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Date: Sat, 19 Aug 2017 09:51:52 -0700
From: Wei Wang <weiwan@...gle.com>
To: Martin KaFai Lau <kafai@...com>
Cc: David Miller <davem@...emloft.net>,
Linux Kernel Network Developers <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] ipv6: add rcu grace period before freeing fib6_node
Hi Martin,
>> +/* Function to safely get fn->sernum for passed in rt
>> + * and store result in passed in cookie.
>> + * Return true if we can get cookie safely
>> + * Return false if not
>> + */
>> +static inline bool rt6_get_cookie_safe(const struct rt6_info *rt,
>> + u32 *cookie)
> Looking at fib6_new_sernum(), fn_sernum should be >0.
>
> Would it further simplify the later changes if we do this instead?:
> static inline u32 rt6_get_cookie_safe(const struct rt6_info *rt)
>
I don't think rt6_check() will work properly if this function only
returns fn_sernum. Because rt6_get_cookie() will return cookie as 0 if
the node is already deleted. And socket will store 0 as its
dst_cookie. And when ip6_dst_check() is called, rt6_check() calls
rt6_get_cookie_safe() to get the current sernum in fib6_node and finds
it is also 0, so it will say the dst is valid. But it is wrong.
Basically, the return status of rt6_get_cookie_safe() indicates if the
rt6i_node is NULL or not. And it needs to be checked in rt6_check().
>> +{
>> + struct fib6_node *fn;
>> + bool status = false;
>> +
>> + rcu_read_lock();
>> + fn = rcu_dereference(rt->rt6i_node);
>> +
>> + if (fn) {
>> + *cookie = fn->fn_sernum;
>> + status = true;
>> + }
>> +
>> + rcu_read_unlock();
>> + return status;
>> +
> extra newline.
>
Thanks. Will remove it in v2.
Wei
On Fri, Aug 18, 2017 at 7:20 PM, Martin KaFai Lau <kafai@...com> wrote:
> On Fri, Aug 18, 2017 at 05:36:55PM -0700, Wei Wang wrote:
>> From: Wei Wang <weiwan@...gle.com>
>>
>> We currently keep rt->rt6i_node pointing to the fib6_node for the route.
>> And some functions make use of this pointer to dereference the fib6_node
>> from rt structure, e.g. rt6_check(). However, as there is neither
>> refcount nor rcu taken when dereferencing rt->rt6i_node, it could
>> potentially cause crashes as rt->rt6i_node could be set to NULL by other
>> CPUs when doing a route deletion.
>> This patch introduces an rcu grace period before freeing fib6_node and
>> makes sure the functions that dereference it takes rcu_read_lock().
>>
>> Note: there is no "Fixes" tag because this bug was there in a very
>> early stage.
>>
>> Signed-off-by: Wei Wang <weiwan@...gle.com>
>> Acked-by: Eric Dumazet <edumazet@...gle.com>
> Looks good. Thanks for the fixing it.
> Only have some nits comments.
>
>> ---
>> include/net/ip6_fib.h | 31 ++++++++++++++++++++++++++++++-
>> net/ipv6/ip6_fib.c | 20 ++++++++++++++++----
>> net/ipv6/route.c | 14 +++++++++++---
>> 3 files changed, 57 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
>> index 71c1646298ae..5691faf6b495 100644
>> --- a/include/net/ip6_fib.h
>> +++ b/include/net/ip6_fib.h
>> @@ -72,6 +72,7 @@ struct fib6_node {
>> __u16 fn_flags;
>> int fn_sernum;
>> struct rt6_info *rr_ptr;
>> + struct rcu_head rcu;
>> };
>>
>> #ifndef CONFIG_IPV6_SUBTREES
>> @@ -171,13 +172,41 @@ static inline void rt6_update_expires(struct rt6_info *rt0, int timeout)
>> rt0->rt6i_flags |= RTF_EXPIRES;
>> }
>>
>> +/* Function to safely get fn->sernum for passed in rt
>> + * and store result in passed in cookie.
>> + * Return true if we can get cookie safely
>> + * Return false if not
>> + */
>> +static inline bool rt6_get_cookie_safe(const struct rt6_info *rt,
>> + u32 *cookie)
> Looking at fib6_new_sernum(), fn_sernum should be >0.
>
> Would it further simplify the later changes if we do this instead?:
> static inline u32 rt6_get_cookie_safe(const struct rt6_info *rt)
>
>> +{
>> + struct fib6_node *fn;
>> + bool status = false;
>> +
>> + rcu_read_lock();
>> + fn = rcu_dereference(rt->rt6i_node);
>> +
>> + if (fn) {
>> + *cookie = fn->fn_sernum;
>> + status = true;
>> + }
>> +
>> + rcu_read_unlock();
>> + return status;
>> +
> extra newline.
>
>> +}
>> +
>> static inline u32 rt6_get_cookie(const struct rt6_info *rt)
>> {
>> + u32 cookie = 0;
>> +
>> if (rt->rt6i_flags & RTF_PCPU ||
>> (unlikely(!list_empty(&rt->rt6i_uncached)) && rt->dst.from))
>> rt = (struct rt6_info *)(rt->dst.from);
>>
>> - return rt->rt6i_node ? rt->rt6i_node->fn_sernum : 0;
>> + rt6_get_cookie_safe(rt, &cookie);
>> +
>> + return cookie;
>> }
>>
>> static inline void ip6_rt_put(struct rt6_info *rt)
>> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
>> index 549aacc3cb2c..a9821c230e4e 100644
>> --- a/net/ipv6/ip6_fib.c
>> +++ b/net/ipv6/ip6_fib.c
>> @@ -149,11 +149,23 @@ static struct fib6_node *node_alloc(void)
>> return fn;
>> }
>>
>> -static void node_free(struct fib6_node *fn)
>> +static void node_free_immediate(struct fib6_node *fn)
>> +{
>> + kmem_cache_free(fib6_node_kmem, fn);
>> +}
>> +
>> +static void node_free_rcu(struct rcu_head *head)
>> {
>> + struct fib6_node *fn = container_of(head, struct fib6_node, rcu);
>> +
>> kmem_cache_free(fib6_node_kmem, fn);
>> }
>>
>> +static void node_free(struct fib6_node *fn)
>> +{
>> + call_rcu(&fn->rcu, node_free_rcu);
>> +}
>> +
>> void rt6_free_pcpu(struct rt6_info *non_pcpu_rt)
>> {
>> int cpu;
>> @@ -697,9 +709,9 @@ static struct fib6_node *fib6_add_1(struct fib6_node *root,
>>
>> if (!in || !ln) {
>> if (in)
>> - node_free(in);
>> + node_free_immediate(in);
>> if (ln)
>> - node_free(ln);
>> + node_free_immediate(ln);
>> return ERR_PTR(-ENOMEM);
>> }
>>
>> @@ -1138,7 +1150,7 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt,
>> root, and then (in failure) stale node
>> in main tree.
>> */
>> - node_free(sfn);
>> + node_free_immediate(sfn);
>> err = PTR_ERR(sn);
>> goto failure;
>> }
>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>> index bec12ae3e6b7..4de2d793c4b8 100644
>> --- a/net/ipv6/route.c
>> +++ b/net/ipv6/route.c
>> @@ -1289,7 +1289,9 @@ static void rt6_dst_from_metrics_check(struct rt6_info *rt)
>>
>> static struct dst_entry *rt6_check(struct rt6_info *rt, u32 cookie)
>> {
>> - if (!rt->rt6i_node || (rt->rt6i_node->fn_sernum != cookie))
>> + u32 rt_cookie;
>> +
>> + if (!rt6_get_cookie_safe(rt, &rt_cookie) || rt_cookie != cookie)
>> return NULL;
>>
>> if (rt6_check_expired(rt))
>> @@ -1357,8 +1359,14 @@ static void ip6_link_failure(struct sk_buff *skb)
>> if (rt->rt6i_flags & RTF_CACHE) {
>> if (dst_hold_safe(&rt->dst))
>> ip6_del_rt(rt);
>> - } else if (rt->rt6i_node && (rt->rt6i_flags & RTF_DEFAULT)) {
>> - rt->rt6i_node->fn_sernum = -1;
>> + } else {
>> + struct fib6_node *fn;
>> +
>> + rcu_read_lock();
>> + fn = rcu_dereference(rt->rt6i_node);
>> + if (fn && (rt->rt6i_flags & RTF_DEFAULT))
>> + fn->fn_sernum = -1;
>> + rcu_read_unlock();
>> }
>> }
>> }
>> --
>> 2.14.1.480.gb18f417b89-goog
>>
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