Re: [PATCH iproute2] iproute2: fix use-after-free

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 12 Sep 2018 16:29:28 -0700
Mahesh Bandewar <mahesh@bandewar.net> wrote:

> From: Mahesh Bandewar <maheshb@google.com>
> 
> A local program using iproute2 lib pointed out the issue and looking
> at the code it is pretty obvious -
> 
>     a = (struct nlmsghdr *)b;
>     ...
>     free(b);
>     if (a->nlmsg_seq == seq)
>     ...
> 
> Fixes: 86bf43c7c2fd ("lib/libnetlink: update rtnl_talk to support malloc buff at run time")
> Signed-off-by: Mahesh Bandewar <maheshb@google.com>

Yes, this is a real problem.

Maybe a minimal patch like this would be enough:
diff --git a/lib/libnetlink.c b/lib/libnetlink.c
index 928de1dd16d8..ab2d8452e4a1 100644
--- a/lib/libnetlink.c
+++ b/lib/libnetlink.c
@@ -661,6 +661,8 @@ next:
 				if (l < sizeof(struct nlmsgerr)) {
 					fprintf(stderr, "ERROR truncated\n");
 				} else if (!err->error) {
+					__u32 err_seq = h->nlmsg_seq;
+
 					/* check messages from kernel */
 					nl_dump_ext_ack(h, errfn);
 
@@ -668,7 +670,8 @@ next:
 						*answer = (struct nlmsghdr *)buf;
 					else
 						free(buf);
-					if (h->nlmsg_seq == seq)
+
+					if (err_seq == seq)
 						return 0;
 					else if (i < iovlen)
 						goto next;