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Message-ID: <2cb34364-0d7c-cf0a-487f-c15ba6568ac8@kernel.org>
Date: Wed, 2 Aug 2023 16:13:57 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Andrew Kanner <andrew.kanner@...il.com>, davem@...emloft.net,
edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
jasowang@...hat.com, netdev@...r.kernel.org, dsahern@...il.com,
jbrouer@...hat.com, john.fastabend@...il.com,
linux-kernel@...r.kernel.org
Cc: linux-kernel-mentees@...ts.linuxfoundation.org,
syzbot+f817490f5bd20541b90a@...kaller.appspotmail.com
Subject: Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed
the packet size limit
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@...il.com>
> ---
>
> 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@...hat.com> 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@...hat.com>. Otherwise extra packets will be dropped
> without falling back to tun_alloc_skb(). And in the discussion of v3
> Jesper Dangaard Brouer <jbrouer@...hat.com> 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@...hat.com>
> * 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@...hat.com>
>
> 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
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