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

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

[Omar Sandoval]

From: Omar Sandoval <osandov@fb.com>

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().

Fixes: 38cfb5a45ee0 ("blk-wbt: improve waking of tasks")
Cc: stable@vger.kernel.org
Signed-off-by: Omar Sandoval <osandov@fb.com>
---
Hi,

I've only managed to reproduce this bug by inserting udelay() calls, so
I don't have a test case, unfortunately. I ran it through fio stress
tests with and without additional udelay() calls.

Note that the bug was introduced in 4.19 in wbt, which was then copied
to the generic rq-qos code in 5.0 in commit 84f603246db9 ("block: add
rq_qos_wait to rq_qos"). If this gets backported to 4.19 LTS, that will
need to be accounted for.

Based on Linus's master branch as of commit
eca631b8fe808748d7585059c4307005ca5c5820.

Thanks,
Omar

 block/blk-rq-qos.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index 2cfb297d9a62..058f92c4f9d5 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -219,8 +219,8 @@ static int rq_qos_wake_function(struct wait_queue_entry *curr,
 
 	data->got_token = true;
 	smp_wmb();
-	list_del_init(&curr->entry);
 	wake_up_process(data->task);
+	list_del_init_careful(&curr->entry);
 	return 1;
 }
 
-- 
2.47.0

Re: [PATCH] blk-rq-qos: fix crash on rq_qos_wait vs. rq_qos_wake_function race

[Tejun Heo]

On Tue, Oct 15, 2024 at 10:59:46AM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> 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().
> 
> Fixes: 38cfb5a45ee0 ("blk-wbt: improve waking of tasks")
> Cc: stable@vger.kernel.org
> Signed-off-by: Omar Sandoval <osandov@fb.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks for debugging this!

-- 
tejun

Re: [PATCH] blk-rq-qos: fix crash on rq_qos_wait vs. rq_qos_wake_function race

[Johannes Thumshirn]

Looks good,
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>

Re: [PATCH] blk-rq-qos: fix crash on rq_qos_wait vs. rq_qos_wake_function race

[Jens Axboe]

On Tue, 15 Oct 2024 10:59:46 -0700, Omar Sandoval wrote:
> 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
>    ...
> 
> [...]

Applied, thanks!

[1/1] blk-rq-qos: fix crash on rq_qos_wait vs. rq_qos_wake_function race
      commit: e972b08b91ef48488bae9789f03cfedb148667fb

Best regards,
-- 
Jens Axboe

Re: [PATCH] blk-rq-qos: fix crash on rq_qos_wait vs. rq_qos_wake_function race

[Jens Axboe]

On 10/15/24 11:59 AM, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> 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().

Thanks Omar, nice debugging!

-- 
Jens Axboe