2018-05-30 19:18:49

by Kees Cook

Subject: [PATCH v2] netfilter: nfnetlink: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this
allocates the maximum size expected for all possible attrs and adds
sanity-checks at both registration and usage to make sure nothing
gets out of sync.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <[email protected]>
---
v2: Use a static value and add a check in nfnetlink_subsys_register()
---
net/netfilter/nfnetlink.c | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nfnetlink.c b/net/netfilter/nfnetlink.c
index 03ead8a9e90c..1a80fdcf008f 100644
--- a/net/netfilter/nfnetlink.c
+++ b/net/netfilter/nfnetlink.c
@@ -37,6 +37,8 @@ MODULE_ALIAS_NET_PF_PROTO(PF_NETLINK, NETLINK_NETFILTER);
rcu_dereference_protected(table[(id)].subsys, \
lockdep_nfnl_is_held((id)))

+#define NFNL_MAX_ATTR_COUNT 32
+
static struct {
struct mutex mutex;
const struct nfnetlink_subsystem __rcu *subsys;
@@ -76,6 +78,13 @@ EXPORT_SYMBOL_GPL(lockdep_nfnl_is_held);

int nfnetlink_subsys_register(const struct nfnetlink_subsystem *n)
{
+ u8 cb_id;
+
+ /* Sanity-check attr_count size to avoid stack buffer overflow. */
+ for (cb_id = 0; cb_id < n->cb_count; cb_id++)
+ if (WARN_ON(n->cb[cb_id].attr_count > NFNL_MAX_ATTR_COUNT))
+ return -EINVAL;
+
nfnl_lock(n->subsys_id);
if (table[n->subsys_id].subsys) {
nfnl_unlock(n->subsys_id);
@@ -185,11 +194,17 @@ static int nfnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
{
int min_len = nlmsg_total_size(sizeof(struct nfgenmsg));
u8 cb_id = NFNL_MSG_TYPE(nlh->nlmsg_type);
- struct nlattr *cda[ss->cb[cb_id].attr_count + 1];
+ struct nlattr *cda[NFNL_MAX_ATTR_COUNT + 1];
struct nlattr *attr = (void *)nlh + min_len;
int attrlen = nlh->nlmsg_len - min_len;
__u8 subsys_id = NFNL_SUBSYS_ID(type);

+ /* Sanity-check NFNL_MAX_ATTR_COUNT */
+ if (ss->cb[cb_id].attr_count > NFNL_MAX_ATTR_COUNT) {
+ rcu_read_unlock();
+ return -ENOMEM;
+ }
+
err = nla_parse(cda, ss->cb[cb_id].attr_count, attr, attrlen,
ss->cb[cb_id].policy, extack);
if (err < 0) {
@@ -379,10 +394,16 @@ static void nfnetlink_rcv_batch(struct sk_buff *skb, struct nlmsghdr *nlh,
{
int min_len = nlmsg_total_size(sizeof(struct nfgenmsg));
u8 cb_id = NFNL_MSG_TYPE(nlh->nlmsg_type);
- struct nlattr *cda[ss->cb[cb_id].attr_count + 1];
+ struct nlattr *cda[NFNL_MAX_ATTR_COUNT + 1];
struct nlattr *attr = (void *)nlh + min_len;
int attrlen = nlh->nlmsg_len - min_len;

+ /* Sanity-check NFTA_MAX_ATTR */
+ if (ss->cb[cb_id].attr_count > NFNL_MAX_ATTR_COUNT) {
+ err = -ENOMEM;
+ goto ack;
+ }
+
err = nla_parse(cda, ss->cb[cb_id].attr_count, attr,
attrlen, ss->cb[cb_id].policy, NULL);
if (err < 0)
--
2.17.0


--
Kees Cook
Pixel Security

2018-06-01 07:51:24

by Pablo Neira Ayuso

Subject: Re: [PATCH v2] netfilter: nfnetlink: Remove VLA usage

On Wed, May 30, 2018 at 12:17:56PM -0700, Kees Cook wrote:
> In the quest to remove all stack VLA usage from the kernel[1], this
> allocates the maximum size expected for all possible attrs and adds
> sanity-checks at both registration and usage to make sure nothing
> gets out of sync.
>
> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Applied, thanks Kees.