[PATCH] mm: Fix vm_brk_flags() to not bail out while holding lock

[Kees Cook]

From: Sebastian Ott <sebott@xxxxxxxxxx>

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.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Fixes: 2e7ce7d354f2 ("mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()")
Signed-off-by: Sebastian Ott <sebott@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 mm/mmap.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 34d2337ace59..c8996fe847c9 100644
--- a/mm/mmap.c
+++ b/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;
-- 
2.34.1