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From:   Paul Lawrence <paullawrence@google.com>
To:     linux-kernel@vger.kernel.org
Cc:     Paul Lawrence <paullawrence@google.com>, stable@vger.kernel.org,
        Ben Hutchings <ben@decadent.org.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?q?Arve=20Hj=C3=B8nnev=C3=A5g?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>, devel@driverdev.osuosl.org
Subject: [PATCH] staging: android: ashmem: Remove deadlock
Date:   Wed,  7 Mar 2018 13:40:30 -0800
Message-Id: <20180307214042.135109-1-paullawrence@google.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Regression introduced in commit ce8a3a9e76d0193e2e8d74a06d275b3c324ca652
("staging: android: ashmem: Fix a race condition in pin ioctls")
causing deadlock.

No need to hold ashmem_mutex while copying from user

Stacks are:

ashmem_mmap+0x53/0x400 drivers/staging/android/ashmem.c:379
mmap_region+0x7dd/0xfd0 mm/mmap.c:1694
do_mmap+0x57b/0xbe0 mm/mmap.c:1473

and

lock_acquire+0x12e/0x410 kernel/locking/lockdep.c:3756
__might_fault+0x14a/0x1d0 mm/memory.c:4014
copy_from_user arch/x86/include/asm/uaccess.h:705 [inline]
ashmem_pin_unpin drivers/staging/android/ashmem.c:719 [inline]

Signed-off-by: Paul Lawrence <paullawrence@google.com>
Cc: <stable@vger.kernel.org> # 4.9.x
Cc: <stable@vger.kernel.org> # 4.4.x
Cc: <stable@vger.kernel.org> # 3.18.x: ce8a3a9e76d01
Cc: <stable@vger.kernel.org> # 3.18.x
Cc: Ben Hutchings <ben@decadent.org.uk>
---
 drivers/staging/android/ashmem.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 6dbba5aff191..8c55706c2cfa 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -702,16 +702,14 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
 	size_t pgstart, pgend;
 	int ret = -EINVAL;
 
+	if (unlikely(copy_from_user(&pin, p, sizeof(pin))))
+		return -EFAULT;
+
 	mutex_lock(&ashmem_mutex);
 
 	if (unlikely(!asma->file))
 		goto out_unlock;
 
-	if (unlikely(copy_from_user(&pin, p, sizeof(pin)))) {
-		ret = -EFAULT;
-		goto out_unlock;
-	}
-
 	/* per custom, you can pass zero for len to mean "everything onward" */
 	if (!pin.len)
 		pin.len = PAGE_ALIGN(asma->size) - pin.offset;
-- 
2.16.2.395.g2e18187dfd-goog