Re: [PATCH v3] drivers: net: prevent tun_get_user() to exceed xdp size limits

[Andrew Kanner <andrew.kanner@gmail.com>]

On Wed, Jul 26, 2023 at 10:09:53AM +0800, 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
> > 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>
> 
> Acked-by: Jason Wang <jasowang@redhat.com>
> 
> Thanks
> 
> >
> >  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
> >
> 

Thanks, Jason.

Can anyone point me to some tests other than
tools/testing/selftests/net/tun.c?

This one shows:
PASSED: 5 / 5 tests passed.

I'm trying to figure out if we're dropping more packets than expected
with this patch. Not sure if the test above is enough.

-- 
Andrew Kanner