ipv6: replace dst_hold() with dst_hold_safe() in routing code

With rwlock, it is safe to call dst_hold() in the read thread because
read thread is guaranteed to be separated from write thread.
However, after we replace rwlock with rcu, it is no longer safe to use
dst_hold(). A dst might already have been deleted but is waiting for the
rcu grace period to pass before freeing the memory when a read thread is
trying to do dst_hold(). This could potentially cause double free issue.

So this commit replaces all dst_hold() with dst_hold_safe() in all read
thread to avoid this double free issue.
And in order to make the code more compact, a new function ip6_hold_safe()
is introduced. It calls dst_hold_safe() first, and if that fails, it will
either fall back to hold and return net->ipv6.ip6_null_entry or set rt to
NULL according to the caller's need.

Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 873afaf..f86e931 100644 (file)
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2333,7 +2333,8 @@ static struct rt6_info *addrconf_get_prefix_route(const struct in6_addr *pfx,
                        continue;
                if ((rt->rt6i_flags & noflags) != 0)
                        continue;
-               dst_hold(&rt->dst);
+               if (!dst_hold_safe(&rt->dst))
+                       rt = NULL;
                break;
        }
 out:

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 941c062..aeb349a 100644 (file)
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -874,6 +874,23 @@ static struct fib6_node* fib6_backtrack(struct fib6_node *fn,
        }
 }
 
+static bool ip6_hold_safe(struct net *net, struct rt6_info **prt,
+                         bool null_fallback)
+{
+       struct rt6_info *rt = *prt;
+
+       if (dst_hold_safe(&rt->dst))
+               return true;
+       if (null_fallback) {
+               rt = net->ipv6.ip6_null_entry;
+               dst_hold(&rt->dst);
+       } else {
+               rt = NULL;
+       }
+       *prt = rt;
+       return false;
+}
+
 static struct rt6_info *ip6_pol_route_lookup(struct net *net,
                                             struct fib6_table *table,
                                             struct flowi6 *fl6, int flags)
@@ -898,7 +915,9 @@ restart:
        if (rt_cache)
                rt = rt_cache;
 
-       dst_use(&rt->dst, jiffies);
+       if (ip6_hold_safe(net, &rt, true))
+               dst_use_noref(&rt->dst, jiffies);
+
        read_unlock_bh(&table->tb6_lock);
 
        trace_fib6_table_lookup(net, rt, table->tb6_id, fl6);
@@ -1061,10 +1080,9 @@ static struct rt6_info *rt6_get_pcpu_route(struct rt6_info *rt)
        p = this_cpu_ptr(rt->rt6i_pcpu);
        pcpu_rt = *p;
 
-       if (pcpu_rt) {
-               dst_hold(&pcpu_rt->dst);
+       if (pcpu_rt && ip6_hold_safe(NULL, &pcpu_rt, false))
                rt6_dst_from_metrics_check(pcpu_rt);
-       }
+
        return pcpu_rt;
 }
 
@@ -1625,12 +1643,17 @@ redo_rt6_select:
        if (rt_cache)
                rt = rt_cache;
 
-       if (rt == net->ipv6.ip6_null_entry || (rt->rt6i_flags & RTF_CACHE)) {
-               dst_use(&rt->dst, jiffies);
+       if (rt == net->ipv6.ip6_null_entry) {
+               read_unlock_bh(&table->tb6_lock);
+               dst_hold(&rt->dst);
+               trace_fib6_table_lookup(net, rt, table->tb6_id, fl6);
+               return rt;
+       } else if (rt->rt6i_flags & RTF_CACHE) {
+               if (ip6_hold_safe(net, &rt, true)) {
+                       dst_use_noref(&rt->dst, jiffies);
+                       rt6_dst_from_metrics_check(rt);
+               }
                read_unlock_bh(&table->tb6_lock);
-
-               rt6_dst_from_metrics_check(rt);
-
                trace_fib6_table_lookup(net, rt, table->tb6_id, fl6);
                return rt;
        } else if (unlikely((fl6->flowi6_flags & FLOWI_FLAG_KNOWN_NH) &&
@@ -1643,7 +1666,13 @@ redo_rt6_select:
 
                struct rt6_info *uncached_rt;
 
-               dst_use(&rt->dst, jiffies);
+               if (ip6_hold_safe(net, &rt, true)) {
+                       dst_use_noref(&rt->dst, jiffies);
+               } else {
+                       read_unlock_bh(&table->tb6_lock);
+                       uncached_rt = rt;
+                       goto uncached_rt_out;
+               }
                read_unlock_bh(&table->tb6_lock);
 
                uncached_rt = ip6_rt_cache_alloc(rt, &fl6->daddr, NULL);
@@ -1659,6 +1688,7 @@ redo_rt6_select:
                        dst_hold(&uncached_rt->dst);
                }
 
+uncached_rt_out:
                trace_fib6_table_lookup(net, uncached_rt, table->tb6_id, fl6);
                return uncached_rt;
 
@@ -1667,8 +1697,7 @@ redo_rt6_select:
 
                struct rt6_info *pcpu_rt;
 
-               rt->dst.lastuse = jiffies;
-               rt->dst.__use++;
+               dst_use_noref(&rt->dst, jiffies);
                pcpu_rt = rt6_get_pcpu_route(rt);
 
                if (pcpu_rt) {
@@ -2130,7 +2159,7 @@ restart:
        }
 
 out:
-       dst_hold(&rt->dst);
+       ip6_hold_safe(net, &rt, true);
 
        read_unlock_bh(&table->tb6_lock);
 
@@ -2841,7 +2870,8 @@ static int ip6_route_del(struct fib6_config *cfg,
                                continue;
                        if (cfg->fc_protocol && cfg->fc_protocol != rt->rt6i_protocol)
                                continue;
-                       dst_hold(&rt->dst);
+                       if (!dst_hold_safe(&rt->dst))
+                               break;
                        read_unlock_bh(&table->tb6_lock);
 
                        /* if gateway was specified only delete the one hop */
@@ -3038,7 +3068,7 @@ static struct rt6_info *rt6_get_route_info(struct net *net,
                        continue;
                if (!ipv6_addr_equal(&rt->rt6i_gateway, gwaddr))
                        continue;
-               dst_hold(&rt->dst);
+               ip6_hold_safe(NULL, &rt, false);
                break;
        }
 out:
@@ -3096,7 +3126,7 @@ struct rt6_info *rt6_get_dflt_router(const struct in6_addr *addr, struct net_dev
                        break;
        }
        if (rt)
-               dst_hold(&rt->dst);
+               ip6_hold_safe(NULL, &rt, false);
        read_unlock_bh(&table->tb6_lock);
        return rt;
 }
@@ -3139,9 +3169,12 @@ restart:
        for (rt = table->tb6_root.leaf; rt; rt = rt->dst.rt6_next) {
                if (rt->rt6i_flags & (RTF_DEFAULT | RTF_ADDRCONF) &&
                    (!rt->rt6i_idev || rt->rt6i_idev->cnf.accept_ra != 2)) {
-                       dst_hold(&rt->dst);
-                       read_unlock_bh(&table->tb6_lock);
-                       ip6_del_rt(rt);
+                       if (dst_hold_safe(&rt->dst)) {
+                               read_unlock_bh(&table->tb6_lock);
+                               ip6_del_rt(rt);
+                       } else {
+                               read_unlock_bh(&table->tb6_lock);
+                       }
                        goto restart;
                }
        }