Re: [PATCH net-next v5] skb_expand_head() adjust skb->truesize incorrectly

On 9/2/21 6:53 PM, Christoph Paasch wrote:

On Thu, Sep 2, 2021 at 4:12 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 may happen because of two reasons:
- skb_set_owner_w() for newly cloned skb is called too early,
before pskb_expand_head() where truesize is adjusted for (!skb-sk) case.
- pskb_expand_head() does not adjust truesize in (skb->sk) case.
In this case sk->sk_wmem_alloc should be adjusted too.

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

Fixes: f1260ff15a71 ("skbuff: introduce skb_expand_head()")
Reported-by: Christoph Paasch <redacted>
Signed-off-by: Vasily Averin <redacted>
---
v5: fixed else condition, thanks to Eric
    reworked update of expanded skb,
    added corresponding comments
v4: decided to use is_skb_wmem() after pskb_expand_head() call
    fixed 'return (EXPRESSION);' in os_skb_wmem according to Eric Dumazet
v3: removed __pskb_expand_head(),
    added is_skb_wmem() helper for skb with wmem-compatible destructors
    there are 2 ways to use it:
     - before pskb_expand_head(), to create skb clones
     - after successfull pskb_expand_head() to change owner on extended skb.
v2: based on patch version from Eric Dumazet,
    added __pskb_expand_head() function, which can be forced
    to adjust skb->truesize and sk->sk_wmem_alloc.
---
 include/net/sock.h |  1 +
 net/core/skbuff.c  | 63 ++++++++++++++++++++++++++++++++++++++++++++++--------
 net/core/sock.c    |  8 +++++++
 3 files changed, 63 insertions(+), 9 deletions(-)

Still the same issues around refcount as I reported in my other email.

Did you try running the syzkaller reproducer on your setup?

no, I do not have 

quoted

+       } else if (sk && skb->destructor != sock_edemux) {
+               bool ref, set_owner;
+
+               ref = false; set_owner = false;
+               delta = osize - skb_end_offset(skb);

error is here, should be instead
delta = skb_end_offset(skb) - osize;

quoted

+               /* skb_set_owner_w() calls current skb destructor.
+                * It can decrease sk_wmem_alloc to 0 and release sk,
+                * To prevnt it we increase sk_wmem_alloc earlier.
+                * Another kind of destructors can release last sk_refcnt,
+                * so it will be impossible to call sock_hold for !fullsock
+                * Take extra sk_refcnt to prevent it.
+                * Otherwise just increase truesize of expanded skb.
+                */
+               refcount_add(delta, &sk->sk_wmem_alloc);
+               if (!is_skb_wmem(skb)) {
+                       set_owner = true;
+                       if (!sk_fullsock(sk) && IS_ENABLED(CONFIG_INET)) {
+                               /* skb_set_owner_w can set sock_edemux */
+                               ref = refcount_inc_not_zero(&sk->sk_refcnt);
+                               if (!ref) {
+                                       set_owner = false;
+                                       WARN_ON(refcount_sub_and_test(delta, &sk->sk_wmem_alloc));
+                               }
+                       }
+               }
+               if (set_owner)
+                       skb_set_owner_w(skb, sk);
+#ifdef CONFIG_INET
+               if (skb->destructor == sock_edemux) {
+                       WARN_ON(refcount_sub_and_test(delta, &sk->sk_wmem_alloc));
+                       if (ref)
+                               WARN_ON(refcount_dec_and_test(&sk->sk_refcnt));
+               }
+#endif
+               skb->truesize += delta;
        }
        return skb;
 }

diff --git a/net/core/sock.c b/net/core/sock.c
index 950f1e7..6cbda43 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c

@@ -2227,6 +2227,14 @@ void skb_set_owner_w(struct sk_buff *skb, struct sock *sk)
 }
 EXPORT_SYMBOL(skb_set_owner_w);

+bool is_skb_wmem(const struct sk_buff *skb)
+{
+       return skb->destructor == sock_wfree ||
+              skb->destructor == __sock_wfree ||
+              (IS_ENABLED(CONFIG_INET) && skb->destructor == tcp_wfree);
+}
+EXPORT_SYMBOL(is_skb_wmem);
+
 static bool can_skb_orphan_partial(const struct sk_buff *skb)
 {
 #ifdef CONFIG_TLS_DEVICE
--

1.8.3.1