[PATCH] dma-buf: udmabuf: validate create-list count before copying

[Samuel Moelius <sam.moelius@trailofbits.com>]

UDMABUF_CREATE_LIST copies a variable-length list using a byte count
derived from head.count. The list_limit module parameter is signed and
writable, so setting it negative lets a large unsigned count bypass the
limit check. The u32 byte-count calculation can then wrap, causing only
a small list to be copied while udmabuf_create() still iterates over the
large count.

Reject negative list_limit values and use checked size_t multiplication
before copying the list.

Assisted-by: Codex:gpt-5.5-cyber-preview
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
---
 drivers/dma-buf/udmabuf.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index 94b8ecb892bb..46b077639bfb 100644
--- a/drivers/dma-buf/udmabuf.c
+++ b/drivers/dma-buf/udmabuf.c
@@ -13,6 +13,7 @@
 #include <linux/hugetlb.h>
 #include <linux/slab.h>
 #include <linux/udmabuf.h>
+#include <linux/overflow.h>
 #include <linux/vmalloc.h>
 #include <linux/iosys-map.h>
 
@@ -489,13 +490,14 @@ static long udmabuf_ioctl_create_list(struct file *filp, unsigned long arg)
 	struct udmabuf_create_list head;
 	struct udmabuf_create_item *list;
 	int ret = -EINVAL;
-	u32 lsize;
+	size_t lsize;
 
 	if (copy_from_user(&head, (void __user *)arg, sizeof(head)))
 		return -EFAULT;
-	if (head.count > list_limit)
+	if (list_limit < 0 || head.count > list_limit)
+		return -EINVAL;
+	if (check_mul_overflow(sizeof(struct udmabuf_create_item), head.count, &lsize))
 		return -EINVAL;
-	lsize = sizeof(struct udmabuf_create_item) * head.count;
 	list = memdup_user((void __user *)(arg + sizeof(head)), lsize);
 	if (IS_ERR(list))
 		return PTR_ERR(list);
-- 
2.43.0