Re: [PATCH v5 2/3] net: add kcov handle to skb extensions

[Florian Westphal <fw@strlen.de>]

Ido Schimmel <idosch@idosch.org> wrote:
> On Thu, Oct 29, 2020 at 05:36:19PM +0000, Aleksandr Nogikh wrote:
> > From: Aleksandr Nogikh <nogikh@google.com>
> > 
> > Remote KCOV coverage collection enables coverage-guided fuzzing of the
> > code that is not reachable during normal system call execution. It is
> > especially helpful for fuzzing networking subsystems, where it is
> > common to perform packet handling in separate work queues even for the
> > packets that originated directly from the user space.
> > 
> > Enable coverage-guided frame injection by adding kcov remote handle to
> > skb extensions. Default initialization in __alloc_skb and
> > __build_skb_around ensures that no socket buffer that was generated
> > during a system call will be missed.
> > 
> > Code that is of interest and that performs packet processing should be
> > annotated with kcov_remote_start()/kcov_remote_stop().
> > 
> > An alternative approach is to determine kcov_handle solely on the
> > basis of the device/interface that received the specific socket
> > buffer. However, in this case it would be impossible to distinguish
> > between packets that originated during normal background network
> > processes or were intentionally injected from the user space.
> > 
> > Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
> > Acked-by: Willem de Bruijn <willemb@google.com>
> 
> [...]
> 
> > @@ -249,6 +249,9 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> >  
> >  		fclones->skb2.fclone = SKB_FCLONE_CLONE;
> >  	}
> > +
> > +	skb_set_kcov_handle(skb, kcov_common_handle());
> 
> Hi,
> 
> This causes skb extensions to be allocated for the allocated skb, but
> there are instances that blindly overwrite 'skb->extensions' by invoking
> skb_copy_header() after __alloc_skb(). For example, skb_copy(),
> __pskb_copy_fclone() and skb_copy_expand(). This results in the skb
> extensions being leaked [1].

[..]
> Other suggestions?

Aleksandr, why was this made into an skb extension in the first place?

AFAIU this feature is usually always disabled at build time.
For debug builds (test farm /debug kernel etc) its always needed.

If thats the case this u64 should be an sk_buff member, not an
extension.