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    Subject[PATCH v3] drivers: net: prevent tun_get_user() to exceed xdp size limits
    Date
    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
    tun_get_user() still provides an execution path with do_xdp_generic()
    and exceed XDP limits for packet size.

    Using the syzkaller repro with reduced packet size it was also
    discovered that XDP_PACKET_HEADROOM is not checked in
    tun_can_build_skb(), although pad may be incremented in
    tun_build_skb().

    If we move the limit check from tun_can_build_skb() to tun_build_skb()
    we will make xdp to be used only in tun_build_skb(), without falling
    in tun_alloc_skb(), etc. And moreover we will drop the packet which
    can't be processed in tun_build_skb().

    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?id=5335c7c62bfff89bbb1c8f14cdabebe91909060f
    Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
    Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
    ---

    Notes:
    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 | 7 +++----
    1 file changed, 3 insertions(+), 4 deletions(-)

    diff --git a/drivers/net/tun.c b/drivers/net/tun.c
    index d75456adc62a..7c2b05ce0421 100644
    --- a/drivers/net/tun.c
    +++ b/drivers/net/tun.c
    @@ -1594,10 +1594,6 @@ 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) +
    - SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
    - return false;
    -
    return true;
    }

    @@ -1673,6 +1669,9 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
    buflen += SKB_DATA_ALIGN(len + pad);
    rcu_read_unlock();

    + if (buflen > PAGE_SIZE)
    + return ERR_PTR(-EFAULT);
    +
    alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES);
    if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, GFP_KERNEL)))
    return ERR_PTR(-ENOMEM);
    --
    2.39.3
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