[PATCH 4.9 037/145] staging: android: ashmem: Fix a race condition in pin ioctls

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ben Hutchings <b...@decadent.org.uk>

commit ce8a3a9e76d0193e2e8d74a06d275b3c324ca652 upstream.

ashmem_pin_unpin() reads asma->file and asma->size before taking the
ashmem_mutex, so it can race with other operations that modify them.

Build-tested only.

Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 drivers/staging/android/ashmem.c |   19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -719,30 +719,32 @@ static int ashmem_pin_unpin(struct ashme
        size_t pgstart, pgend;
        int ret = -EINVAL;
 
+       mutex_lock(&ashmem_mutex);
+
        if (unlikely(!asma->file))
-               return -EINVAL;
+               goto out_unlock;
 
-       if (unlikely(copy_from_user(&pin, p, sizeof(pin))))
-               return -EFAULT;
+       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;
 
        if (unlikely((pin.offset | pin.len) & ~PAGE_MASK))
-               return -EINVAL;
+               goto out_unlock;
 
        if (unlikely(((__u32)-1) - pin.offset < pin.len))
-               return -EINVAL;
+               goto out_unlock;
 
        if (unlikely(PAGE_ALIGN(asma->size) < pin.offset + pin.len))
-               return -EINVAL;
+               goto out_unlock;
 
        pgstart = pin.offset / PAGE_SIZE;
        pgend = pgstart + (pin.len / PAGE_SIZE) - 1;
 
-       mutex_lock(&ashmem_mutex);
-
        switch (cmd) {
        case ASHMEM_PIN:
                ret = ashmem_pin(asma, pgstart, pgend);
@@ -755,6 +757,7 @@ static int ashmem_pin_unpin(struct ashme
                break;
        }
 
+out_unlock:
        mutex_unlock(&ashmem_mutex);
 
        return ret;