Re: [PATCH NET-NEXT] ipv6: skb_expand_head() adjust skb->truesize incorrectly

On 8/24/21 1:23 AM, Eric Dumazet wrote:

On 8/23/21 2:51 PM, Eric Dumazet wrote:

quoted

On 8/23/21 2:45 PM, Eric Dumazet wrote:

quoted

On 8/23/21 10:25 AM, Christoph Paasch wrote:

quoted

Hello,

On Mon, Aug 23, 2021 at 12:56 AM Vasily Averin [off-list ref] wrote:

quoted

Christoph Paasch reports [1] about incorrect skb->truesize
after skb_expand_head() call in ip6_xmit.
This happen because skb_set_owner_w() for newly clone skb is called
too early, before pskb_expand_head() where truesize is adjusted for
(!skb-sk) case.

[1] https://lkml.org/lkml/2021/8/20/1082

Reported-by: Christoph Paasch <redacted>
Signed-off-by: Vasily Averin <redacted>
---
 net/core/skbuff.c | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f931176..508d5c4 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -1803,6 +1803,8 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)

 struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
 {
+       struct sk_buff *oskb = skb;
+       struct sk_buff *nskb = NULL;
        int delta = headroom - skb_headroom(skb);

        if (WARN_ONCE(delta <= 0,

@@ -1811,21 +1813,21 @@ struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)

        /* pskb_expand_head() might crash, if skb is shared */
        if (skb_shared(skb)) {
-               struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
-
-               if (likely(nskb)) {
-                       if (skb->sk)
-                               skb_set_owner_w(nskb, skb->sk);
-                       consume_skb(skb);
-               } else {
-                       kfree_skb(skb);
-               }
+               nskb = skb_clone(skb, GFP_ATOMIC);
                skb = nskb;
        }
        if (skb &&
-           pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
-               kfree_skb(skb);
+           pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC))
                skb = NULL;
+
+       if (!skb) {
+               kfree_skb(oskb);
+               if (nskb)
+                       kfree_skb(nskb);
+       } else if (nskb) {
+               if (oskb->sk)
+                       skb_set_owner_w(nskb, oskb->sk);
+               consume_skb(oskb);

sorry, this does not fix the problem. The syzkaller repro still
triggers the WARN.

When it happens, the skb in ip6_xmit() is not shared as it comes from
__tcp_transmit_skb, where it is skb_clone()'d.

Old code (in skb_realloc_headroom())
was first calling skb2 = skb_clone(skb, GFP_ATOMIC); 

At this point, skb2->sk was NULL
So pskb_expand_head(skb2, SKB_DATA_ALIGN(delta), 0, ...) was able to tweak skb2->truesize

I would try :

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f9311762cc475bd38d87c33e988d7c983b902e56..326749a8938637b044a616cc33b6a19ed191ac41 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -1804,6 +1804,7 @@ EXPORT_SYMBOL(skb_realloc_headroom);
 struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
 {
        int delta = headroom - skb_headroom(skb);
+       struct sk_buff *oskb = NULL;
 
        if (WARN_ONCE(delta <= 0,
                      "%s is expecting an increase in the headroom", __func__))

@@ -1813,19 +1814,21 @@ struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
        if (skb_shared(skb)) {
                struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
 
-               if (likely(nskb)) {
-                       if (skb->sk)
-                               skb_set_owner_w(nskb, skb->sk);
-                       consume_skb(skb);
-               } else {
+               if (unlikely(!nskb)) {
                        kfree_skb(skb);
+                       return NULL;
                }
+               oskb = skb;
                skb = nskb;
        }
-       if (skb &&
-           pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
+       if (pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
                kfree_skb(skb);
-               skb = NULL;
+               kfree_skb(oskb);
+               return NULL;
+       }
+       if (oskb) {
+               skb_set_owner_w(skb, oskb->sk);
+               consume_skb(oskb);
        }
        return skb;
 }

Oh well, probably not going to work.

We have to find a way to properly increase skb->truesize, even if skb_clone() is _not_ called.

I also note that current use of skb_set_owner_w(), forcing skb->destructor to sock_wfree()
is probably breaking TCP Small queues, since original skb->destructor would be tcp_wfree() or __sock_wfree()

I asked Alexey Kuznetsov to look at this problem. Below is his answer:
"I think the current scheme is obsolete. It was created
when we had only two kinds of skb accounting (rmem & wmem)
and with more kinds of accounting it just does not work.
Even there we had ignored problems with adjusting accounting.

Logically the best solution would be replacing ->destructor,
set_owner* etc with skb_ops. Something like:

struct skb_ops
{
        void init(struct sk_buff * skb, struct skb_ops * ops, struct
sock * owner);
        void fini(struct sk_buff * skb);
        void update(struct sk_buff * skb, int adjust);
        void inherit(struct sk_buff * skb2, struct sk_buff * skb);
};

init - is replacement for skb_set_owner_r|w
fini - is replacement for skb_orphan
update - is new operation to be used in places where skb->truesize changes,
       instead of awful constructions like:

       if (!skb->sk || skb->destructor == sock_edemux)
            skb->truesize += size - osize;

       Now it will look like:

       if (skb->ops)
            skb->ops->update(skb, size - osize);

inherit - is replacement for also awful constructs like:

      if (skb->sk)
            skb_set_owner_w(skb2, skb->sk);

      Now it will be:

      if (skb->ops)
            skb->ops->inherit(skb2, skb);

The implementation looks mostly obvious.
Some troubles can be only with new functionality:
update of accounting was never done before.


More efficient, functionally equivalent, but uglier and less flexible
alternative would be removal of ->destructor, replaced with
a small numeric indicator of ownership:

enum
{
        SKB_OWNER_NONE,  /* aka destructor == NULL */
        SKB_OWNER_WMEM,  /* aka destructor == sk_wfree */
        SKB_OWNER_RMEM,  /* aka destructor == sk_rfree */
        SKB_OWNER_SK,    /* aka destructor == sk_edemux */
        SKB_OWNER_TCP,   /* aka destructor == tcp_wfree */
}

And the same init,fini,inherit,update become functions
w/o any inidirect calls. Not sure it is really more efficient though."

Thank you,
	Vasily Averin