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>

1 files changed, 6 insertions, 7 deletions

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index b2d1838897c9..db4984e13f2f 100644
--- 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);