Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[Jesper Dangaard Brouer]

On 02/08/2023 00.07, Andrew Kanner wrote:
> Using the syzkaller repro with reduced packet size it was discovered
> that XDP_PACKET_HEADROOM is not checked in tun_can_build_skb(),
> although pad may be incremented in tun_build_skb(). This may end up
> with exceeding the PAGE_SIZE limit in tun_build_skb().
>
> Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
> Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
> Signed-off-by: Andrew Kanner <andrew.kanner@xxxxxxxxx>
> ---
>
> Notes:
>      v3 -> v4:
>      * fall back to v1, fixing only missing XDP_PACKET_HEADROOM in pad and
>        removing bpf_xdp_adjust_tail() check for frame_sz.
>      * added rcu read lock, noted by Jason Wang <jasowang@xxxxxxxxxx> in v1
>      * I decided to leave the packet length check in tun_can_build_skb()
>        instead of moving to tun_build_skb() suggested by Jason Wang
>        <jasowang@xxxxxxxxxx>. Otherwise extra packets will be dropped
>        without falling back to tun_alloc_skb(). And in the discussion of v3
>        Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx> noticed that XDP is ok
>        with a higher order pages if it's a contiguous physical memory
>        allocation, so falling to tun_alloc_skb() -> do_xdp_generic() should
>        be ok.
>      
>      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>
>
>   drivers/net/tun.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index d75456adc62a..a1d04bc9485f 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1582,6 +1582,9 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
>   static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
>   			      int len, int noblock, bool zerocopy)
>   {
> +	struct bpf_prog *xdp_prog;
> +	int pad = TUN_RX_PAD;
> +
>   	if ((tun->flags & TUN_TYPE_MASK) != IFF_TAP)
>   		return false;
>   
> @@ -1594,7 +1597,13 @@ 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) +
> +	rcu_read_lock();
> +	xdp_prog = rcu_dereference(tun->xdp_prog);
> +	if (xdp_prog)
> +		pad += XDP_PACKET_HEADROOM;
> +	rcu_read_unlock();
> +

Isolated seen, I guess, this is a correct fix to 7df13219d757.

> +	if (SKB_DATA_ALIGN(len + pad) +
>   	    SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
>   		return false;
>   

Question to Jason Wang:
Why fall back (to e.g. tun_alloc_skb()) when size is above PAGE_SIZE?

AFAIK tun_build_skb() *can* create get larger packets than PAGE_SIZE
from it's page_frag.  Is there a reason for this limitation?

(To Andrew, I assume a change in this area is another patch).

--Jesper