CVE-2026-74506: afs: Fix UAF when sending a message

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

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

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

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

afs: Fix UAF when sending a message

In afs_make_call(), there's a race with async call reception and
destruction.  If a call is dispatched that doesn't have call->write_iter
set (used to specify the data content for FS.StoreData), then the first
rxrpc_kernel_send_data() will not set MSG_MORE in the msghdr.

Once rxrpc_send_data() queues the last request packet, the response could
come in at any time and cause the call to be completed and put.  However,
afs_make_call() will look at the call again to see it ->write_iter should
be handled - something it's only allowed to do if it has its own ref on the
call.  Whilst this is the case for synchronous calls, it isn't true for
async calls such as FS.FetchData.

There's also a potential UAF in afs_make_call() in the event that an
asynchronous call is being sent, but the call fails in some way (e.g. it
gets aborted from the server).  The problem there is that afs_make_call()
tries to abort a call if the rxrpc send fails, but the asynchronous
notification from rxrpc may have caused the afs_call to be torn down.

generic/650 plays games with randomly taking CPUs offline, and can
interject a significant delay such that the call is deallocated before
afs_make_call() gets to check call->write_iter - and a UAF ensues (caught
by KASAN).

   BUG: KASAN: slab-use-after-free in afs_make_call+0x1c90/0x2210 [kafs]
   Read of size 8 at addr ffff888035e050e8 by task fsstress/1409

Fix this by making afs_make_op_call() give the op->call its own ref rather
than transferring the caller's ref to it and then dropping the ref when
afs_make_call() returns.

This also means that the afs_make_call() func never loses its ref on the
call now.

The Linux kernel CVE team has assigned CVE-2026-74506 to this issue.


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

	Issue introduced in 5.8 with commit e49c7b2f6de7ff81ca34c56e4eeb4fa740c099f2 and fixed in 6.18.44 with commit c0d3b81f703b2a9e37fe1347610a50cdf0078c27
	Issue introduced in 5.8 with commit e49c7b2f6de7ff81ca34c56e4eeb4fa740c099f2 and fixed in 7.1.8 with commit daaa726b14fc3026a6b328614d312b698f62f391
	Issue introduced in 5.8 with commit e49c7b2f6de7ff81ca34c56e4eeb4fa740c099f2 and fixed in 7.2-rc6 with commit 4af1ec68d54b3871155914d584fb10669c41a861

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-2026-74506
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/internal.h


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/c0d3b81f703b2a9e37fe1347610a50cdf0078c27
	https://git.kernel.org/stable/c/daaa726b14fc3026a6b328614d312b698f62f391
	https://git.kernel.org/stable/c/4af1ec68d54b3871155914d584fb10669c41a861