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Date: Sat, 19 Aug 2017 21:33:04 -0700
From: Martin KaFai Lau <kafai@...com>
To: Wei Wang <weiwan@...gle.com>
CC: David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net v2] ipv6: add rcu grace period before freeing
fib6_node
On Sat, Aug 19, 2017 at 05:34:08PM -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>
Acked-by: Martin KaFai Lau <kafai@...com>
> ---
> v2: removed one extra empty line
>
> include/net/ip6_fib.h | 30 +++++++++++++++++++++++++++++-
> net/ipv6/ip6_fib.c | 20 ++++++++++++++++----
> net/ipv6/route.c | 14 +++++++++++---
> 3 files changed, 56 insertions(+), 8 deletions(-)
>
> diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
> index 71c1646298ae..977a86e3a8d9 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 +172,40 @@ 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)
> +{
> + 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;
> +}
> +
> 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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