net: fib6: fib6_commit_metrics: fix potential NULL pointer dereference

When IPv6 host routes with metrics attached are being added, we fetch
the metrics store from the dst via COW through dst_metrics_write_ptr(),
added through commit e5fd387ad5b3.

One remaining problem here is that we actually call into inet_getpeer()
and may end up allocating/creating a new peer from the kmemcache, which
may fail.

Example trace from perf probe (inet_getpeer:41) where create is 1:

ip 6877 [002] 4221.391591: probe:inet_getpeer: (ffffffff8165e293)
  85e294 inet_getpeer.part.7 (<- kmem_cache_alloc())
  85e578 inet_getpeer
  8eb333 ipv6_cow_metrics
  8f10ff fib6_commit_metrics

Therefore, a check for NULL on the return of dst_metrics_write_ptr()
is necessary here.

Joint work with Florian Westphal.

Fixes: e5fd387ad5b3 ("ipv6: do not overwrite inetpeer metrics prematurely")
Cc: Michal Kubeček <mkubecek@suse.cz>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index b2d1838897c933f6f1ceb2a6f64a2f67495ce3c9..db4984e13f2f4bf27eb7284439478755f5bcb3a6 100644 (file)
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -633,18 +633,17 @@ static bool rt6_qualify_for_ecmp(struct rt6_info *rt)
 static int fib6_commit_metrics(struct dst_entry *dst,
                               struct nlattr *mx, int mx_len)
 {
+       bool dst_host = dst->flags & DST_HOST;
        struct nlattr *nla;
        int remaining;
        u32 *mp;
 
-       if (dst->flags & DST_HOST) {
-               mp = dst_metrics_write_ptr(dst);
-       } else {
-               mp = kzalloc(sizeof(u32) * RTAX_MAX, GFP_ATOMIC);
-               if (!mp)
-                       return -ENOMEM;
+       mp = dst_host ? dst_metrics_write_ptr(dst) :
+                       kzalloc(sizeof(u32) * RTAX_MAX, GFP_ATOMIC);
+       if (unlikely(!mp))
+               return -ENOMEM;
+       if (!dst_host)
                dst_init_metrics(dst, mp, 0);
-       }
 
        nla_for_each_attr(nla, mx, mx_len, remaining) {
                int type = nla_type(nla);