[PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

[Suren Baghdasaryan <surenb@google.com>]

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.

Fixes: 884a7e5964e0 ("mm: introduce process_mrelease system call")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/oom_kill.c | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 831340e7ad8b..989f35a2bbb1 100644
--- a/mm/oom_kill.c
+++ b/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:
-- 
2.33.0.1079.g6e70778dc9-goog