Subject: CVE-2024-50082: blk-rq-qos: fix crash on rq_qos_wait vs. rq_qos_wake_function race

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

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

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

blk-rq-qos: fix crash on rq_qos_wait vs. rq_qos_wake_function race

We're seeing crashes from rq_qos_wake_function that look like this:

  BUG: unable to handle page fault for address: ffffafe180a40084
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 100000067 P4D 100000067 PUD 10027c067 PMD 10115d067 PTE 0
  Oops: Oops: 0002 [#1] PREEMPT SMP PTI
  CPU: 17 UID: 0 PID: 0 Comm: swapper/17 Not tainted 6.12.0-rc3-00013-geca631b8fe80 #11
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
  RIP: 0010:_raw_spin_lock_irqsave+0x1d/0x40
  Code: 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 41 54 9c 41 5c fa 65 ff 05 62 97 30 4c 31 c0 ba 01 00 00 00 <f0> 0f b1 17 75 0a 4c 89 e0 41 5c c3 cc cc cc cc 89 c6 e8 2c 0b 00
  RSP: 0018:ffffafe180580ca0 EFLAGS: 00010046
  RAX: 0000000000000000 RBX: ffffafe180a3f7a8 RCX: 0000000000000011
  RDX: 0000000000000001 RSI: 0000000000000003 RDI: ffffafe180a40084
  RBP: 0000000000000000 R08: 00000000001e7240 R09: 0000000000000011
  R10: 0000000000000028 R11: 0000000000000888 R12: 0000000000000002
  R13: ffffafe180a40084 R14: 0000000000000000 R15: 0000000000000003
  FS:  0000000000000000(0000) GS:ffff9aaf1f280000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: ffffafe180a40084 CR3: 000000010e428002 CR4: 0000000000770ef0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  PKRU: 55555554
  Call Trace:
   <IRQ>
   try_to_wake_up+0x5a/0x6a0
   rq_qos_wake_function+0x71/0x80
   __wake_up_common+0x75/0xa0
   __wake_up+0x36/0x60
   scale_up.part.0+0x50/0x110
   wb_timer_fn+0x227/0x450
   ...

So rq_qos_wake_function() calls wake_up_process(data->task), which calls
try_to_wake_up(), which faults in raw_spin_lock_irqsave(&p->pi_lock).

p comes from data->task, and data comes from the waitqueue entry, which
is stored on the waiter's stack in rq_qos_wait(). Analyzing the core
dump with drgn, I found that the waiter had already woken up and moved
on to a completely unrelated code path, clobbering what was previously
data->task. Meanwhile, the waker was passing the clobbered garbage in
data->task to wake_up_process(), leading to the crash.

What's happening is that in between rq_qos_wake_function() deleting the
waitqueue entry and calling wake_up_process(), rq_qos_wait() is finding
that it already got a token and returning. The race looks like this:

rq_qos_wait()                           rq_qos_wake_function()
==============================================================
prepare_to_wait_exclusive()
                                        data->got_token = true;
                                        list_del_init(&curr->entry);
if (data.got_token)
        break;
finish_wait(&rqw->wait, &data.wq);
  ^- returns immediately because
     list_empty_careful(&wq_entry->entry)
     is true
... return, go do something else ...
                                        wake_up_process(data->task)
                                          (NO LONGER VALID!)-^

Normally, finish_wait() is supposed to synchronize against the waker.
But, as noted above, it is returning immediately because the waitqueue
entry has already been removed from the waitqueue.

The bug is that rq_qos_wake_function() is accessing the waitqueue entry
AFTER deleting it. Note that autoremove_wake_function() wakes the waiter
and THEN deletes the waitqueue entry, which is the proper order.

Fix it by swapping the order. We also need to use
list_del_init_careful() to match the list_empty_careful() in
finish_wait().

The Linux kernel CVE team has assigned CVE-2024-50082 to this issue.

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

	Issue introduced in 4.19 with commit 38cfb5a45ee0 and fixed in 5.10.228 with commit 3bc6d0f8b70a
	Issue introduced in 4.19 with commit 38cfb5a45ee0 and fixed in 5.15.169 with commit 455a469758e5
	Issue introduced in 4.19 with commit 38cfb5a45ee0 and fixed in 6.1.114 with commit b5e900a3612b
	Issue introduced in 4.19 with commit 38cfb5a45ee0 and fixed in 6.6.58 with commit 4c5b123ab289
	Issue introduced in 4.19 with commit 38cfb5a45ee0 and fixed in 6.11.5 with commit 04f283fc16c8
	Issue introduced in 4.19 with commit 38cfb5a45ee0 and fixed in 6.12-rc4 with commit e972b08b91ef

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-2024-50082
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:
	block/blk-rq-qos.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/3bc6d0f8b70a9101456cf02ab99acb75254e1852
	https://git.kernel.org/stable/c/455a469758e57a6fe070e3e342db12e4a629e0eb
	https://git.kernel.org/stable/c/b5e900a3612b69423a0e1b0ab67841a1fb4af80f
	https://git.kernel.org/stable/c/4c5b123ab289767afe940389dbb963c5c05e594e
	https://git.kernel.org/stable/c/04f283fc16c8d5db641b6bffd2d8310aa7eccebc
	https://git.kernel.org/stable/c/e972b08b91ef48488bae9789f03cfedb148667fb