Messages in this thread | | | From | Jason Wang <> | Date | Thu, 27 Jul 2023 14:07:38 +0800 | Subject | Re: [PATCH v3] drivers: net: prevent tun_get_user() to exceed xdp size limits |
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On Thu, Jul 27, 2023 at 8:27 AM David Ahern <dsahern@gmail.com> wrote: > > On 7/26/23 1:37 PM, David Ahern wrote: > > On 7/26/23 3:02 AM, Jesper Dangaard Brouer wrote: > >> Cc. John and Ahern > >> > >> On 26/07/2023 04.09, Jason Wang wrote: > >>> On Tue, Jul 25, 2023 at 11:54 PM Andrew Kanner > >>> <andrew.kanner@gmail.com> wrote: > >>>> > >>>> Syzkaller reported the following issue: > >>>> ======================================= > >>>> Too BIG xdp->frame_sz = 131072 > >> > >> Is this a contiguous physical memory allocation? > >> > >> 131072 bytes equal order 5 page. > >> > >> Looking at tun.c code I cannot find a code path that could create > >> order-5 skb->data, but only SKB with order-0 fragments. But I guess it > >> is the netif_receive_generic_xdp() what will realloc to make this linear > >> (via skb_linearize()) > > > > > > get_tun_user is passed an iov_iter with a single segment of 65007 > > total_len. The alloc_skb path is hit with an align size of only 64. That > > is insufficient for XDP so the netif_receive_generic_xdp hits the > > pskb_expand_head path. Something is off in the math in > > netif_receive_generic_xdp resulting in the skb markers being off. That > > causes bpf_prog_run_generic_xdp to compute the wrong frame_sz. > > > BTW, it is pskb_expand_head that turns it from a 64kB to a 128 kB > allocation. But the 128kB part is not relevant to the "bug" here really. > > The warn on getting tripped in bpf_xdp_adjust_tail is because xdp > generic path is skb based and can have a frame_sz > 4kB. That's what the > splat is about.
Other possibility:
tun_can_build_skb() doesn't count XDP_PACKET_HEADROOM this may end up with producing a frame_sz which is greater than PAGE_SIZE as well in tun_build_skb().
And rethink this patch, it looks wrong since it basically drops all packets whose buflen is greater than PAGE_SIZE since it can't fall back to tun_alloc_skb().
> > Perhaps the solution is to remove the WARN_ON.
Yes, that is what I'm asking if this warning still makes sense in V1.
Thanks
> >
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