[ 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@xxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxx> Cc: Dan Williams <dan.j.williams@xxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> --- 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
