[PATCH] mm/gup: fix null pointer dereference detected by coverity

[Miles Chen <miles.chen@xxxxxxxxxxxx>]

In fixup_user_fault(), it is possible that unlocked is NULL,
so we should test unlocked before using it.

For example, in arch/arc/kernel/process.c, NULL is passed
to fixup_user_fault().

SYSCALL_DEFINE3(arc_usr_cmpxchg, int *, uaddr, int, expected, int, new)
{
...
	ret = fixup_user_fault(current, current->mm, (unsigned long) uaddr,
			       FAULT_FLAG_WRITE, NULL);
...
}

Fixes: 4a9e1cda2748 ("mm: bring in additional flag for fixup_user_fault to signal unlock")
Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
---
 mm/gup.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index da3e03185144..a68d11dc232d 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1230,7 +1230,8 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
 	if (ret & VM_FAULT_RETRY) {
 		down_read(&mm->mmap_sem);
 		if (!(fault_flags & FAULT_FLAG_TRIED)) {
-			*unlocked = true;
+			if (unlocked)
+				*unlocked = true;
 			fault_flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
-- 
2.18.0