mm/mempolicy: Allow lookup_node() to handle fatal signal

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.

Fixes: 4426e945df58 ("mm/gup: allow VM_FAULT_RETRY for multiple times")
Reported-by: syzbot+693dc11fcb53120b5559@syzkaller.appspotmail.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b4a039d..fb678a7 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -896,12 +896,15 @@ 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);
        }