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

[Vasily Averin]

On 9/2/21 6:53 PM, Christoph Paasch wrote:
> On Thu, Sep 2, 2021 at 4:12 AM Vasily Averin <vvs@xxxxxxxxxxxxx> wrote:
>>
>> 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 <christoph.paasch@xxxxxxxxx>
>> Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx>
>> ---
>> 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 

>> +       } 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;

>> +               /* 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
>>