From: Naoya Horiguchi n-horiguchi@ah.jp.nec.com Subject: 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.)

Link: http://lkml.kernel.org/r/20190116093046.GA29835@hori1.linux.bs1.fc.nec.co.jp Signed-off-by: Naoya Horiguchi n-horiguchi@ah.jp.nec.com Reported-by: Jane Chu jane.chu@oracle.com Reviewed-by: Dan Williams dan.j.williams@intel.com Reviewed-by: William Kucharski william.kucharski@oracle.com Cc: Oleg Nesterov oleg@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memory-failure.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/memory-failure.c~mm-hwpoison-use-do_send_sig_info-instead-of-force_sig-re-pmem-error-handling-forces-sigkill-causes-kernel-panic +++ a/mm/memory-failure.c @@ -372,7 +372,8 @@ static void kill_procs(struct list_head 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); }

/* _