[PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal signal

[Peter Xu]

lookup_node() uses gup to pin the page and get node information.  It
checks against ret>=0 assuming the page will be filled in.  However
it's also possible that gup will return zero, for example, when the
thread is quickly killed with a fatal signal.  Teach lookup_node() to
gracefully return an error -EFAULT if it happens.

Meanwhile, initialize "page" to NULL to avoid potential risk of
exploiting the pointer.

Reported-by: syzbot+693dc11fcb53120b5559@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 4426e945df58 ("mm/gup: allow VM_FAULT_RETRY for multiple times")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 mm/mempolicy.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 5fb427aed612..c7ca6a808fb1 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -897,12 +897,15 @@ static void get_policy_nodemask(struct mempolicy *p, nodemask_t *nodes)
 
 static int lookup_node(struct mm_struct *mm, unsigned long addr)
 {
-	struct page *p;
+	struct page *p = NULL;
 	int err;
 
 	int locked = 1;
 	err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked);
-	if (err >= 0) {
+	if (err == 0) {
+		/* E.g. GUP interrupted by fatal signal */
+		err = -EFAULT;
+	} else if (err > 0) {
 		err = page_to_nid(p);
 		put_page(p);
 	}
-- 
2.24.1