[PATCH] dma-buf: udmabuf: avoid list copy size overflow

[Yousef Alhouseen <alhouseenyousef@gmail.com>]

UDMABUF_CREATE_LIST copies an array whose element count comes from
userspace. The count is compared against list_limit, but list_limit is a
signed module parameter while the count is u32.

If the limit is raised too far or made negative, that comparison no
longer bounds the count to a range where sizeof(*list) * count fits in
the u32 temporary used for the copy length. A wrapped copy length lets
memdup_user() copy fewer entries than udmabuf_create() subsequently
walks, leading to out-of-bounds reads from the copied list.

Take a positive snapshot of the module limit and use memdup_array_user()
so the multiplication is checked before copying.

Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
---
 drivers/dma-buf/udmabuf.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index bced421c0..b4078ec84 100644
--- a/drivers/dma-buf/udmabuf.c
+++ b/drivers/dma-buf/udmabuf.c
@@ -469,14 +469,15 @@ 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;
+	int limit;
 
 	if (copy_from_user(&head, (void __user *)arg, sizeof(head)))
 		return -EFAULT;
-	if (head.count > list_limit)
+	limit = READ_ONCE(list_limit);
+	if (!head.count || limit <= 0 || head.count > limit)
 		return -EINVAL;
-	lsize = sizeof(struct udmabuf_create_item) * head.count;
-	list = memdup_user((void __user *)(arg + sizeof(head)), lsize);
+	list = memdup_array_user((void __user *)(arg + sizeof(head)),
+				 head.count, sizeof(*list));
 	if (IS_ERR(list))
 		return PTR_ERR(list);
 
-- 
2.54.0