linux-kernel - [PATCH v2] binder: Don't use mmput() from shrinker function.

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Message-ID: <4ba9adb2-43f5-2de0-22de-f6075c1fab50@i-love.sakura.ne.jp>
Date:   Fri, 17 Jul 2020 00:12:15 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjonnevag <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <christian@...uner.io>
Cc:     Michal Hocko <mhocko@...nel.org>,
        syzbot <syzbot+e5344baa319c9a96edec@...kaller.appspotmail.com>,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, linux-kernel@...r.kernel.org,
        mark.rutland@....com, mingo@...hat.com, namhyung@...nel.org,
        peterz@...radead.org, syzkaller-bugs@...glegroups.com,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        linux-mm <linux-mm@...ck.org>
Subject: [PATCH v2] binder: Don't use mmput() from shrinker function.

syzbot is reporting that mmput() from shrinker function has a risk of
deadlock [1], for delayed_uprobe_add() from update_ref_ctr() calls
kzalloc(GFP_KERNEL) with delayed_uprobe_lock held, and
uprobe_clear_state() from __mmput() also holds delayed_uprobe_lock.

Commit a1b2289cef92ef0e ("android: binder: drop lru lock in isolate
callback") replaced mmput() with mmput_async() in order to avoid sleeping
with spinlock held. But this patch replaces mmput() with mmput_async() in
order not to start __mmput() from shrinker context.

[1] https://syzkaller.appspot.com/bug?id=bc9e7303f537c41b2b0cc2dfcea3fc42964c2d45

Reported-by: syzbot <syzbot+1068f09c44d151250c33@...kaller.appspotmail.com>
Reported-by: syzbot <syzbot+e5344baa319c9a96edec@...kaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
 drivers/android/binder_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 42c672f1584e..cbe6aa77d50d 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -947,7 +947,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
 		trace_binder_unmap_user_end(alloc, index);
 	}
 	mmap_read_unlock(mm);
-	mmput(mm);
+	mmput_async(mm);
 
 	trace_binder_unmap_kernel_start(alloc, index);
 
-- 
2.18.4