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

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

[ 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 <jane.chu@oracle.com> 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 <n-horiguchi@ah.jp.nec.com>
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 <jane.chu@oracle.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 mm/memory-failure.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

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);
 			}
 
 			/*
-- 
2.7.5