libbpf: Fix alen calculation in libbpf_nla_dump_errormsg()

The code assumes that everything that comes after nlmsgerr are nlattrs.
When calculating their size, it does not account for the initial
nlmsghdr. This may lead to accessing uninitialized memory.

Fixes: bbf48c18ee0c ("libbpf: add error reporting in XDP")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230210001210.395194-8-iii@linux.ibm.com

diff --git a/tools/lib/bpf/nlattr.c b/tools/lib/bpf/nlattr.c
index 3900d05..975e265 100644 (file)
--- a/tools/lib/bpf/nlattr.c
+++ b/tools/lib/bpf/nlattr.c
@@ -178,7 +178,7 @@ int libbpf_nla_dump_errormsg(struct nlmsghdr *nlh)
                hlen += nlmsg_len(&err->msg);
 
        attr = (struct nlattr *) ((void *) err + hlen);
-       alen = nlh->nlmsg_len - hlen;
+       alen = (void *)nlh + nlh->nlmsg_len - (void *)attr;
 
        if (libbpf_nla_parse(tb, NLMSGERR_ATTR_MAX, attr, alen,
                             extack_policy) != 0) {