* [PATCH] ext4: avoid huge mmp update interval value
@ 2021-08-05 15:14 Pavel Skripkin
2021-08-05 19:45 ` Theodore Ts'o
0 siblings, 1 reply; 5+ messages in thread
From: Pavel Skripkin @ 2021-08-05 15:14 UTC (permalink / raw)
To: tytso, adilger.kernel, johann
Cc: linux-ext4, linux-kernel, Pavel Skripkin, syzbot+c9ff4822a62eee994ea3
Syzbot reported task hung bug in ext4_fill_super(). The problem was in
too huge mmp update interval.
Syzkaller reproducer setted s_mmp_update_interval to 39785 seconds. This
update interaval is unreasonable huge and it can cause tasks to hung on
kthread_stop() call, since it will wait until timeout timer expires.
To avoid this sutiation, I've added MIN and MAX constants for kmmp
interval and clamped s_mmp_update_interval within the boundaries
Reported-and-tested-by: syzbot+c9ff4822a62eee994ea3@syzkaller.appspotmail.com
Fixes: c5e06d101aaf ("ext4: add support for multiple mount protection")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---
Hi, Ted and ext4 maintainers!
I am not sure about min/max values for interval, so I look forward to
receiving your views on these values and patch in general!
With regards,
Pavel Skripkin
---
fs/ext4/mmp.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index bc364c119af6..160abee66dce 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -7,6 +7,9 @@
#include "ext4.h"
+#define EXT4_KMMP_MAX_INTERVAL 100
+#define EXT4_KMMP_MIN_INTERVAL 5
+
/* Checksumming functions */
static __le32 ext4_mmp_csum(struct super_block *sb, struct mmp_struct *mmp)
{
@@ -140,6 +143,12 @@ static int kmmpd(void *data)
unsigned long diff;
int retval;
+ /* We should avoid unreasonable huge update interval, since it can cause
+ * task hung bug on umount or on error handling path in ext4_fill_super()
+ */
+ mmp_update_interval = clamp(mmp_update_interval, EXT4_KMMP_MIN_INTERVAL,
+ EXT4_KMMP_MAX_INTERVAL);
+
mmp_block = le64_to_cpu(es->s_mmp_block);
mmp = (struct mmp_struct *)(bh->b_data);
mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
@@ -156,6 +165,9 @@ static int kmmpd(void *data)
memcpy(mmp->mmp_nodename, init_utsname()->nodename,
sizeof(mmp->mmp_nodename));
+ ext4_msg(sb, KERN_INFO, "Started kmmp thread with update interval = %u\n",
+ mmp_update_interval);
+
while (!kthread_should_stop() && !sb_rdonly(sb)) {
if (!ext4_has_feature_mmp(sb)) {
ext4_warning(sb, "kmmpd being stopped since MMP feature"
--
2.32.0
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH] ext4: avoid huge mmp update interval value
2021-08-05 15:14 [PATCH] ext4: avoid huge mmp update interval value Pavel Skripkin
@ 2021-08-05 19:45 ` Theodore Ts'o
2021-08-05 20:12 ` Pavel Skripkin
0 siblings, 1 reply; 5+ messages in thread
From: Theodore Ts'o @ 2021-08-05 19:45 UTC (permalink / raw)
To: Pavel Skripkin
Cc: adilger.kernel, johann, linux-ext4, linux-kernel,
syzbot+c9ff4822a62eee994ea3
On Thu, Aug 05, 2021 at 06:14:18PM +0300, Pavel Skripkin wrote:
> Syzbot reported task hung bug in ext4_fill_super(). The problem was in
> too huge mmp update interval.
>
> Syzkaller reproducer setted s_mmp_update_interval to 39785 seconds. This
> update interaval is unreasonable huge and it can cause tasks to hung on
> kthread_stop() call, since it will wait until timeout timer expires.
I must be missing something. kthread_stop() should wake up the
kmmpd() thread, which should see kthread_should_stop(), and then it
should exit. What is causing it to wait until the timeout timer
expires?
- Ted
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] ext4: avoid huge mmp update interval value
2021-08-05 19:45 ` Theodore Ts'o
@ 2021-08-05 20:12 ` Pavel Skripkin
2021-08-05 22:59 ` Dave Chinner
0 siblings, 1 reply; 5+ messages in thread
From: Pavel Skripkin @ 2021-08-05 20:12 UTC (permalink / raw)
To: Theodore Ts'o
Cc: adilger.kernel, johann, linux-ext4, linux-kernel,
syzbot+c9ff4822a62eee994ea3
On 8/5/21 10:45 PM, Theodore Ts'o wrote:
> On Thu, Aug 05, 2021 at 06:14:18PM +0300, Pavel Skripkin wrote:
>> Syzbot reported task hung bug in ext4_fill_super(). The problem was in
>> too huge mmp update interval.
>>
>> Syzkaller reproducer setted s_mmp_update_interval to 39785 seconds. This
>> update interaval is unreasonable huge and it can cause tasks to hung on
>> kthread_stop() call, since it will wait until timeout timer expires.
>
> I must be missing something. kthread_stop() should wake up the
> kmmpd() thread, which should see kthread_should_stop(), and then it
> should exit. What is causing it to wait until the timeout timer
> expires?
>
> - Ted
>
Hi, Ted!
I guess, I've explained my idea badly, sorry :)
I mean, that there is a chance to hit this situation:
CPU0 CPU1
kthread_should_stop() <-- false
kthread_stop()
set_bit(KTHREAD_SHOULD_STOP)
wake_up_process()
wait_for_completion()
schedule_timeout_interruptible()
*waits until timer expires*
Since there wasn't any validation checks for mmp_update_interval, CPU0
will wait for up to (1 << 16) seconds (s_mmp_update_interval it __le16).
With regards,
Pavel Skripkin
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] ext4: avoid huge mmp update interval value
2021-08-05 20:12 ` Pavel Skripkin
@ 2021-08-05 22:59 ` Dave Chinner
2021-08-06 10:20 ` Pavel Skripkin
0 siblings, 1 reply; 5+ messages in thread
From: Dave Chinner @ 2021-08-05 22:59 UTC (permalink / raw)
To: Pavel Skripkin
Cc: Theodore Ts'o, adilger.kernel, johann, linux-ext4,
linux-kernel, syzbot+c9ff4822a62eee994ea3
On Thu, Aug 05, 2021 at 11:12:42PM +0300, Pavel Skripkin wrote:
> On 8/5/21 10:45 PM, Theodore Ts'o wrote:
> > On Thu, Aug 05, 2021 at 06:14:18PM +0300, Pavel Skripkin wrote:
> > > Syzbot reported task hung bug in ext4_fill_super(). The problem was in
> > > too huge mmp update interval.
> > >
> > > Syzkaller reproducer setted s_mmp_update_interval to 39785 seconds. This
> > > update interaval is unreasonable huge and it can cause tasks to hung on
> > > kthread_stop() call, since it will wait until timeout timer expires.
> >
> > I must be missing something. kthread_stop() should wake up the
> > kmmpd() thread, which should see kthread_should_stop(), and then it
> > should exit. What is causing it to wait until the timeout timer
> > expires?
> >
> > - Ted
> >
>
>
> Hi, Ted!
>
> I guess, I've explained my idea badly, sorry :)
>
> I mean, that there is a chance to hit this situation:
>
> CPU0 CPU1
> kthread_should_stop() <-- false
> kthread_stop()
> set_bit(KTHREAD_SHOULD_STOP)
> wake_up_process()
> wait_for_completion()
> schedule_timeout_interruptible()
>
> *waits until timer expires*
Yeah, so the bug here is checking kthread_should_stop() while
the task state is TASK_RUNNING.
What you need to do here is:
while (run) {
....
set_current_state(TASK_INTERRUPTIBLE);
if (kthread_should_stop()) {
__set_current_state(TASK_RUNNING);
break;
}
schedule_timeout(tout);
.....
}
That means in the case above where schedule() occurs after the
kthread_should_stop() check has raced with kthread_stop(), then
wake_up_process() will handle any races with schedule() correctly.
i.e. wake_up_process() will set the task state to TASK_RUNNING and
schedule() will not sleep if it is called after wake_up_process().
Or if schedule() runs first then wake_up_process() will wake it
correctly after setting the state to TASK_RUNNING.
Either way, the loop then runs around again straight away to the next
kthread_should_stop() call, at which point it breaks out.
I note that the "wait_to_exit:" code in the same function does this
properly....
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] ext4: avoid huge mmp update interval value
2021-08-05 22:59 ` Dave Chinner
@ 2021-08-06 10:20 ` Pavel Skripkin
0 siblings, 0 replies; 5+ messages in thread
From: Pavel Skripkin @ 2021-08-06 10:20 UTC (permalink / raw)
To: Dave Chinner, Theodore Ts'o
Cc: adilger.kernel, johann, linux-ext4, linux-kernel,
syzbot+c9ff4822a62eee994ea3
On 8/6/21 1:59 AM, Dave Chinner wrote:
> On Thu, Aug 05, 2021 at 11:12:42PM +0300, Pavel Skripkin wrote:
>> On 8/5/21 10:45 PM, Theodore Ts'o wrote:
>> > On Thu, Aug 05, 2021 at 06:14:18PM +0300, Pavel Skripkin wrote:
>> > > Syzbot reported task hung bug in ext4_fill_super(). The problem was in
>> > > too huge mmp update interval.
>> > >
>> > > Syzkaller reproducer setted s_mmp_update_interval to 39785 seconds. This
>> > > update interaval is unreasonable huge and it can cause tasks to hung on
>> > > kthread_stop() call, since it will wait until timeout timer expires.
>> >
>> > I must be missing something. kthread_stop() should wake up the
>> > kmmpd() thread, which should see kthread_should_stop(), and then it
>> > should exit. What is causing it to wait until the timeout timer
>> > expires?
>> >
>> > - Ted
>> >
>>
>>
>> Hi, Ted!
>>
>> I guess, I've explained my idea badly, sorry :)
>>
>> I mean, that there is a chance to hit this situation:
>>
>> CPU0 CPU1
>> kthread_should_stop() <-- false
>> kthread_stop()
>> set_bit(KTHREAD_SHOULD_STOP)
>> wake_up_process()
>> wait_for_completion()
>> schedule_timeout_interruptible()
>>
>> *waits until timer expires*
>
> Yeah, so the bug here is checking kthread_should_stop() while
> the task state is TASK_RUNNING.
>
> What you need to do here is:
>
> while (run) {
>
> ....
> set_current_state(TASK_INTERRUPTIBLE);
> if (kthread_should_stop()) {
> __set_current_state(TASK_RUNNING);
> break;
> }
> schedule_timeout(tout);
>
> .....
> }
>
>
> That means in the case above where schedule() occurs after the
> kthread_should_stop() check has raced with kthread_stop(), then
> wake_up_process() will handle any races with schedule() correctly.
>
> i.e. wake_up_process() will set the task state to TASK_RUNNING and
> schedule() will not sleep if it is called after wake_up_process().
> Or if schedule() runs first then wake_up_process() will wake it
> correctly after setting the state to TASK_RUNNING.
>
> Either way, the loop then runs around again straight away to the next
> kthread_should_stop() call, at which point it breaks out.
>
> I note that the "wait_to_exit:" code in the same function does this
> properly....
>
Hi, Dave!
I've tested your suggestion with syzbot and it works, thank you!
Anyway, @Ted, does it make sense to add boundaries for
s_mmp_update_interval? I think, too big update interval for mmp isn't
reasonable. I can send patch series with Dave's suggestion and previous
patch. What do you think?
With regards,
Pavel Skripkin
^ permalink raw reply [flat|nested] 5+ messages in thread
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2021-08-05 15:14 [PATCH] ext4: avoid huge mmp update interval value Pavel Skripkin
2021-08-05 19:45 ` Theodore Ts'o
2021-08-05 20:12 ` Pavel Skripkin
2021-08-05 22:59 ` Dave Chinner
2021-08-06 10:20 ` Pavel Skripkin
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