Re: [PATCH] netlink: add buffer boundary checking

[Eric Dumazet]

On 7/23/20 11:21 AM, Mark Salyzyn wrote:
> Many of the nla_get_* inlines fail to check attribute's length before
> copying the content resulting in possible out-of-boundary accesses.
> Adjust the inlines to perform nla_len checking, for the most part
> using the nla_memcpy function to faciliate since these are not
> necessarily performance critical and do not need a likely fast path.
> 
> Signed-off-by: Mark Salyzyn <salyzyn@xxxxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kernel-team@xxxxxxxxxxx
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: Thomas Graf <tgraf@xxxxxxx>
> Fixes: bfa83a9e03cf ("[NETLINK]: Type-safe netlink messages/attributes interface")
> ---
>  include/net/netlink.h | 66 +++++++++++++++++++++++++++++++++++--------
>  1 file changed, 54 insertions(+), 12 deletions(-)
> 
> diff --git a/include/net/netlink.h b/include/net/netlink.h
> index c0411f14fb53..11c0f153be7c 100644
> --- a/include/net/netlink.h
> +++ b/include/net/netlink.h
> @@ -1538,7 +1538,11 @@ static inline int nla_put_bitfield32(struct sk_buff *skb, int attrtype,
>   */
>  static inline u32 nla_get_u32(const struct nlattr *nla)
>  {
> -	return *(u32 *) nla_data(nla);
> +	u32 tmp;
> +
> +	nla_memcpy(&tmp, nla, sizeof(tmp));
> +
> +	return tmp;

I believe this will hide bugs, that syzbot was able to catch.

Instead, you could perhaps introduce a CONFIG_DEBUG_NETLINK option,
and add a WARN_ON_ONCE(nla_len(nla) < sizeof(u32)) so that we can detect bugs in callers.