mm/memory-failure: send SIGBUS(BUS_MCEERR_AR) only to current thread

Action Required memory error should happen only when a processor is
about to access to a corrupted memory, so it's synchronous and only
affects current process/thread.

Recently commit 872e9a20 ("mm, memory_failure: don't send
BUS_MCEERR_AO for action required error") fixed the issue that Action
Required memory could unnecessarily send SIGBUS to the processes which
share the error memory.  But we still have another issue that we could
send SIGBUS to a wrong thread.

This is because collect_procs() and task_early_kill() fails to add the
current process to "to-kill" list.  So this patch is suggesting to fix
it.  With this fix, SIGBUS(BUS_MCEERR_AR) is never sent to non-current
process/thread.
Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Tony Luck <tony.luck@intel.com>
Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Link: http://lkml.kernel.org/r/1591321039-22141-3-git-send-email-naoya.horiguchi@nec.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory-failure.c

@@ -212,15 +212,13 @@ static int kill_proc(struct to_kill *tk, unsigned long pfn, int flags)
 	short addr_lsb = tk->size_shift;
 	int ret = 0;
 
-	if ((t->mm == current->mm) || !(flags & MF_ACTION_REQUIRED))
-		pr_err("Memory failure: %#lx: Sending SIGBUS to %s:%d due to hardware memory corruption\n",
+	pr_err("Memory failure: %#lx: Sending SIGBUS to %s:%d due to hardware memory corruption\n",
 			pfn, t->comm, t->pid);
 
 	if (flags & MF_ACTION_REQUIRED) {
-		if (t->mm == current->mm)
-			ret = force_sig_mceerr(BUS_MCEERR_AR,
+		WARN_ON_ONCE(t != current);
+		ret = force_sig_mceerr(BUS_MCEERR_AR,
 					 (void __user *)tk->addr, addr_lsb);
-		/* send no signal to non-current processes */
 	} else {
 		/*
 		 * Don't use force here, it's convenient if the signal
@@ -419,14 +417,25 @@ static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
  * to be signaled when some page under the process is hwpoisoned.
  * Return task_struct of the dedicated thread (main thread unless explicitly
  * specified) if the process is "early kill," and otherwise returns NULL.
+ *
+ * Note that the above is true for Action Optional case, but not for Action
+ * Required case where SIGBUS should sent only to the current thread.
  */
 static struct task_struct *task_early_kill(struct task_struct *tsk,
 					   int force_early)
 {
 	if (!tsk->mm)
 		return NULL;
-	if (force_early)
-		return tsk;
+	if (force_early) {
+		/*
+		 * Comparing ->mm here because current task might represent
+		 * a subthread, while tsk always points to the main thread.
+		 */
+		if (tsk->mm == current->mm)
+			return current;
+		else
+			return NULL;
+	}
 	return find_early_kill_thread(tsk);
 }