[PATCH net] inet: frags: better deal with smp races

[PATCH net] inet: frags: better deal with smp races

[Eric Dumazet]

Multiple cpus might attempt to insert a new fragment in rhashtable,
if for example RPS is buggy, as reported by 배석진in
https://patchwork.ozlabs.org/patch/994601/

We use rhashtable_lookup_get_insert_key() instead of
rhashtable_insert_fast() to let cpus losing the race
free their own inet_frag_queue and use the one that
was inserted by another cpu.

Fixes: 648700f76b03 ("inet: frags: use rhashtables for reassembly units")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: 배석진 <soukjin.bae@samsung.com>
---
 net/ipv4/inet_fragment.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index bcb11f3a27c0c34115af05034a5a20f57842eb0a..ced9abd4bec6cd494e352c1d6a97da8f67cf6073 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -178,21 +178,22 @@ static struct inet_frag_queue *inet_frag_alloc(struct netns_frags *nf,
 }
 
 static struct inet_frag_queue *inet_frag_create(struct netns_frags *nf,
-						void *arg)
+						void *arg,
+						struct inet_frag_queue **prev)
 {
 	struct inet_frags *f = nf->f;
 	struct inet_frag_queue *q;
-	int err;
 
 	q = inet_frag_alloc(nf, f, arg);
-	if (!q)
+	if (!q) {
+		*prev = ERR_PTR(-ENOMEM);
 		return NULL;
-
+	}
 	mod_timer(&q->timer, jiffies + nf->timeout);
 
-	err = rhashtable_insert_fast(&nf->rhashtable, &q->node,
-				     f->rhash_params);
-	if (err < 0) {
+	*prev = rhashtable_lookup_get_insert_key(&nf->rhashtable, &q->key,
+						 &q->node, f->rhash_params);
+	if (*prev) {
 		q->flags |= INET_FRAG_COMPLETE;
 		inet_frag_kill(q);
 		inet_frag_destroy(q);
@@ -204,22 +205,22 @@ static struct inet_frag_queue *inet_frag_create(struct netns_frags *nf,
 /* TODO : call from rcu_read_lock() and no longer use refcount_inc_not_zero() */
 struct inet_frag_queue *inet_frag_find(struct netns_frags *nf, void *key)
 {
-	struct inet_frag_queue *fq;
+	struct inet_frag_queue *fq, *prev;
 
 	if (!nf->high_thresh || frag_mem_limit(nf) > nf->high_thresh)
 		return NULL;
 
 	rcu_read_lock();
 
-	fq = rhashtable_lookup(&nf->rhashtable, key, nf->f->rhash_params);
-	if (fq) {
+	prev = rhashtable_lookup(&nf->rhashtable, key, nf->f->rhash_params);
+	if (!prev)
+		fq = inet_frag_create(nf, key, &prev);
+	if (prev && !IS_ERR(prev)) {
+		fq = prev;
 		if (!refcount_inc_not_zero(&fq->refcnt))
 			fq = NULL;
-		rcu_read_unlock();
-		return fq;
 	}
 	rcu_read_unlock();
-
-	return inet_frag_create(nf, key);
+	return fq;
 }
 EXPORT_SYMBOL(inet_frag_find);
-- 
2.19.1.930.g4563a0d9d0-goog

Re: [PATCH net] inet: frags: better deal with smp races

[David Miller]

From: Eric Dumazet <edumazet@google.com>
Date: Wed,  7 Nov 2018 22:10:53 -0800

> @@ -204,22 +205,22 @@ static struct inet_frag_queue *inet_frag_create(struct netns_frags *nf,
>  /* TODO : call from rcu_read_lock() and no longer use refcount_inc_not_zero() */
>  struct inet_frag_queue *inet_frag_find(struct netns_frags *nf, void *key)
>  {
> -	struct inet_frag_queue *fq;
> +	struct inet_frag_queue *fq, *prev;
>  
>  	if (!nf->high_thresh || frag_mem_limit(nf) > nf->high_thresh)
>  		return NULL;
>  
>  	rcu_read_lock();
>  
> -	fq = rhashtable_lookup(&nf->rhashtable, key, nf->f->rhash_params);
> -	if (fq) {
> +	prev = rhashtable_lookup(&nf->rhashtable, key, nf->f->rhash_params);
> +	if (!prev)
> +		fq = inet_frag_create(nf, key, &prev);
> +	if (prev && !IS_ERR(prev)) {
> +		fq = prev;
>  		if (!refcount_inc_not_zero(&fq->refcnt))
>  			fq = NULL;
> -		rcu_read_unlock();
> -		return fq;
>  	}
>  	rcu_read_unlock();
> -
> -	return inet_frag_create(nf, key);
> +	return fq;

GCC is unwilling to see that all paths leading to that final return
statement do in fact set 'fq' one way or another:

net/ipv4/inet_fragment.c: In function ‘inet_frag_find’:
net/ipv4/inet_fragment.c:224:9: warning: ‘fq’ may be used uninitialized in this function [-Wmaybe-uninitialized]

This is with:

gcc (GCC) 8.2.1 20181011 (Red Hat 8.2.1-4)

Please adjust your patch so that the warning is eliminated.

Thanks.

Re: [PATCH net] inet: frags: better deal with smp races

[Eric Dumazet]

On 11/08/2018 05:02 PM, David Miller wrote:

> GCC is unwilling to see that all paths leading to that final return
> statement do in fact set 'fq' one way or another:
> 
> net/ipv4/inet_fragment.c: In function ‘inet_frag_find’:
> net/ipv4/inet_fragment.c:224:9: warning: ‘fq’ may be used uninitialized in this function [-Wmaybe-uninitialized]
> 
> This is with:
> 
> gcc (GCC) 8.2.1 20181011 (Red Hat 8.2.1-4)
> 
> Please adjust your patch so that the warning is eliminated.
> 

Interesting, I will init *fq to NULL in v2, thanks.