mm: fix vm_brk_flags() to not bail out while holding lock

Calling vm_brk_flags() with flags set other than VM_EXEC will exit the
function without releasing the mmap_write_lock.

Just do the sanity check before the lock is acquired.  This doesn't fix an
actual issue since no caller sets a flag other than VM_EXEC.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 2e7ce7d ("mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()")
Signed-off-by: Sebastian Ott <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Cc: Yu Zhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Sebastian Ott

mm/mmap.c

@@ -3143,13 +3143,13 @@ int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
 	if (!len)
 		return 0;
 
-	if (mmap_write_lock_killable(mm))
-		return -EINTR;
-
 	/* Until we need other flags, refuse anything except VM_EXEC. */
 	if ((flags & (~VM_EXEC)) != 0)
 		return -EINVAL;
 
+	if (mmap_write_lock_killable(mm))
+		return -EINTR;
+
 	ret = check_brk_limits(addr, len);
 	if (ret)
 		goto limits_failed;