Re: [PATCH bpf-next 1/2] bpf, test_run: fix crashes due to XDP frame overwriting/corruption

[Toke Høiland-Jørgensen]

Alexander Lobakin <aleksander.lobakin@xxxxxxxxx> writes:

> syzbot and Ilya faced the splats when %XDP_PASS happens for bpf_test_run
> after skb PP recycling was enabled for {__,}xdp_build_skb_from_frame():
>
> BUG: kernel NULL pointer dereference, address: 0000000000000d28
> RIP: 0010:memset_erms+0xd/0x20 arch/x86/lib/memset_64.S:66
> [...]
> Call Trace:
>  <TASK>
>  __finalize_skb_around net/core/skbuff.c:321 [inline]
>  __build_skb_around+0x232/0x3a0 net/core/skbuff.c:379
>  build_skb_around+0x32/0x290 net/core/skbuff.c:444
>  __xdp_build_skb_from_frame+0x121/0x760 net/core/xdp.c:622
>  xdp_recv_frames net/bpf/test_run.c:248 [inline]
>  xdp_test_run_batch net/bpf/test_run.c:334 [inline]
>  bpf_test_run_xdp_live+0x1289/0x1930 net/bpf/test_run.c:362
>  bpf_prog_test_run_xdp+0xa05/0x14e0 net/bpf/test_run.c:1418
> [...]
>
> This happens due to that it calls xdp_scrub_frame(), which nullifies
> xdpf->data. bpf_test_run code doesn't reinit the frame when the XDP
> program doesn't adjust head or tail. Previously, %XDP_PASS meant the
> page will be released from the pool and returned to the MM layer, but
> now it does return to the Pool with the nullified xdpf->data, which
> doesn't get reinitialized then.
> So, in addition to checking whether the head and/or tail have been
> adjusted, check also for a potential XDP frame corruption. xdpf->data
> is 100% affected and also xdpf->flags is the field closest to the
> metadata / frame start. Checking for these two should be enough for
> non-extreme cases.
>
> Fixes: 9c94bbf9a87b ("xdp: recycle Page Pool backed skbs built from XDP frames")
> Reported-by: syzbot+e1d1b65f7c32f2a86a9f@xxxxxxxxxxxxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/bpf/000000000000f1985705f6ef2243@xxxxxxxxxx
> Reported-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/bpf/e07dd94022ad5731705891b9487cc9ed66328b94.camel@xxxxxxxxxxxxx
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>

Acked-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>