Re: [PATCH v3] drivers: net: prevent tun_get_user() to exceed xdp size limits

[Jason Wang]

On Tue, Jul 25, 2023 at 11:54 PM 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
> tun_get_user() still provides an execution path with do_xdp_generic()
> and exceed XDP limits for packet size.
>
> Using the syzkaller repro with reduced packet size it was also
> discovered that XDP_PACKET_HEADROOM is not checked in
> tun_can_build_skb(), although pad may be incremented in
> tun_build_skb().
>
> If we move the limit check from tun_can_build_skb() to tun_build_skb()
> we will make xdp to be used only in tun_build_skb(), without falling
> in tun_alloc_skb(), etc. And moreover we will drop the packet which
> can't be processed in tun_build_skb().
>
> Reported-and-tested-by: syzbot+f817490f5bd20541b90a@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@xxxxxxxxxx/T/
> Link: https://syzkaller.appspot.com/bug?id=5335c7c62bfff89bbb1c8f14cdabebe91909060f
> Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
> Signed-off-by: Andrew Kanner <andrew.kanner@xxxxxxxxx>
> ---
>
> Notes:
>     V2 -> V3:
>     * attach the forgotten changelog
>     V1 -> V2:
>     * merged 2 patches in 1, fixing both issues: WARN_ON_ONCE with
>       syzkaller repro and missing XDP_PACKET_HEADROOM in pad
>     * changed the title and description of the execution path, suggested
>       by Jason Wang <jasowang@xxxxxxxxxx>
>     * move the limit check from tun_can_build_skb() to tun_build_skb() to
>       remove duplication and locking issue, and also drop the packet in
>       case of a failed check - noted by Jason Wang <jasowang@xxxxxxxxxx>

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>

Thanks

>
>  drivers/net/tun.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index d75456adc62a..7c2b05ce0421 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1594,10 +1594,6 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
>         if (zerocopy)
>                 return false;
>
> -       if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> -           SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
> -               return false;
> -
>         return true;
>  }
>
> @@ -1673,6 +1669,9 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>         buflen += SKB_DATA_ALIGN(len + pad);
>         rcu_read_unlock();
>
> +       if (buflen > PAGE_SIZE)
> +               return ERR_PTR(-EFAULT);
> +
>         alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES);
>         if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, GFP_KERNEL)))
>                 return ERR_PTR(-ENOMEM);
> --
> 2.39.3
>