linux-kernel - [PATCH v4 3/8] mm: validate mm in do

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Date:   Wed, 12 Feb 2020 15:39:41 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        linux-api@...r.kernel.org, oleksandr@...hat.com,
        Suren Baghdasaryan <surenb@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Dias <joaodias@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Minchan Kim <minchan@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: [PATCH v4 3/8] mm: validate mm in do_madvise

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

Cc: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 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