CVE-2026-80697: erofs: ensure valid f_path for page cache sharing

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

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

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

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

erofs: ensure valid f_path for page cache sharing

Previously, backing files for page cache sharing were set up with
f_path left as NULL (only f_inode was valid).  It worked, but a recent
mincore fix relies on f_path.mnt and crashes (found by "erofs/028" on
7.2-rc4):

 BUG: kernel NULL pointer dereference, address: 0000000000000018
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: Oops: 0000 [#1] SMP PTI
 CPU: 3 UID: 0 PID: 675528 Comm: fincore Not tainted 7.2.0-rc4-00002-g[]-dirty #1 PREEMPT(lazy)
 Hardware name: Red Hat KVM, BIOS 1.16.0-4.al8 04/01/2014
 RIP: 0010:__do_sys_mincore+0xc0/0x2c0
 ...

Specify valid paths using valid disconnected dentries together with
erofs_ishare_mnt instead of leaving f_path empty, so they are more
like real backing files in a pseudo filesystem and standard
backing_file_open() can be used directly.

The Linux kernel CVE team has assigned CVE-2026-80697 to this issue.


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

	Issue introduced in 7.1.4 with commit 04ba248d02d9eaa3d9077b00a6134caa75fa3e90 and fixed in 7.1.8 with commit 3879657c4ffd81b9a42cf575fc6cb60201032587
	Issue introduced in 5.15.217 with commit 7368bec565bac3e536cd43579dbde1e715e6ba61
	Issue introduced in 6.1.184 with commit b2f3d94ea310bea9d36d53e9d9b3f45e86c1d893
	Issue introduced in 6.6.145 with commit 744b23aa430d52f5c8e4dbff7d71496d6643bed2
	Issue introduced in 6.12.96 with commit 8344bdf0629457e532797b42d9d2bbf2a2900bbf
	Issue introduced in 6.18.39 with commit 5c942ad7df75925ee166e7f0fb36892d8dde376b

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-2026-80697
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/erofs/Kconfig
	fs/erofs/internal.h
	fs/erofs/ishare.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/3879657c4ffd81b9a42cf575fc6cb60201032587
	https://git.kernel.org/stable/c/96b2dbbe58a1ea5df8d29c2fe24b5f04715f4443