linux-kernel - [PATCH] mtd: cfi_cmdset_0001: Fix a potential double mutex

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Message-Id: <20171022082301.26220-1-christophe.jaillet@wanadoo.fr>
Date:   Sun, 22 Oct 2017 10:23:01 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     dwmw2@...radead.org, computersforpeace@...il.com,
        boris.brezillon@...e-electrons.com, marek.vasut@...il.com,
        richard@....at, cyrille.pitchen@...ev4u.fr
Cc:     linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Subject: [PATCH] mtd: cfi_cmdset_0001: Fix a potential double mutex_lock

If 'chip->state == FL_SYNCING', we will 'goto retry' with the mutex
'&shared->lock' already taken.
In such a case, the 'mutex_lock' at line 927 can never succeed.

In order to avoid a deadlock, move the 'mutex_lock(&shared->lock)' at the
very end of the block.

This has been spotted with the following coccinelle script:

@find@
expression x, t;
@@

	mutex_lock(x);
	... when != mutex_unlock(x)
	mutex_lock(t);

@@
expression find.t;
expression find.x;
@@

*	mutex_lock(t);
	... when != mutex_unlock(t)
*	mutex_lock(x);


Fixes: 3afe7eb37f4d ("[MTD] [NOR] fix cfi_cmdset_0001 FL_SYNCING race (take 2)")
Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
Review carefuly, untested.
---
 drivers/mtd/chips/cfi_cmdset_0001.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/mtd/chips/cfi_cmdset_0001.c b/drivers/mtd/chips/cfi_cmdset_0001.c
index 5e1b68cbcd0a..bddf407a366d 100644
--- a/drivers/mtd/chips/cfi_cmdset_0001.c
+++ b/drivers/mtd/chips/cfi_cmdset_0001.c
@@ -952,7 +952,6 @@ static int get_chip(struct map_info *map, struct flchip *chip, unsigned long adr
 				mutex_unlock(&contender->mutex);
 				return ret;
 			}
-			mutex_lock(&shared->lock);
 
 			/* We should not own chip if it is already
 			 * in FL_SYNCING state. Put contender and retry. */
@@ -962,6 +961,8 @@ static int get_chip(struct map_info *map, struct flchip *chip, unsigned long adr
 				goto retry;
 			}
 			mutex_unlock(&contender->mutex);
+
+			mutex_lock(&shared->lock);
 		}
 
 		/* Check if we already have suspended erase
-- 
2.14.1