Re: [PATCH net-next v4] skb_expand_head() adjust skb->truesize incorrectly

[Vasily Averin]

On 9/1/21 10:17 PM, Eric Dumazet wrote:
> 
> 
> On 9/1/21 1:11 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()")
>> Reported-by: Christoph Paasch <christoph.paasch@xxxxxxxxx>
>> Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx>
>> ---
>> 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, 35 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/net/sock.h b/include/net/sock.h
>> index 95b2577..173d58c 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 f931176..09991cb 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -1804,28 +1804,45 @@ 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)
>>  {
>>  	int delta = headroom - skb_headroom(skb);
>> +	int osize = skb_end_offset(skb);
>> +	struct sk_buff *oskb = NULL;
>> +	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 */
>> +	delta = SKB_DATA_ALIGN(delta);
>> +	/* pskb_expand_head() might crash, if skb is shared.
>> +	 * Also we should clone skb if its destructor does
>> +	 * not adjust skb->truesize and sk->sk_wmem_alloc
>> + 	 */
>>  	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, delta, 0, GFP_ATOMIC)) {
>>  		kfree_skb(skb);
>> -		skb = NULL;
>> +		kfree_skb(oskb);
>> +		return NULL;
>> +	}
>> +	if (oskb) {
>> +		if (sk)
> 
> if (is_skb_wmem(oskb))
> Again, it is not valid to call skb_set_owner_w(skb, sk) on all kind of sockets.

I'm disagree.

In this particular case we have new skb with skb->sk = NULL,
In this case skb_orphan() called inside skb_set_owner_w(() will do nothing,
we just properly set destructor to sock_wfree and adjust sk->sk_wmem_alloc,

It is 100% equivalent of code used with skb_realloc_headroom(),
and there was no claims on this.
Cristoph's reproducer do not use shared skb and to not check this path,
so it cannot be the reason of troubles in his experiments.

Old destructor (sock_edemux?) can be calleda bit later, for old skb, inside consume_skb().
It can decrement last refcount and can trigger sk_free(). However in this case
adjusted sk_wmem_alloc did not allow to free sk.

So I'm sure it is safe.

>> +			skb_set_owner_w(skb, sk);
>> +		consume_skb(oskb);
>> +	} else if (sk) {
> 
> && (skb->destructor != sock_edemux)
> (Because in this case , pskb_expand_head() already adjusted skb->truesize)

Agree, thank you, my fault, I've missed it.
I think it was the reason of the troubles in last Cristoph's experiment.

>> +		delta = osize - skb_end_offset(skb);
> 
>> +		if (!is_skb_wmem(skb))
>> +			skb_set_owner_w(skb, sk);
> 
> This is dangerous, even if a socket is there, its sk->sk_wmem_alloc could be zero.
> We can not add skb->truesize to a refcount_t that already reached 0 (sk_free())
> 
> If is_skb_wmem() is false, you probably should do nothing, and leave
> current destructor as it is.

I;m still not sure and think it is tricky too.
I've found few destructors called sock_wfree inside, they require sk_wmem_alloc adjustement.
sctp_wfree, unix_destruct_scm and tpacket_destruct_skb

In the same time another ones do not use sk_wmem_alloc and I do not know how to detect proper ones.
Potentially there are some 3rd party protocols out-of-tree, and I cannot list all of them here.

However I think I can use the same trick as one described above:
I can increase sk_wmem_alloc before skb_orphan(), so sk_free() called by old destuctor 
cannot call __sk_free() and release sk.

I hope this should work, 
otherwise we'll need to clone skb for !is_skb_wmem(skb) before pskb_expand_head() call.

Thank you,
	Vasily Averin