Re: [PATCH v4 2/2] net: core: remove unnecessary frame_sz check in bpf_xdp_adjust_tail()

[Jason Wang]

On Wed, Aug 2, 2023 at 6:09 AM Andrew Kanner <andrew.kanner@xxxxxxxxx> wrote:
>
> Syzkaller reported the following issue:
> =======================================
> Too BIG xdp->frame_sz = 131072
> WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
>   ____bpf_xdp_adjust_tail net/core/filter.c:4121 [inline]
> WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
>   bpf_xdp_adjust_tail+0x466/0xa10 net/core/filter.c:4103
> ...
> Call Trace:
>  <TASK>
>  bpf_prog_4add87e5301a4105+0x1a/0x1c
>  __bpf_prog_run include/linux/filter.h:600 [inline]
>  bpf_prog_run_xdp include/linux/filter.h:775 [inline]
>  bpf_prog_run_generic_xdp+0x57e/0x11e0 net/core/dev.c:4721
>  netif_receive_generic_xdp net/core/dev.c:4807 [inline]
>  do_xdp_generic+0x35c/0x770 net/core/dev.c:4866
>  tun_get_user+0x2340/0x3ca0 drivers/net/tun.c:1919
>  tun_chr_write_iter+0xe8/0x210 drivers/net/tun.c:2043
>  call_write_iter include/linux/fs.h:1871 [inline]
>  new_sync_write fs/read_write.c:491 [inline]
>  vfs_write+0x650/0xe40 fs/read_write.c:584
>  ksys_write+0x12f/0x250 fs/read_write.c:637
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> xdp->frame_sz > PAGE_SIZE check was introduced in commit c8741e2bfe87
> ("xdp: Allow bpf_xdp_adjust_tail() to grow packet size"). But Jesper
> Dangaard Brouer <jbrouer@xxxxxxxxxx> noted that after introducing the
> xdp_init_buff() which all XDP driver use - it's safe to remove this
> check. The original intend was to catch cases where XDP drivers have
> not been updated to use xdp.frame_sz, but that is not longer a concern
> (since xdp_init_buff).
>
> Running the initial syzkaller repro it was discovered that the
> contiguous physical memory allocation is used for both xdp paths in
> tun_get_user(), e.g. tun_build_skb() and tun_alloc_skb(). It was also
> stated by Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx> that XDP can
> work on higher order pages, as long as this is contiguous physical
> memory (e.g. a page).
>
> Reported-and-tested-by: syzbot+f817490f5bd20541b90a@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@xxxxxxxxxx/T/
> Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
> Link: https://lore.kernel.org/all/20230725155403.796-1-andrew.kanner@xxxxxxxxx/T/
> Fixes: 43b5169d8355 ("net, xdp: Introduce xdp_init_buff utility routine")
> Signed-off-by: Andrew Kanner <andrew.kanner@xxxxxxxxx>

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>

Thanks

> ---
>  net/core/filter.c | 6 ------
>  1 file changed, 6 deletions(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 06ba0e56e369..28a59596987a 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4116,12 +4116,6 @@ BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
>         if (unlikely(data_end > data_hard_end))
>                 return -EINVAL;
>
> -       /* ALL drivers MUST init xdp->frame_sz, chicken check below */
> -       if (unlikely(xdp->frame_sz > PAGE_SIZE)) {
> -               WARN_ONCE(1, "Too BIG xdp->frame_sz = %d\n", xdp->frame_sz);
> -               return -EINVAL;
> -       }
> -
>         if (unlikely(data_end < xdp->data + ETH_HLEN))
>                 return -EINVAL;
>
> --
> 2.39.3
>