linux-kernel - Re: [PATCH net v9] skb_expand_head() adjust skb->truesize incorrectly

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Date:   Tue, 5 Oct 2021 08:57:33 +0300
From:   Vasily Averin <vvs@...tuozzo.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Julian Wiedmann <jwi@...ux.ibm.com>,
        Christoph Paasch <christoph.paasch@...il.com>,
        linux-kernel@...r.kernel.org, kernel@...nvz.org
Subject: Re: [PATCH net v9] skb_expand_head() adjust skb->truesize incorrectly

On 10/4/21 10:26 PM, Eric Dumazet wrote:
> 
> 
> On 10/4/21 6:00 AM, Vasily Averin 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()")
>> Fixes: 2d85a1b31dde ("ipv6: ip6_finish_output2: set sk into newly allocated nskb")
>> Reported-by: Christoph Paasch <christoph.paasch@...il.com>
>> Signed-off-by: Vasily Averin <vvs@...tuozzo.com>
>> ---
>> v9: restored sock_edemux check
>> v8: clone non-wmem skb
>> v7 (from kuba@):
>>     shift more magic into helpers,
>>     follow Eric's advice and don't inherit non-wmem skbs for now
>> v6: fixed delta,
>>     improved comments
>> 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  | 35 ++++++++++++++++++++++-------------
>>  net/core/sock.c    |  8 ++++++++
>>  3 files changed, 31 insertions(+), 13 deletions(-)
>>
>> diff --git a/include/net/sock.h b/include/net/sock.h
>> index 66a9a90f9558..a547651d46a7 100644
>> --- a/include/net/sock.h
>> +++ b/include/net/sock.h
>> @@ -1695,6 +1695,7 @@ struct sk_buff *sock_wmalloc(struct sock *sk, unsigned long size, int force,
>>  			     gfp_t priority);
>>  void __sock_wfree(struct sk_buff *skb);
>>  void sock_wfree(struct sk_buff *skb);
>> +bool is_skb_wmem(const struct sk_buff *skb);
>>  struct sk_buff *sock_omalloc(struct sock *sk, unsigned long size,
>>  			     gfp_t priority);
>>  void skb_orphan_partial(struct sk_buff *skb);
>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> index 2170bea2c7de..8cb6d360cda5 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -1804,30 +1804,39 @@ EXPORT_SYMBOL(skb_realloc_headroom);
>>  struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
>>  {
>>  	int delta = headroom - skb_headroom(skb);
>> +	int osize = skb_end_offset(skb);
>> +	struct sock *sk = skb->sk;
>>  
>>  	if (WARN_ONCE(delta <= 0,
>>  		      "%s is expecting an increase in the headroom", __func__))
>>  		return skb;
>>  
>> -	/* pskb_expand_head() might crash, if skb is shared */
>> -	if (skb_shared(skb)) {
>> +	delta = SKB_DATA_ALIGN(delta);
>> +	/* pskb_expand_head() might crash, if skb is shared. */
>> +	if (skb_shared(skb) || !is_skb_wmem(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);
>> -		}
>> +		if (unlikely(!nskb))
>> +			goto fail;
>> +
>> +		if (sk)
>> +			skb_set_owner_w(nskb, sk);
>> +		consume_skb(skb);
>>  		skb = nskb;
>>  	}
>> -	if (skb &&
>> -	    pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
>> -		kfree_skb(skb);
>> -		skb = NULL;
>> +	if (pskb_expand_head(skb, delta, 0, GFP_ATOMIC))
>> +		goto fail;
>> +
>> +	if (sk && skb->destructor != sock_edemux) {
> 
>     Why not re-using is_skb_wmem() here ?
>     Testing != sock_edemux looks strange.

All non-wmem skbs was cloned and then was freed already.
After pskb_expand_head() call we can have:
(1) either original wmem skbs 
(2) or cloned skbs: 
 (2a) either without sk at all,
 (2b) or with sock_edemux destructor (that was set inside skb_set_owner_w() for !sk_fullsock(sk))
 (2c) or with sock_wfree destructor (that was set inside skb_set_owner_w() for sk_fullsock(sk))

(2a) and (2b) do not require truesize/sk_wmem_alloc update, it was handled inside pskb_expand_head()
(1) and (2c) cases are processed here.

If required I can add this explanation either into patch description or as comment.

Btw I just noticed that we can avoid cloning for original skbs without sk.
How do you think should I do it?

>> +		delta = skb_end_offset(skb) - osize;
>> +		refcount_add(delta, &sk->sk_wmem_alloc);
>> +		skb->truesize += delta;
>>  	}
>>  	return skb;
>> +
>> +fail:
>> +	kfree_skb(skb);
>> +	return NULL;
>>  }
>>  EXPORT_SYMBOL(skb_expand_head);
>>  
>> diff --git a/net/core/sock.c b/net/core/sock.c
>> index 62627e868e03..1932755ae9ba 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);
>> +
> 
> This probably should be inlined.

David Miller pointed me out in the comments to an early version of the patch
"Please do not use inline in foo.c files, let the compiler decide."

>>  static bool can_skb_orphan_partial(const struct sk_buff *skb)
>>  {
>>  #ifdef CONFIG_TLS_DEVICE
>>