Subject: Re: [PATCH] net: sk_prot_alloc() should not blindly overwrite memory

From: Eric Dumazet <[email-masked]>
Date: Thu, 09 Jul 2009 22:50:00 +0200

Paul E. McKenney a écrit :
> On Thu, Jul 09, 2009 at 07:36:05AM +0200, Eric Dumazet wrote:
>> Eric Dumazet a écrit :
>>> David Miller a écrit :
>>>> From: Eric Dumazet <[email-masked]>
>>>> Date: Wed, 08 Jul 2009 00:33:29 +0200
>>>>
>>>>> [PATCH] net: sk_prot_alloc() should not blindly overwrite memory
>>>>>
>>>>> Some sockets use SLAB_DESTROY_BY_RCU, and our RCU code rely that some
>>>>> fields should not be blindly overwritten, even with null.
>>>>>
>>>>> These fields are sk->sk_refcnt and sk->sk_nulls_node.next
>>>>>
>>>>> Current sk_prot_alloc() implementation doesnt respect this hypothesis,
>>>>> calling kmem_cache_alloc() with __GFP_ZERO and setting sk_refcnt to 1
>>>>> instead of atomically increment it.
>>>>>
>>>>> Reported-by: Emil S Tantilov <[email-masked]>
>>>>> Signed-off-by: Eric Dumazet <[email-masked]>
>>>> I've applied this but will wait for some more testing before
>>>> I push it out for real to kernel.org
>>> Thanks David
>>>
>>> I forgot to CC Paul and Patrick, so I'll ask them to look at this patch.
>>>
>>> Patrick, a similar fix is needed in conntrack as well, we currently
>>> uses "ct = kmem_cache_zalloc(nf_conntrack_cachep, gfp);" and thus
>>>  overwrite struct hlist_nulls_node hnnode; contained
>>> in "struct nf_conntrack_tuple_hash", while lockless readers still
>>> potentialy need them. Setting hnnode.next to NULL is dangerous
>>> since last bit is not set (not a nulls value), a reader could
>>> try to dereference this NULL pointer and trap.
>>>
>>>
>>> Here is the patch again so that Paul & Patrick can comment on it.
>>>
>>> I am not sure about the refcnt thing (blindly setting it to 0 again
>>> should be OK in fact, since no reader should/can to the 
>>> atomic_inc_if_not_zero on it), but the nulls.next thing is problematic.
>> Here is an updated and much simpler patch, taking care of sk_node.next being not set to 0
>>
>> This patch applies to >= 2.6.29 kernels
> 
> Does this one also need the rearrangement of struct elements in the
> earlier patch?  (And apologies about being slow to get to that one.)
> 

No, because only one field (sk_node.next) needs special attention, I felt
it was not really necessary to reorder fields. First memset
is inlined because of constant size, so small cost.

Thanks

> 						Thanx, Paul
> 
>> [PATCH] net: sk_prot_alloc() should not blindly overwrite memory
>>
>> Some sockets use SLAB_DESTROY_BY_RCU, and our RCU code correctness
>> depends on sk->sk_nulls_node.next being always valid. A NULL
>> value is not allowed as it might fault a lockless reader.
>>
>> Current sk_prot_alloc() implementation doesnt respect this hypothesis,
>> calling kmem_cache_alloc() with __GFP_ZERO. Just call memset() around
>> the forbidden field.
>>
>> Signed-off-by: Eric Dumazet <[email-masked]>
>> ---
>> diff --git a/net/core/sock.c b/net/core/sock.c
>> index b0ba569..7b87ec0 100644
>> --- a/net/core/sock.c
>> +++ b/net/core/sock.c
>> @@ -939,8 +939,23 @@ static struct sock *sk_prot_alloc(struct proto *prot, gfp_t priority,
>>  	struct kmem_cache *slab;
>>
>>  	slab = prot->slab;
>> -	if (slab != NULL)
>> -		sk = kmem_cache_alloc(slab, priority);
>> +	if (slab != NULL) {
>> +		sk = kmem_cache_alloc(slab, priority & ~__GFP_ZERO);
>> +		if (!sk)
>> +			return sk;
>> +		if (priority & __GFP_ZERO) {
>> +			/*
>> +			 * caches using SLAB_DESTROY_BY_RCU should let
>> +			 * sk_node.next un-modified. Special care is taken
>> +			 * when initializing object to zero.
>> +			 */
>> +			if (offsetof(struct sock, sk_node.next) != 0)
>> +				memset(sk, 0, offsetof(struct sock, sk_node.next));
>> +			memset(&sk->sk_node.pprev, 0,
>> +			       prot->obj_size - offsetof(struct sock,
>> +							 sk_node.pprev));
>> +		}
>> +	}
>>  	else
>>  		sk = kmalloc(prot->obj_size, priority);
>>