Re: [PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

[Dan Carpenter]

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);
 }