Re: Linux 4.14 - regression: broken tun/tap / bridge network with virtio - bisected

[Andreas Hartmann]

On 12/20/2017 at 11:44 PM Willem de Bruijn wrote:
On Wed, Dec 20, 2017 at 10:56 AM, Andreas Hartmann
<andihartm...@01019freenet.de> wrote:
On 12/18/2017 at 06:11 PM Andreas Hartmann wrote:
On 12/17/2017 at 11:33 PM Willem de Bruijn wrote:
[...]
I have been able to reproduce the hang by sending a UFO packet
between two guests running v4.13 on a host running v4.15-rc1.
The vhost_net_ubuf_ref refcount indeed hits overflow (-1) from
vhost_zerocopy_callback being called for each segment of a
segmented UFO skb. This refcount is decremented then on each
segment, but incremented only once for the entire UFO skb.

Before v4.14, these packets would be converted in skb_segment to
regular copy packets with skb_orphan_frags and the callback function
called once at this point. v4.14 added support for reference counted
zerocopy skb that can pass through skb_orphan_frags unmodified and
have their zerocopy state safely cloned with skb_zerocopy_clone.

The call to skb_zerocopy_clone must come after skb_orphan_frags
to limit cloning of this state to those skbs that can do so safely.

Please try a host with the following patch. This fixes it for me. I intend to
send it to net.

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a592ca025fc4..d2d985418819 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3654,8 +3654,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,

                 skb_shinfo(nskb)->tx_flags |= skb_shinfo(head_skb)->tx_flags &
                                               SKBTX_SHARED_FRAG;
-               if (skb_zerocopy_clone(nskb, head_skb, GFP_ATOMIC))
-                       goto err;

                 while (pos < offset + len) {
                         if (i >= nfrags) {
@@ -3681,6 +3679,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,

                         if (unlikely(skb_orphan_frags(frag_skb, GFP_ATOMIC)))
                                 goto err;
+                       if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
+                               goto err;

                         *nskb_frag = *frag;
                         __skb_frag_ref(nskb_frag);


This is relatively inefficient, as it calls skb_zerocopy_clone for each frag
in the frags[] array. I will follow-up with a patch to net-next that only
checks once per skb:

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 466581cf4cdc..a293a33604ec 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3662,7 +3662,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,

                 skb_shinfo(nskb)->tx_flags |= skb_shinfo(head_skb)->tx_flags &
                                               SKBTX_SHARED_FRAG;
-               if (skb_zerocopy_clone(nskb, head_skb, GFP_ATOMIC))
+               if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
+                   skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
                         goto err;

                 while (pos < offset + len) {
@@ -3676,6 +3677,11 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,

                                 BUG_ON(!nfrags);

+                               if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
+                                   skb_zerocopy_clone(nskb, frag_skb,
+                                                      GFP_ATOMIC))
+                                       goto err;
+
                                 list_skb = list_skb->next;
                         }

@@ -3687,9 +3693,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
                                 goto err;
                         }

-                       if (unlikely(skb_orphan_frags(frag_skb, GFP_ATOMIC)))
-                               goto err;
-

I'm currently testing this one.


Test is in progress. I'm testing w/ 4.14.7, which already contains "net:
accept UFO datagrams from tuntap and packet".

At first, I tested an unpatched 4.14.7 - the problem (no more killable
qemu-process) did occur promptly on shutdown of the machine. This was
expected.

Next, I applied the above patch (the second one). Until now, I didn't
face any problem any more on shutdown of VMs. Looks promising.

Thanks for testing.

I sent the first, simpler, one to net together with another fix.

   http://patchwork.ozlabs.org/patch/851715/


If I'm using the second patch above (the more efficient one and not 
"[net,1/2] skbuff: orphan frags before zerocopy clone"), which I'm 
already testing here: Is it still necessary to apply this patch 
"[net,2/2] skbuff: skb_copy_ubufs must release uarg even without user 
frags"?


Thanks,
Andreas