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    SubjectRe: [PATCH v4 2/2] net: core: remove unnecessary frame_sz check in bpf_xdp_adjust_tail()
    On Wed, Aug 2, 2023 at 6:09 AM Andrew Kanner <andrew.kanner@gmail.com> 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 Jesper
    > Dangaard Brouer <jbrouer@redhat.com> noted that after introducing the
    > xdp_init_buff() which all XDP driver use - it's safe to remove this
    > check. The original intend was to catch cases where XDP drivers have
    > not been updated to use xdp.frame_sz, but that is not longer a concern
    > (since xdp_init_buff).
    >
    > Running the initial syzkaller repro it was discovered that the
    > contiguous physical memory allocation is used for both xdp paths in
    > tun_get_user(), e.g. tun_build_skb() and tun_alloc_skb(). It was also
    > stated by Jesper Dangaard Brouer <jbrouer@redhat.com> that XDP can
    > work on higher order pages, as long as this is contiguous physical
    > memory (e.g. a page).
    >
    > Reported-and-tested-by: syzbot+f817490f5bd20541b90a@syzkaller.appspotmail.com
    > Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@google.com/T/
    > Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
    > Link: https://lore.kernel.org/all/20230725155403.796-1-andrew.kanner@gmail.com/T/
    > Fixes: 43b5169d8355 ("net, xdp: Introduce xdp_init_buff utility routine")
    > Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>

    Acked-by: Jason Wang <jasowang@redhat.com>

    Thanks

    > ---
    > net/core/filter.c | 6 ------
    > 1 file changed, 6 deletions(-)
    >
    > diff --git a/net/core/filter.c b/net/core/filter.c
    > index 06ba0e56e369..28a59596987a 100644
    > --- a/net/core/filter.c
    > +++ b/net/core/filter.c
    > @@ -4116,12 +4116,6 @@ BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
    > if (unlikely(data_end > data_hard_end))
    > return -EINVAL;
    >
    > - /* ALL drivers MUST init xdp->frame_sz, chicken check below */
    > - if (unlikely(xdp->frame_sz > PAGE_SIZE)) {
    > - WARN_ONCE(1, "Too BIG xdp->frame_sz = %d\n", xdp->frame_sz);
    > - return -EINVAL;
    > - }
    > -
    > if (unlikely(data_end < xdp->data + ETH_HLEN))
    > return -EINVAL;
    >
    > --
    > 2.39.3
    >

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