staging: android: ashmem: Fix possible deadlock in ashmem_ioctl

ashmem_mutex may create a chain of dependencies like:

CPU0                                    CPU1
 mmap syscall                           ioctl syscall
 -> mmap_sem (acquired)                 -> ashmem_ioctl
 -> ashmem_mmap                            -> ashmem_mutex (acquired)
    -> ashmem_mutex (try to acquire)       -> copy_from_user
                                              -> mmap_sem (try to acquire)

There is a lock odering problem between mmap_sem and ashmem_mutex causing
a lockdep splat[1] during a syzcaller test. This patch fixes the problem
by move copy_from_user out of ashmem_mutex.

[1] https://www.spinics.net/lists/kernel/msg2733200.html

Fixes: ce8a3a9e76d0 (staging: android: ashmem: Fix a race condition in pin ioctls)
Reported-by: syzbot+d7a918a7a8e1c952bc36@syzkaller.appspotmail.com
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 40761f6cd62764b13ad8881b5e5fe041349035fa..69df278e9aa471e9cd9d4b68d939ac8f8bc4cbef 100644 (file)
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -701,16 +701,14 @@ 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;