net: filter: fix possible use after free

[ No appicable upstream commit, this bug has been subsequently been
  fixed as a side effect of other changes. ]

If kmemdup() fails, we free fp->orig_prog and return -ENOMEM

sk_attach_filter()
 -> sk_filter_uncharge(sk, fp)
  -> sk_filter_release(fp)
   -> call_rcu(&fp->rcu, sk_filter_release_rcu)
    -> sk_filter_release_rcu()
     -> sk_release_orig_filter()
        fprog = fp->orig_prog; // not NULL, but points to freed memory
	  kfree(fprog->filter); // use after free, potential corruption
          kfree(fprog); // double free or corruption

Note: This was fixed in 3.17+ with commit 278571ba
("net: filter: simplify socket charging")

Found by AddressSanitizer
Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: a3ea269b ("net: filter: keep original BPF program around")
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/core/filter.c

@@ -1318,6 +1318,7 @@ static int sk_store_orig_filter(struct sk_filter *fp,
 	fkprog->filter = kmemdup(fp->insns, fsize, GFP_KERNEL);
 	if (!fkprog->filter) {
 		kfree(fp->orig_prog);
+		fp->orig_prog = NULL;
 		return -ENOMEM;
 	}