linux-cve-announce - CVE-2025-71074: functionfs: fix the open/removal races

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Message-ID: <2026011326-CVE-2025-71074-f3ed@gregkh>
Date: Tue, 13 Jan 2026 16:31:32 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-71074: functionfs: fix the open/removal races

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

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

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

functionfs: fix the open/removal races

ffs_epfile_open() can race with removal, ending up with file->private_data
pointing to freed object.

There is a total count of opened files on functionfs (both ep0 and
dynamic ones) and when it hits zero, dynamic files get removed.
Unfortunately, that removal can happen while another thread is
in ffs_epfile_open(), but has not incremented the count yet.
In that case open will succeed, leaving us with UAF on any subsequent
read() or write().

The root cause is that ffs->opened is misused; atomic_dec_and_test() vs.
atomic_add_return() is not a good idea, when object remains visible all
along.

To untangle that
	* serialize openers on ffs->mutex (both for ep0 and for dynamic files)
	* have dynamic ones use atomic_inc_not_zero() and fail if we had
zero ->opened; in that case the file we are opening is doomed.
	* have the inodes of dynamic files marked on removal (from the
callback of simple_recursive_removal()) - clear ->i_private there.
	* have open of dynamic ones verify they hadn't been already removed,
along with checking that state is FFS_ACTIVE.

The Linux kernel CVE team has assigned CVE-2025-71074 to this issue.


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

	Fixed in 6.18.3 with commit b49c766856fb5901490de577e046149ebf15e39d
	Fixed in 6.19-rc1 with commit e5bf5ee266633cb18fff6f98f0b7d59a62819eee

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-2025-71074
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:
	drivers/usb/gadget/function/f_fs.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/b49c766856fb5901490de577e046149ebf15e39d
	https://git.kernel.org/stable/c/e5bf5ee266633cb18fff6f98f0b7d59a62819eee