[PATCH net-next 06/12] Revert "net: ptr_ring: otherwise safe empty checks can overrun array bounds"

[Michael S. Tsirkin]

This reverts commit bcecb4bbf88aa03171c30652bca761cf27755a6b.

If we try to allocate an extra entry as the above commit did, and when
the requested size is UINT_MAX, addition overflows causing zero size to
be passed to kmalloc().

kmalloc then returns ZERO_SIZE_PTR with a subsequent crash.

Reported-by: syzbot+87678bcf753b44c39b67@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
 include/linux/ptr_ring.h | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index f175846..3a19ebd 100644
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -466,12 +466,7 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
 
 static inline void **__ptr_ring_init_queue_alloc(unsigned int size, gfp_t gfp)
 {
-	/* Allocate an extra dummy element at end of ring to avoid consumer head
-	 * or produce head access past the end of the array. Possible when
-	 * producer/consumer operations and __ptr_ring_peek operations run in
-	 * parallel.
-	 */
-	return kcalloc(size + 1, sizeof(void *), gfp);
+	return kcalloc(size, sizeof(void *), gfp);
 }
 
 static inline void __ptr_ring_set_size(struct ptr_ring *r, int size)
-- 
MST