/stable/c/7e64474aba78d240f7804f48f2d454dcca78b15f
	https://git.kernel.org/stable/c/ac5cda4fae8818cf1963317bb699f7f2f85b60af
	https://git.kernel.org/stable/c/3c9ba2777d6c86025e1ba4186dc5cd930e40ec5f