CVE-2024-50082
Publication date:
29/10/2024
In the Linux kernel, the following vulnerability has been resolved:<br />
<br />
blk-rq-qos: fix crash on rq_qos_wait vs. rq_qos_wake_function race<br />
<br />
We&#39;re seeing crashes from rq_qos_wake_function that look like this:<br />
<br />
BUG: unable to handle page fault for address: ffffafe180a40084<br />
#PF: supervisor write access in kernel mode<br />
#PF: error_code(0x0002) - not-present page<br />
PGD 100000067 P4D 100000067 PUD 10027c067 PMD 10115d067 PTE 0<br />
Oops: Oops: 0002 [#1] PREEMPT SMP PTI<br />
CPU: 17 UID: 0 PID: 0 Comm: swapper/17 Not tainted 6.12.0-rc3-00013-geca631b8fe80 #11<br />
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014<br />
RIP: 0010:_raw_spin_lock_irqsave+0x1d/0x40<br />
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 0f b1 17 75 0a 4c 89 e0 41 5c c3 cc cc cc cc 89 c6 e8 2c 0b 00<br />
RSP: 0018:ffffafe180580ca0 EFLAGS: 00010046<br />
RAX: 0000000000000000 RBX: ffffafe180a3f7a8 RCX: 0000000000000011<br />
RDX: 0000000000000001 RSI: 0000000000000003 RDI: ffffafe180a40084<br />
RBP: 0000000000000000 R08: 00000000001e7240 R09: 0000000000000011<br />
R10: 0000000000000028 R11: 0000000000000888 R12: 0000000000000002<br />
R13: ffffafe180a40084 R14: 0000000000000000 R15: 0000000000000003<br />
FS: 0000000000000000(0000) GS:ffff9aaf1f280000(0000) knlGS:0000000000000000<br />
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033<br />
CR2: ffffafe180a40084 CR3: 000000010e428002 CR4: 0000000000770ef0<br />
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000<br />
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400<br />
PKRU: 55555554<br />
Call Trace:<br />
<br />
try_to_wake_up+0x5a/0x6a0<br />
rq_qos_wake_function+0x71/0x80<br />
__wake_up_common+0x75/0xa0<br />
__wake_up+0x36/0x60<br />
scale_up.part.0+0x50/0x110<br />
wb_timer_fn+0x227/0x450<br />
...<br />
<br />
So rq_qos_wake_function() calls wake_up_process(data->task), which calls<br />
try_to_wake_up(), which faults in raw_spin_lock_irqsave(&p->pi_lock).<br />
<br />
p comes from data->task, and data comes from the waitqueue entry, which<br />
is stored on the waiter&#39;s stack in rq_qos_wait(). Analyzing the core<br />
dump with drgn, I found that the waiter had already woken up and moved<br />
on to a completely unrelated code path, clobbering what was previously<br />
data->task. Meanwhile, the waker was passing the clobbered garbage in<br />
data->task to wake_up_process(), leading to the crash.<br />
<br />
What&#39;s happening is that in between rq_qos_wake_function() deleting the<br />
waitqueue entry and calling wake_up_process(), rq_qos_wait() is finding<br />
that it already got a token and returning. The race looks like this:<br />
<br />
rq_qos_wait() rq_qos_wake_function()<br />
==============================================================<br />
prepare_to_wait_exclusive()<br />
data->got_token = true;<br />
list_del_init(&curr->entry);<br />
if (data.got_token)<br />
break;<br />
finish_wait(&rqw->wait, &data.wq);<br />
^- returns immediately because<br />
list_empty_careful(&wq_entry->entry)<br />
is true<br />
... return, go do something else ...<br />
wake_up_process(data->task)<br />
(NO LONGER VALID!)-^<br />
<br />
Normally, finish_wait() is supposed to synchronize against the waker.<br />
But, as noted above, it is returning immediately because the waitqueue<br />
entry has already been removed from the waitqueue.<br />
<br />
The bug is that rq_qos_wake_function() is accessing the waitqueue entry<br />
AFTER deleting it. Note that autoremove_wake_function() wakes the waiter<br />
and THEN deletes the waitqueue entry, which is the proper order.<br />
<br />
Fix it by swapping the order. We also need to use<br />
list_del_init_careful() to match the list_empty_careful() in<br />
finish_wait().
Severity CVSS v4.0: Pending analysis
Last modification:
03/11/2025
