Re: [syzbot] WARNING in u32_change

[Kees Cook <keescook@chromium.org>]

On Sun, Sep 25, 2022 at 07:39:23PM -0700, Kees Cook wrote:
> On Sun, Sep 25, 2022 at 10:34:37AM -0700, Eric Dumazet wrote:
> > Sure, please look at:
> > 
> > commit 54d9469bc515dc5fcbc20eecbe19cea868b70d68
> > Author: Kees Cook <keescook@chromium.org>
> > Date:   Thu Jun 24 15:39:26 2021 -0700
> > 
> >     fortify: Add run-time WARN for cross-field memcpy()
> > [...]
> > Here, we might switch to unsafe_memcpy() instead of memcpy()
> 
> I would tend to agree. Something like:
> 
> diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> index 4d27300c287c..21e0e6206ecc 100644
> --- a/net/sched/cls_u32.c
> +++ b/net/sched/cls_u32.c
> @@ -1040,7 +1040,9 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
>  	}
>  #endif
>  
> -	memcpy(&n->sel, s, sel_size);
> +	unsafe_memcpy(&n->sel, s, sel_size,
> +		      /* A composite flex-array structure destination,
> +		       * which was correctly sized and allocated above. */);
>  	RCU_INIT_POINTER(n->ht_up, ht);
>  	n->handle = handle;
>  	n->fshift = s->hmask ? ffs(ntohl(s->hmask)) - 1 : 0;

Ah, there is another in the same source file, in u32_init_knode():

        memcpy(&new->sel, s, struct_size(s, keys, s->nkeys));

(I've been trying to convince Coccinelle to produce a list of all the
composite structure targets, but I keep running into weird glitches.
That it hadn't found this one let me track down the latest issue, so now
I should be able to find more! Whew.)

-- 
Kees Cook