[PATCH v4 3/8] mm: validate mm in do_madvise

[Minchan Kim]

Oleg pointed out mm could be nulllified right after mm_access succeeds.
This patch validates it before the using.

Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/madvise.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 8611f1d39289..bb04c7897eb9 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1073,7 +1073,11 @@ int do_madvise(struct task_struct *task, unsigned long start,
 	int write;
 	size_t len;
 	struct blk_plug plug;
-	struct mm_struct *mm = task->mm;
+	struct mm_struct *mm = READ_ONCE(task->mm);
+
+	/* task can exit and nullify its ->mm right after mm_access() */
+	if (!mm)
+		return -ESRCH;
 
 	start = untagged_addr(start);
 
-- 
2.25.0.225.g125e21ebc7-goog