mm/oom_kill.c: prevent a race between process_mrelease and exit_mmap

Race between process_mrelease and exit_mmap, where free_pgtables is
called while __oom_reap_task_mm is in progress, leads to kernel crash
during pte_offset_map_lock call.  oom-reaper avoids this race by setting
MMF_OOM_VICTIM flag and causing exit_mmap to take and release
mmap_write_lock, blocking it until oom-reaper releases mmap_read_lock.

Reusing MMF_OOM_VICTIM for process_mrelease would be the simplest way to
fix this race, however that would be considered a hack.  Fix this race
by elevating mm->mm_users and preventing exit_mmap from executing until
process_mrelease is finished.  Patch slightly refactors the code to
adapt for a possible mmget_not_zero failure.

This fix has considerable negative impact on process_mrelease
performance and will likely need later optimization.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 884a7e5 ("mm: introduce process_mrelease system call")
Signed-off-by: Suren Baghdasaryan <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: Jan Engelhardt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: surenbaghdasaryan

mm/oom_kill.c

@@ -1150,7 +1150,7 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 	struct task_struct *task;
 	struct task_struct *p;
 	unsigned int f_flags;
-	bool reap = true;
+	bool reap = false;
 	struct pid *pid;
 	long ret = 0;
 
@@ -1177,15 +1177,15 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 		goto put_task;
 	}
 
-	mm = p->mm;
-	mmgrab(mm);
-
-	/* If the work has been done already, just exit with success */
-	if (test_bit(MMF_OOM_SKIP, &mm->flags))
-		reap = false;
-	else if (!task_will_free_mem(p)) {
-		reap = false;
-		ret = -EINVAL;
+	if (mmget_not_zero(p->mm)) {
+		mm = p->mm;
+		if (task_will_free_mem(p))
+			reap = true;
+		else {
+			/* Error only if the work has not been done already */
+			if (!test_bit(MMF_OOM_SKIP, &mm->flags))
+				ret = -EINVAL;
+		}
 	}
 	task_unlock(p);
 
@@ -1201,7 +1201,8 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 	mmap_read_unlock(mm);
 
 drop_mm:
-	mmdrop(mm);
+	if (mm)
+		mmput(mm);
 put_task:
 	put_task_struct(task);
 put_pid: