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

From: Eric Dumazet <[email-masked]>
Date: Thu, 09 Jul 2009 07:36:05 +0200

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

[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);