On Wed, Jan 22, 2025 at 02:52:39PM +0100, Przemek Kitszel wrote:
> On 1/22/25 14:49, Dan Carpenter wrote:
> > The "payload" variable is type size_t, however the nlmsg_total_size()
> > function will a few bytes to it and then truncate the result to type
> > int. That means that if "payload" is more than UINT_MAX the alloc_skb()
>
> In the code it's INT_MAX, would be best to have the same used in both
> places (or explain it so it's obvious)
>
Yeah. It's not probably not obvious.
I don't like using UINT_MAX as a limit because why push so close to the
edge? Normal allocation functions are capped at INT_MAX to avoid
integer overflows. You'd have to use vmalloc() to allocate more than
2GB of RAM. So it's not like we gain anything by using a higher, riskier
number.
The nlmsg_total_size() function adds potentially 19 bytes to the
payload.
INT_MAX plus anything less than 2 million number can't overflow to zero.
It could overflow to negative but you can't allocate negative bytes so
that's fine.
The vfs_read/write() functions use MAX_RW_COUNT to avoid integer
overflows. That's basically INT_MAX - PAGE_SIZE. There are quite
a few places like this in the kernel which assume small numbers like
sizeof() are generally going to return less than PAGE_SIZE. Would
that be better to do this. Then it couldn't overflow to negative.
regards,
dan carpenter
diff --git a/include/net/netlink.h b/include/net/netlink.h
index e015ffbed819..ceeea04fae4a 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -1015,6 +1015,9 @@ static inline struct nlmsghdr *nlmsg_put_answer(struct sk_buff *skb,
*/
static inline struct sk_buff *nlmsg_new(size_t payload, gfp_t flags)
{
+ /* Prevent integer overflow */
+ if (payload > INT_MAX - PAGE_SIZE)
+ return NULL;
return alloc_skb(nlmsg_total_size(payload), flags);
}
