[PATCH 21/46] mm/gup: fix out-of-bounds write for nr_pages == 0 in GUP-fast

[David Hildenbrand (Arm)]

From: Chris Mason <mason at kernel.org>

With nr_pages == 0, GUP-fast still enters its page table walk and can
write past the caller's page array. The old end <= start check caught
empty ranges, but was replaced with an overflow check.

Users such as OrangeFS can pass a zero page count derived from a
userspace-supplied length. So let's return zero for nr_pages == 0 before
starting the walk, just like GUP-slow.

Fix generated by an LLM and sent by Chris Mason in private.

Fixes: c28b1fc70390 ("mm/gup: reorganize internal_get_user_pages_fast()")
Cc: stable at vger.kernel.org
Signed-off-by: Chris Mason <mason at kernel.org>
[ david: simplify description ]
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
 mm/gup.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 47e7ae152cdf1..9906965893431 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -3204,6 +3204,9 @@ static int gup_fast_fallback(unsigned long start, unsigned long nr_pages,
 				       FOLL_PCI_P2PDMA | FOLL_HONOR_NUMA_FAULT)))
 		return -EINVAL;
 
+	if (unlikely(!nr_pages))
+		return 0;
+
 	if (gup_flags & FOLL_PIN)
 		mm_set_has_pinned_flag(current->mm);
 

-- 
2.43.0