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

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Thu, 2023-03-16 at 18:50 +0100, Alexander Lobakin wrote:
> 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@syzkaller.appspotmail.com
> Link:
> https://lore.kernel.org/bpf/000000000000f1985705f6ef2243@google.com
> Reported-by: Ilya Leoshkevich <iii@linux.ibm.com>
> Link:
> https://lore.kernel.org/bpf/e07dd94022ad5731705891b9487cc9ed66328b94.camel@linux.ibm.com
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
> ---
>  net/bpf/test_run.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 71226f68270d..8d6b31209bd6 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -208,6 +208,16 @@ static void xdp_test_run_teardown(struct
> xdp_test_data *xdp)
>         kfree(xdp->skbs);
>  }
>  
> +static bool frame_was_changed(const struct xdp_page_head *head)
> +{
> +       /* xdp_scrub_frame() zeroes the data pointer, flags is the
> last field,
> +        * i.e. has the highest chances to be overwritten. If those
> two are
> +        * untouched, it's most likely safe to skip the context
> reset.
> +        */
> +       return head->frm.data != head->orig_ctx.data ||
> +              head->frm.flags != head->orig_ctx.flags;
> +}
> +
>  static bool ctx_was_changed(struct xdp_page_head *head)
>  {
>         return head->orig_ctx.data != head->ctx.data ||
> @@ -217,7 +227,7 @@ static bool ctx_was_changed(struct xdp_page_head
> *head)
>  
>  static void reset_ctx(struct xdp_page_head *head)
>  {
> -       if (likely(!ctx_was_changed(head)))
> +       if (likely(!frame_was_changed(head) &&
> !ctx_was_changed(head)))
>                 return;
>  
>         head->ctx.data = head->orig_ctx.data;

With this test begins to work on s390x:

# ./test_progs -t xdp_do_redirect
IPv6: ADDRCONF(NETDEV_CHANGE): veth_dst: link becomes ready
IPv6: ADDRCONF(NETDEV_CHANGE): veth_src: link becomes ready
#290     xdp_do_redirect:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Thanks!

Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>