CVE-2026-64057: afs: Fix the locking used by afs_get_link()

From: Greg Kroah-Hartman <[email protected]>

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

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

afs: Fix the locking used by afs_get_link()

The afs filesystem in the kernel doesn't do locking correctly for symbolic
links.  There are a number of problems:

 (1) It doesn't do any locking around afs_read_single() to prevent races
     between multiple ->get_link() calls, thereby allowing the possibility
     of leaks.

 (2) It doesn't use RCU barriering when accessing the buffer pointers
     during RCU pathwalk.

 (3) It can race with another thread updating the contents of the symlink
     if a third party updated it on the server.

Fix this by the following means:

 (0) Move symlink handling into its own file as this makes it more
     complicated.

 (1) Take the validate_lock around afs_read_single() to prevent races
     between multiple ->get_link() calls.

 (2) Keep a separate copy of the symlink contents with an rcu_head.  This
     is always going to be a lot smaller than a page, so it can be
     kmalloc'd and save quite a bit of memory.  It also needs a refcount
     for non-RCU pathwalk.

 (3) Split the symlink read and write-to-cache routines in afs from those
     for directories.

 (4) Discard the I/O buffer as soon as the write-to-cache completes as this
     is a full page (plus a folio_queue).

 (5) If there's no cache, discard the I/O buffer immediately after reading
     and copying if there is no cache.

The Linux kernel CVE team has assigned CVE-2026-64057 to this issue.


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

	Issue introduced in 6.14 with commit eae9e78951bb02a7b94a9adef6e981413d13c564 and fixed in 7.0.11 with commit 77ea917cbed62882a33114b1e23ededb977e4287
	Issue introduced in 6.14 with commit eae9e78951bb02a7b94a9adef6e981413d13c564 and fixed in 7.1 with commit c0410adf3da6db46f3513411fcf95e63c2f1d1ad

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-2026-64057
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/afs/Makefile
	fs/afs/dir.c
	fs/afs/fsclient.c
	fs/afs/inode.c
	fs/afs/internal.h
	fs/afs/symlink.c
	fs/afs/validation.c
	fs/afs/yfsclient.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/77ea917cbed62882a33114b1e23ededb977e4287
	https://git.kernel.org/stable/c/c0410adf3da6db46f3513411fcf95e63c2f1d1ad