Subject: CVE-2024-39510: cachefiles: fix slab-use-after-free in cachefiles_ondemand_daemon_read()

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

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

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

cachefiles: fix slab-use-after-free in cachefiles_ondemand_daemon_read()

We got the following issue in a fuzz test of randomly issuing the restore
command:

==================================================================
BUG: KASAN: slab-use-after-free in cachefiles_ondemand_daemon_read+0xb41/0xb60
Read of size 8 at addr ffff888122e84088 by task ondemand-04-dae/963

CPU: 13 PID: 963 Comm: ondemand-04-dae Not tainted 6.8.0-dirty #564
Call Trace:
 kasan_report+0x93/0xc0
 cachefiles_ondemand_daemon_read+0xb41/0xb60
 vfs_read+0x169/0xb50
 ksys_read+0xf5/0x1e0

Allocated by task 116:
 kmem_cache_alloc+0x140/0x3a0
 cachefiles_lookup_cookie+0x140/0xcd0
 fscache_cookie_state_machine+0x43c/0x1230
 [...]

Freed by task 792:
 kmem_cache_free+0xfe/0x390
 cachefiles_put_object+0x241/0x480
 fscache_cookie_state_machine+0x5c8/0x1230
 [...]
==================================================================

Following is the process that triggers the issue:

     mount  |   daemon_thread1    |    daemon_thread2
------------------------------------------------------------
cachefiles_withdraw_cookie
 cachefiles_ondemand_clean_object(object)
  cachefiles_ondemand_send_req
   REQ_A = kzalloc(sizeof(*req) + data_len)
   wait_for_completion(&REQ_A->done)

            cachefiles_daemon_read
             cachefiles_ondemand_daemon_read
              REQ_A = cachefiles_ondemand_select_req
              msg->object_id = req->object->ondemand->ondemand_id
                                  ------ restore ------
                                  cachefiles_ondemand_restore
                                  xas_for_each(&xas, req, ULONG_MAX)
                                   xas_set_mark(&xas, CACHEFILES_REQ_NEW)

                                  cachefiles_daemon_read
                                   cachefiles_ondemand_daemon_read
                                    REQ_A = cachefiles_ondemand_select_req
              copy_to_user(_buffer, msg, n)
               xa_erase(&cache->reqs, id)
               complete(&REQ_A->done)
              ------ close(fd) ------
              cachefiles_ondemand_fd_release
               cachefiles_put_object
 cachefiles_put_object
  kmem_cache_free(cachefiles_object_jar, object)
                                    REQ_A->object->ondemand->ondemand_id
                                     // object UAF !!!

When we see the request within xa_lock, req->object must not have been
freed yet, so grab the reference count of object before xa_unlock to
avoid the above issue.

The Linux kernel CVE team has assigned CVE-2024-39510 to this issue.

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

	Issue introduced in 6.8 with commit 0a7e54c1959c and fixed in 6.9.6 with commit 93064676a282
	Issue introduced in 6.8 with commit 0a7e54c1959c and fixed in 6.10 with commit da4a82741606

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-2024-39510
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/cachefiles/ondemand.c
	include/trace/events/cachefiles.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/cb55625f8eb9d2de8be4da0c4580d48cbb32058e
	https://git.kernel.org/stable/c/3958679c49152391209b32be3357193300a51abd
	https://git.kernel.org/stable/c/93064676a2820420a2d37d7c8289f277fe20793d
	https://git.kernel.org/stable/c/da4a827416066191aafeeccee50a8836a826ba10