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/20211022014658.263508-1-surenb@google.com
Fixes: 884a7e59 ("mm: introduce process_mrelease system call")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Christian Brauner <christian@brauner.io>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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: