mm/gup: do not return 0 from pin_user_pages_fast() for bad args

These routines are not intended to return zero, the callers cannot do
anything sane with a 0 return.  They should return an error which means
future calls to GUP will not succeed, or they should return some non-zero
number of pinned pages which means GUP should be called again.

If start + nr_pages overflows it should return -EOVERFLOW to signal the
arguments are invalid.

Syzkaller keeps tripping on this when fuzzing GUP arguments.

Link: https://lkml.kernel.org/r/0-v1-3d5ed1f20d50+104-gup_overflow_jgg@nvidia.comSigned-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reported-by: syzbot+353c7be4964c6253f24a@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000094fdd05faa4d3a4@google.comReviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/gup.c

@@ -3080,7 +3080,7 @@ static int internal_get_user_pages_fast(unsigned long start,
 	start = untagged_addr(start) & PAGE_MASK;
 	len = nr_pages << PAGE_SHIFT;
 	if (check_add_overflow(start, len, &end))
-		return 0;
+		return -EOVERFLOW;
 	if (end > TASK_SIZE_MAX)
 		return -EFAULT;
 	if (unlikely(!access_ok((void __user *)start, len)))