[PATCH 4.4 02/66] ipv6: add rcu grace period before freeing fib6_node

Greg Kroah-Hartman
2017-09-24T21:30:19Z

From: Wei Wang <weiwan@google.com>

[ Upstream commit c5cff8561d2d0006e972bd114afd51f082fee77c ]

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@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/net/ip6_fib.h | 30 +++++++++++++++++++++++++++++-
net/ipv6/ip6_fib.c | 20 ++++++++++++++++----
net/ipv6/route.c | 14 +++++++++++---
3 files changed, 56 insertions(+), 8 deletions(-)

--- a/include/net/ip6_fib.h
+++ b/include/net/ip6_fib.h
@@ -68,6 +68,7 @@ struct fib6_node {
__u16 fn_flags;
int fn_sernum;
struct rt6_info *rr_ptr;
+ struct rcu_head rcu;
};

#ifndef CONFIG_IPV6_SUBTREES
@@ -165,13 +166,40 @@ static inline void rt6_update_expires(st
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(rt->dst.flags & DST_NOCACHE) && 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)
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -150,11 +150,23 @@ 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);
+}
+
static void rt6_rcu_free(struct rt6_info *rt)
{
call_rcu(&rt->dst.rcu_head, dst_rcu_free);
@@ -588,9 +600,9 @@ insert_above:

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);
}

@@ -1015,7 +1027,7 @@ int fib6_add(struct fib6_node *root, str
root, and then (in failure) stale node
in main tree.
*/
- node_free(sfn);
+ node_free_immediate(sfn);
err = PTR_ERR(sn);
goto failure;
}
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1248,7 +1248,9 @@ static void rt6_dst_from_metrics_check(s

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))
@@ -1316,8 +1318,14 @@ static void ip6_link_failure(struct sk_b
if (rt->rt6i_flags & RTF_CACHE) {
dst_hold(&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();
}
}
}