splice: fix user pointer access in get_iovec_page_array()

Commit 8811930d ("splice: missing user
pointer access verification") added the proper access_ok() calls to
copy_from_user_mmap_sem() which ensures we can copy the struct iovecs
from userspace to the kernel.

But we also must check whether we can access the actual memory region
pointed to by the struct iovec to fix the access checks properly.
Signed-off-by: NBastian Blank <waldi@debian.org>
Acked-by: NOliver Pinter <oliver.pntr@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NPekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/splice.c

@@ -1234,7 +1234,7 @@ static int get_iovec_page_array(const struct iovec __user *iov,
 		if (unlikely(!len))
 			break;
 		error = -EFAULT;
-		if (unlikely(!base))
+		if (!access_ok(VERIFY_READ, base, len))
 			break;
 
 		/*