Vulnerability: The XFRM dump policy implementation in net/xfrm/xfrm_user.c in the Linux kernel before 4.13.11 allows local users to gain privileges or cause a denial of service (use-after-free) via a crafted SO_RCVBUF setsockopt system call in conjunction with XFRM_MSG_GETPOLICY Netlink messages.
Vulnerability ID: CVE-2017-16939
CWE: CWE-416
Severity score: 10
Project: https://github.com/torvalds/linux
Published: 2017-11-24
File: net/xfrm/xfrm_user.c
Commit message: ipsec: Fix aborted xfrm policy dump crash

An independent security researcher, Mohamed Ghannam, has reported
this vulnerability to Beyond Security's SecuriTeam Secure Disclosure
program.

The xfrm_dump_policy_done function expects xfrm_dump_policy to
have been called at least once or it will crash.  This can be
triggered if a dump fails because the target socket's receive
buffer is full.

This patch fixes it by using the cb->start mechanism to ensure that
the initialisation is always done regardless of the buffer situation.

Fixes: 12a169e7d8f4 ("ipsec: Put dumpers on the dump list")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Commit author: Herbert Xu

Vulnerable version (before fix):
 static int dump_one_policy(struct xfrm_policy *xp, int dir, int count, void *ptr  static int xfrm_dump_policy_done(struct netlink_callback *cb) { 	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *) &cb->args[1]; 	struct net *net = sock_net(cb->skb->sk);  	xfrm_policy_walk_done(walk, net); 	return 0; }  static int xfrm_dump_policy(struct sk_buff *skb, struct netlink_callback *cb) { 	struct net *net = sock_net(skb->sk); 	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *) &cb->args[1]; 	struct xfrm_dump_info info;  	BUILD_BUG_ON(sizeof(struct xfrm_policy_walk) > 		     sizeof(cb->args) - sizeof(cb->args[0]));  	info.in_skb = cb->skb; 	info.out_skb = skb; 	info.nlmsg_seq = cb->nlh->nlmsg_seq; 	info.nlmsg_flags = NLM_F_MULTI;  	if (!cb->args[0]) { 		cb->args[0] = 1; 		xfrm_policy_walk_init(walk, XFRM_POLICY_TYPE_ANY); 	}  	(void) xfrm_policy_walk(net, walk, dump_one_policy, &info);  	return skb->len; static const struct nla_policy xfrma_spd_policy[XFRMA_SPD_MAX+1] = {  static const struct xfrm_link { 	int (*doit)(struct sk_buff *, struct nlmsghdr *, struct nlattr **); 	int (*dump)(struct sk_buff *, struct netlink_callback *); 	int (*done)(struct netlink_callback *); 	const struct nla_policy *nla_pol; static const struct xfrm_link { 	[XFRM_MSG_NEWPOLICY   - XFRM_MSG_BASE] = { .doit = xfrm_add_policy    }, 	[XFRM_MSG_DELPOLICY   - XFRM_MSG_BASE] = { .doit = xfrm_get_policy    }, 	[XFRM_MSG_GETPOLICY   - XFRM_MSG_BASE] = { .doit = xfrm_get_policy, 						   .dump = xfrm_dump_policy, 						   .done = xfrm_dump_policy_done }, 	[XFRM_MSG_ALLOCSPI    - XFRM_MSG_BASE] = { .doit = xfrm_alloc_userspi }, static int xfrm_user_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,  		{ 			struct netlink_dump_control c = { 				.dump = link->dump, 				.done = link->done, 			};

Fixed version (after fix):
 static int dump_one_policy(struct xfrm_policy *xp, int dir, int count, void *ptr  static int xfrm_dump_policy_done(struct netlink_callback *cb) { 	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *)cb->args; 	struct net *net = sock_net(cb->skb->sk);  	xfrm_policy_walk_done(walk, net); 	return 0; }  static int xfrm_dump_policy_start(struct netlink_callback *cb) { 	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *)cb->args;  	BUILD_BUG_ON(sizeof(*walk) > sizeof(cb->args));  	xfrm_policy_walk_init(walk, XFRM_POLICY_TYPE_ANY); 	return 0; }  static int xfrm_dump_policy(struct sk_buff *skb, struct netlink_callback *cb) { 	struct net *net = sock_net(skb->sk); 	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *)cb->args; 	struct xfrm_dump_info info;  	info.in_skb = cb->skb; 	info.out_skb = skb; 	info.nlmsg_seq = cb->nlh->nlmsg_seq; 	info.nlmsg_flags = NLM_F_MULTI;  	(void) xfrm_policy_walk(net, walk, dump_one_policy, &info);  	return skb->len; static const struct nla_policy xfrma_spd_policy[XFRMA_SPD_MAX+1] = {  static const struct xfrm_link { 	int (*doit)(struct sk_buff *, struct nlmsghdr *, struct nlattr **); 	int (*start)(struct netlink_callback *); 	int (*dump)(struct sk_buff *, struct netlink_callback *); 	int (*done)(struct netlink_callback *); 	const struct nla_policy *nla_pol; static const struct xfrm_link { 	[XFRM_MSG_NEWPOLICY   - XFRM_MSG_BASE] = { .doit = xfrm_add_policy    }, 	[XFRM_MSG_DELPOLICY   - XFRM_MSG_BASE] = { .doit = xfrm_get_policy    }, 	[XFRM_MSG_GETPOLICY   - XFRM_MSG_BASE] = { .doit = xfrm_get_policy, 						   .start = xfrm_dump_policy_start, 						   .dump = xfrm_dump_policy, 						   .done = xfrm_dump_policy_done }, 	[XFRM_MSG_ALLOCSPI    - XFRM_MSG_BASE] = { .doit = xfrm_alloc_userspi }, static int xfrm_user_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,  		{ 			struct netlink_dump_control c = { 				.start = link->start, 				.dump = link->dump, 				.done = link->done, 			};

Patch:
patch: @@ -1693,32 +1693,34 @@ static int dump_one_policy(struct xfrm_policy *xp, int dir, int count, void *ptr
 
 static int xfrm_dump_policy_done(struct netlink_callback *cb)
 {
-	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *) &cb->args[1];
+	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *)cb->args;
 	struct net *net = sock_net(cb->skb->sk);
 
 	xfrm_policy_walk_done(walk, net);
 	return 0;
 }
 
+static int xfrm_dump_policy_start(struct netlink_callback *cb)
+{
+	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *)cb->args;
+
+	BUILD_BUG_ON(sizeof(*walk) > sizeof(cb->args));
+
+	xfrm_policy_walk_init(walk, XFRM_POLICY_TYPE_ANY);
+	return 0;
+}
+
 static int xfrm_dump_policy(struct sk_buff *skb, struct netlink_callback *cb)
 {
 	struct net *net = sock_net(skb->sk);
-	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *) &cb->args[1];
+	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *)cb->args;
 	struct xfrm_dump_info info;
 
-	BUILD_BUG_ON(sizeof(struct xfrm_policy_walk) >
-		     sizeof(cb->args) - sizeof(cb->args[0]));
-
 	info.in_skb = cb->skb;
 	info.out_skb = skb;
 	info.nlmsg_seq = cb->nlh->nlmsg_seq;
 	info.nlmsg_flags = NLM_F_MULTI;
 
-	if (!cb->args[0]) {
-		cb->args[0] = 1;
-		xfrm_policy_walk_init(walk, XFRM_POLICY_TYPE_ANY);
-	}
-
 	(void) xfrm_policy_walk(net, walk, dump_one_policy, &info);
 
 	return skb->len;
@@ -2474,6 +2476,7 @@ static const struct nla_policy xfrma_spd_policy[XFRMA_SPD_MAX+1] = {
 
 static const struct xfrm_link {
 	int (*doit)(struct sk_buff *, struct nlmsghdr *, struct nlattr **);
+	int (*start)(struct netlink_callback *);
 	int (*dump)(struct sk_buff *, struct netlink_callback *);
 	int (*done)(struct netlink_callback *);
 	const struct nla_policy *nla_pol;
@@ -2487,6 +2490,7 @@ static const struct xfrm_link {
 	[XFRM_MSG_NEWPOLICY   - XFRM_MSG_BASE] = { .doit = xfrm_add_policy    },
 	[XFRM_MSG_DELPOLICY   - XFRM_MSG_BASE] = { .doit = xfrm_get_policy    },
 	[XFRM_MSG_GETPOLICY   - XFRM_MSG_BASE] = { .doit = xfrm_get_policy,
+						   .start = xfrm_dump_policy_start,
 						   .dump = xfrm_dump_policy,
 						   .done = xfrm_dump_policy_done },
 	[XFRM_MSG_ALLOCSPI    - XFRM_MSG_BASE] = { .doit = xfrm_alloc_userspi },
@@ -2539,6 +2543,7 @@ static int xfrm_user_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 
 		{
 			struct netlink_dump_control c = {
+				.start = link->start,
 				.dump = link->dump,
 				.done = link->done,
 			};
path: net/xfrm/xfrm_user.c