Messages in this thread | | | From | Jason Wang <> | Date | Thu, 3 Aug 2023 11:19:47 +0800 | Subject | Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit |
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On Wed, Aug 2, 2023 at 10:14 PM Jesper Dangaard Brouer <hawk@kernel.org> wrote: > > > > 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@gmail.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@redhat.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@redhat.com>. Otherwise extra packets will be dropped > > without falling back to tun_alloc_skb(). And in the discussion of v3 > > Jesper Dangaard Brouer <jbrouer@redhat.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@redhat.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@redhat.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.
I think so.
Actually, I think we can probably always count XDP_PACKET_HEADROOM here. Since there's a window that XDP program might be attached in the middle of tun_can_build_skb() and tun_build_skb().
> > > + 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?
I couldn't recall but I think we can relax.
Thanks
> > (To Andrew, I assume a change in this area is another patch). > > --Jesper > > >
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