mm: hwpoison: use do_send_sig_info() instead of force_sig() (Re: PMEM error-handling forces SIGKILL causes kernel panic)

Commit Message

Naoya Horiguchi Jan. 16, 2019, 9:30 a.m. UTC

[ CCed Andrew and linux-mm ]

On Fri, Jan 11, 2019 at 08:14:02AM +0000, Horiguchi Naoya(堀口 直也) wrote:
> Hi Dan, Jane,
> 
> Thanks for the report.
> 
> On Wed, Jan 09, 2019 at 03:49:32PM -0800, Dan Williams wrote:
> > [ switch to text mail, add lkml and Naoya ]
> > 
> > On Wed, Jan 9, 2019 at 12:19 PM Jane Chu <[e-mail]> wrote:
> ...
> > > 3. The hardware consists the latest revision CPU and Intel NVDIMM, we suspected
> > >    the CPU faulty because it generated MCE over PMEM UE in a unlikely high
> > >    rate for any reasonable NVDIMM (like a few per 24hours).
> > >
> > > After swapping the CPU, the problem stopped reproducing.
> > >
> > > But one could argue that perhaps the faulty CPU exposed a small race window
> > > from collect_procs() to unmap_mapping_range() and to kill_procs(), hence
> > > caught the kernel  PMEM error handler off guard.
> > 
> > There's definitely a race, and the implementation is buggy as can be
> > seen in __exit_signal:
> > 
> >         sighand = rcu_dereference_check(tsk->sighand,
> >                                         lockdep_tasklist_lock_is_held());
> >         spin_lock(&sighand->siglock);
> > 
> > ...the memory-failure path needs to hold the proper locks before it
> > can assume that de-referencing tsk->sighand is valid.
> > 
> > > Also note, the same workload on the same faulty CPU were run on Linux prior to
> > > the 4.19 PMEM error handling and did not encounter kernel crash, probably because
> > > the prior HWPOISON handler did not force SIGKILL?
> > 
> > Before 4.19 this test should result in a machine-check reboot, not
> > much better than a kernel crash.
> > 
> > > Should we not to force the SIGKILL, or find a way to close the race window?
> > 
> > The race should be closed by holding the proper tasklist and rcu read lock(s).
> 
> This reasoning and proposal sound right to me. I'm trying to reproduce
> this race (for non-pmem case,) but no luck for now. I'll investigate more.

I wrote/tested a patch for this issue.
I think that switching signal API effectively does proper locking.

Thanks,
Naoya Horiguchi
---
From 16dbf6105ff4831f73276d79d5df238ab467de76 Mon Sep 17 00:00:00 2001
From: Naoya Horiguchi <[e-mail]>
Date: Wed, 16 Jan 2019 16:59:27 +0900
Subject: [PATCH] mm: hwpoison: use do_send_sig_info() instead of force_sig()

Currently memory_failure() is racy against process's exiting,
which results in kernel crash by null pointer dereference.

The root cause is that memory_failure() uses force_sig() to forcibly
kill asynchronous (meaning not in the current context) processes.  As
discussed in thread https://lkml.org/lkml/2010/6/8/236 years ago for
OOM fixes, this is not a right thing to do.  OOM solves this issue by
using do_send_sig_info() as done in commit d2d393099de2 ("signal:
oom_kill_task: use SEND_SIG_FORCED instead of force_sig()"), so this
patch is suggesting to do the same for hwpoison.  do_send_sig_info()
properly accesses to siglock with lock_task_sighand(), so is free from
the reported race.

I confirmed that the reported bug reproduces with inserting some delay
in kill_procs(), and it never reproduces with this patch.

Note that memory_failure() can send another type of signal using
force_sig_mceerr(), and the reported race shouldn't happen on it
because force_sig_mceerr() is called only for synchronous processes
(i.e. BUS_MCEERR_AR happens only when some process accesses to the
corrupted memory.)

Reported-by: Jane Chu <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: [e-mail]
Signed-off-by: Naoya Horiguchi <[e-mail]>
---
 mm/memory-failure.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Comments

Dan Williams Jan. 16, 2019, 4:55 p.m. UTC | #1

On Wed, Jan 16, 2019 at 1:33 AM Naoya Horiguchi
<[e-mail]> wrote:
>
> [ CCed Andrew and linux-mm ]
>
> On Fri, Jan 11, 2019 at 08:14:02AM +0000, Horiguchi Naoya(堀口 直也) wrote:
> > Hi Dan, Jane,
> >
> > Thanks for the report.
> >
> > On Wed, Jan 09, 2019 at 03:49:32PM -0800, Dan Williams wrote:
> > > [ switch to text mail, add lkml and Naoya ]
> > >
> > > On Wed, Jan 9, 2019 at 12:19 PM Jane Chu <[e-mail]> wrote:
> > ...
> > > > 3. The hardware consists the latest revision CPU and Intel NVDIMM, we suspected
> > > >    the CPU faulty because it generated MCE over PMEM UE in a unlikely high
> > > >    rate for any reasonable NVDIMM (like a few per 24hours).
> > > >
> > > > After swapping the CPU, the problem stopped reproducing.
> > > >
> > > > But one could argue that perhaps the faulty CPU exposed a small race window
> > > > from collect_procs() to unmap_mapping_range() and to kill_procs(), hence
> > > > caught the kernel  PMEM error handler off guard.
> > >
> > > There's definitely a race, and the implementation is buggy as can be
> > > seen in __exit_signal:
> > >
> > >         sighand = rcu_dereference_check(tsk->sighand,
> > >                                         lockdep_tasklist_lock_is_held());
> > >         spin_lock(&sighand->siglock);
> > >
> > > ...the memory-failure path needs to hold the proper locks before it
> > > can assume that de-referencing tsk->sighand is valid.
> > >
> > > > Also note, the same workload on the same faulty CPU were run on Linux prior to
> > > > the 4.19 PMEM error handling and did not encounter kernel crash, probably because
> > > > the prior HWPOISON handler did not force SIGKILL?
> > >
> > > Before 4.19 this test should result in a machine-check reboot, not
> > > much better than a kernel crash.
> > >
> > > > Should we not to force the SIGKILL, or find a way to close the race window?
> > >
> > > The race should be closed by holding the proper tasklist and rcu read lock(s).
> >
> > This reasoning and proposal sound right to me. I'm trying to reproduce
> > this race (for non-pmem case,) but no luck for now. I'll investigate more.
>
> I wrote/tested a patch for this issue.
> I think that switching signal API effectively does proper locking.
>
> Thanks,
> Naoya Horiguchi
> ---
> From 16dbf6105ff4831f73276d79d5df238ab467de76 Mon Sep 17 00:00:00 2001
> From: Naoya Horiguchi <[e-mail]>
> Date: Wed, 16 Jan 2019 16:59:27 +0900
> Subject: [PATCH] mm: hwpoison: use do_send_sig_info() instead of force_sig()
>
> Currently memory_failure() is racy against process's exiting,
> which results in kernel crash by null pointer dereference.
>
> The root cause is that memory_failure() uses force_sig() to forcibly
> kill asynchronous (meaning not in the current context) processes.  As
> discussed in thread https://lkml.org/lkml/2010/6/8/236 years ago for
> OOM fixes, this is not a right thing to do.  OOM solves this issue by
> using do_send_sig_info() as done in commit d2d393099de2 ("signal:
> oom_kill_task: use SEND_SIG_FORCED instead of force_sig()"), so this
> patch is suggesting to do the same for hwpoison.  do_send_sig_info()
> properly accesses to siglock with lock_task_sighand(), so is free from
> the reported race.
>
> I confirmed that the reported bug reproduces with inserting some delay
> in kill_procs(), and it never reproduces with this patch.
>
> Note that memory_failure() can send another type of signal using
> force_sig_mceerr(), and the reported race shouldn't happen on it
> because force_sig_mceerr() is called only for synchronous processes
> (i.e. BUS_MCEERR_AR happens only when some process accesses to the
> corrupted memory.)
>
> Reported-by: Jane Chu <[e-mail]>
> Cc: Dan Williams <[e-mail]>
> Cc: [e-mail]
> Signed-off-by: Naoya Horiguchi <[e-mail]>
> ---

Looks good to me.

Reviewed-by: Dan Williams <[e-mail]>

...but it would still be good to get a Tested-by from Jane.

Jane Chu Jan. 16, 2019, 5:20 p.m. UTC | #2

On 1/16/2019 8:55 AM, Dan Williams wrote:
> On Wed, Jan 16, 2019 at 1:33 AM Naoya Horiguchi
> <[e-mail]> wrote:
>> [ CCed Andrew and linux-mm ]
>>
>> On Fri, Jan 11, 2019 at 08:14:02AM +0000, Horiguchi Naoya(堀口 直也) wrote:
>>> Hi Dan, Jane,
>>>
>>> Thanks for the report.
>>>
>>> On Wed, Jan 09, 2019 at 03:49:32PM -0800, Dan Williams wrote:
>>>> [ switch to text mail, add lkml and Naoya ]
>>>>
>>>> On Wed, Jan 9, 2019 at 12:19 PM Jane Chu <[e-mail]> wrote:
>>> ...
>>>>> 3. The hardware consists the latest revision CPU and Intel NVDIMM, we suspected
>>>>>     the CPU faulty because it generated MCE over PMEM UE in a unlikely high
>>>>>     rate for any reasonable NVDIMM (like a few per 24hours).
>>>>>
>>>>> After swapping the CPU, the problem stopped reproducing.
>>>>>
>>>>> But one could argue that perhaps the faulty CPU exposed a small race window
>>>>> from collect_procs() to unmap_mapping_range() and to kill_procs(), hence
>>>>> caught the kernel  PMEM error handler off guard.
>>>> There's definitely a race, and the implementation is buggy as can be
>>>> seen in __exit_signal:
>>>>
>>>>          sighand = rcu_dereference_check(tsk->sighand,
>>>>                                          lockdep_tasklist_lock_is_held());
>>>>          spin_lock(&sighand->siglock);
>>>>
>>>> ...the memory-failure path needs to hold the proper locks before it
>>>> can assume that de-referencing tsk->sighand is valid.
>>>>
>>>>> Also note, the same workload on the same faulty CPU were run on Linux prior to
>>>>> the 4.19 PMEM error handling and did not encounter kernel crash, probably because
>>>>> the prior HWPOISON handler did not force SIGKILL?
>>>> Before 4.19 this test should result in a machine-check reboot, not
>>>> much better than a kernel crash.
>>>>
>>>>> Should we not to force the SIGKILL, or find a way to close the race window?
>>>> The race should be closed by holding the proper tasklist and rcu read lock(s).
>>> This reasoning and proposal sound right to me. I'm trying to reproduce
>>> this race (for non-pmem case,) but no luck for now. I'll investigate more.
>> I wrote/tested a patch for this issue.
>> I think that switching signal API effectively does proper locking.
>>
>> Thanks,
>> Naoya Horiguchi
>> ---
>>  From 16dbf6105ff4831f73276d79d5df238ab467de76 Mon Sep 17 00:00:00 2001
>> From: Naoya Horiguchi <[e-mail]>
>> Date: Wed, 16 Jan 2019 16:59:27 +0900
>> Subject: [PATCH] mm: hwpoison: use do_send_sig_info() instead of force_sig()
>>
>> Currently memory_failure() is racy against process's exiting,
>> which results in kernel crash by null pointer dereference.
>>
>> The root cause is that memory_failure() uses force_sig() to forcibly
>> kill asynchronous (meaning not in the current context) processes.  As
>> discussed in thread https://lkml.org/lkml/2010/6/8/236 years ago for
>> OOM fixes, this is not a right thing to do.  OOM solves this issue by
>> using do_send_sig_info() as done in commit d2d393099de2 ("signal:
>> oom_kill_task: use SEND_SIG_FORCED instead of force_sig()"), so this
>> patch is suggesting to do the same for hwpoison.  do_send_sig_info()
>> properly accesses to siglock with lock_task_sighand(), so is free from
>> the reported race.
>>
>> I confirmed that the reported bug reproduces with inserting some delay
>> in kill_procs(), and it never reproduces with this patch.
>>
>> Note that memory_failure() can send another type of signal using
>> force_sig_mceerr(), and the reported race shouldn't happen on it
>> because force_sig_mceerr() is called only for synchronous processes
>> (i.e. BUS_MCEERR_AR happens only when some process accesses to the
>> corrupted memory.)
>>
>> Reported-by: Jane Chu <[e-mail]>
>> Cc: Dan Williams <[e-mail]>
>> Cc: [e-mail]
>> Signed-off-by: Naoya Horiguchi <[e-mail]>
>> ---
> Looks good to me.
>
> Reviewed-by: Dan Williams <[e-mail]>
>
> ...but it would still be good to get a Tested-by from Jane.

Sure, will let you know how the test goes.

Thanks!
-jane

Jane Chu Jan. 16, 2019, 5:56 p.m. UTC | #3

Hi, Naoya,

On 1/16/2019 1:30 AM, Naoya Horiguchi wrote:
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 7c72f2a95785..831be5ff5f4d 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -372,7 +372,8 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
>   			if (fail || tk->addr_valid == 0) {
>   				pr_err("Memory failure: %#lx: forcibly killing %s:%d because of failure to unmap corrupted page\n",
>   				       pfn, tk->tsk->comm, tk->tsk->pid);
> -				force_sig(SIGKILL, tk->tsk);
> +				do_send_sig_info(SIGKILL, SEND_SIG_PRIV,
> +						 tk->tsk, PIDTYPE_PID);
>   			}
>   

Since we don't care the return from do_send_sig_info(), would you mind to
prefix it with (void) ?

thanks!
-jane

Naoya Horiguchi Jan. 16, 2019, 11:32 p.m. UTC | #4

Hi Jane,

On Wed, Jan 16, 2019 at 09:56:02AM -0800, Jane Chu wrote:
> Hi, Naoya,
> 
> On 1/16/2019 1:30 AM, Naoya Horiguchi wrote:
> 
>     diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>     index 7c72f2a95785..831be5ff5f4d 100644
>     --- a/mm/memory-failure.c
>     +++ b/mm/memory-failure.c
>     @@ -372,7 +372,8 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
>                             if (fail || tk->addr_valid == 0) {
>                                     pr_err("Memory failure: %#lx: forcibly killing %s:%d because of failure to unmap corrupted page\n",
>                                            pfn, tk->tsk->comm, tk->tsk->pid);
>     -                               force_sig(SIGKILL, tk->tsk);
>     +                               do_send_sig_info(SIGKILL, SEND_SIG_PRIV,
>     +                                                tk->tsk, PIDTYPE_PID);
>                             }
> 
> 
> Since we don't care the return from do_send_sig_info(), would you mind to
> prefix it with (void) ?

Sorry, I'm not sure about the benefit to do casting the return value
just being ignored, so personally I'd like keeping the code simple.
Do you have some in mind?

- Naoya

Jane Chu Jan. 17, 2019, 1:07 a.m. UTC | #5

On 1/16/2019 3:32 PM, Naoya Horiguchi wrote:
> Hi Jane,
> 
> On Wed, Jan 16, 2019 at 09:56:02AM -0800, Jane Chu wrote:
>> Hi, Naoya,
>>
>> On 1/16/2019 1:30 AM, Naoya Horiguchi wrote:
>>
>>      diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>      index 7c72f2a95785..831be5ff5f4d 100644
>>      --- a/mm/memory-failure.c
>>      +++ b/mm/memory-failure.c
>>      @@ -372,7 +372,8 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
>>                              if (fail || tk->addr_valid == 0) {
>>                                      pr_err("Memory failure: %#lx: forcibly killing %s:%d because of failure to unmap corrupted page\n",
>>                                             pfn, tk->tsk->comm, tk->tsk->pid);
>>      -                               force_sig(SIGKILL, tk->tsk);
>>      +                               do_send_sig_info(SIGKILL, SEND_SIG_PRIV,
>>      +                                                tk->tsk, PIDTYPE_PID);
>>                              }
>>
>>
>> Since we don't care the return from do_send_sig_info(), would you mind to
>> prefix it with (void) ?
> 
> Sorry, I'm not sure about the benefit to do casting the return value
> just being ignored, so personally I'd like keeping the code simple.
> Do you have some in mind?

It's just coding style I'm used to, no big deal.
Up to you to decide. :)

thanks,
-jane

> 
> - Naoya
>

William Kucharski Jan. 17, 2019, 9:44 a.m. UTC | #6

> On Jan 16, 2019, at 6:07 PM, Jane Chu <[e-mail]> wrote:
> 
> It's just coding style I'm used to, no big deal.
> Up to you to decide. :)

Personally I like a (void) cast as it's pretty long-standing syntactic sugar to cast a call that returns a value we don't care about to (void) to show we know it returns a value and we don't care.

Without it, it may suggest we either didn't know it returned a value or that we neglected to check the return value.

However, in current use elsewhere (e.g. in send_sig_all() and __oom_kill_process()), no such (void) cast is added, so it seems better to match current usage elsewhere in the kernel.

Reviewed-by: William Kucharski <[e-mail]>

Patch

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 7c72f2a95785..831be5ff5f4d 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -372,7 +372,8 @@  static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
 			if (fail || tk->addr_valid == 0) {
 				pr_err("Memory failure: %#lx: forcibly killing %s:%d because of failure to unmap corrupted page\n",
 				       pfn, tk->tsk->comm, tk->tsk->pid);
-				force_sig(SIGKILL, tk->tsk);
+				do_send_sig_info(SIGKILL, SEND_SIG_PRIV,
+						 tk->tsk, PIDTYPE_PID);
 			}
 
 			/*