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Linux ip6gre_err() invalid reads

CVE	Category	Price	Severity
CVE-2019-16231	CWE-119	$5,000	High

Author	Risk	Exploitation Type	Date
Qualys Security Advisory	High	Remote	2017-02-06

CPE
cpe:cpe:2.3:a:linux:linux_kernel:6.0:::::::*

CVSS	EPSS	EPSSP
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L	0.02192	0.50148

CVSS vector description

Metric	Value	Metric Description	Value Description
Attack vector	Local	AV	The vulnerable system is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or through terminal emulation (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., using social engineering techniques to trick a legitimate user into opening a malicious document).
Attack Complexity	Low	AC	The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Privileges Required	None	PR	The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction	None	UI	The vulnerable system can be exploited without interaction from any human user, other than the attacker. Examples include: a remote attacker is able to send packets to a target system a locally authenticated attacker executes code to elevate privileges
Scope	Unchanged	S	An exploited vulnerability can only affect resources managed by the same security authority. In the case of a vulnerability in a virtualized environment, an exploited vulnerability in one guest instance would not affect neighboring guest instances.
Confidentiality	Low	C	There is some impact on confidentiality, but the attacker either does not gain control of any data, or the information obtained does not have a significant impact on the system or its operations.
Integrity	Low	I	Modification of data is possible, but the attacker does not have control over what can be modified, or the extent of what the attacker can affect is limited. The data modified does not have a direct, serious impact on the system.
Availability	Low	A	There is reduced performance or interruptions in resource availability. However, the attacker does not have the ability to completely prevent access to the resources or services; the impact is limited.

Our sensors found this exploit at: https://cxsecurity.com/ascii/WLB-2017020050

Below is a copy:

Linux ip6gre_err() invalid readsAndrey Konovalov reported out of bound accesses in ip6gre_err()

If GRE flags contains GRE_KEY, the following expression
*(((__be32 *)p) + (grehlen / 4) - 1)

accesses data ~40 bytes after the expected point, since grehlen includes the size of IPv6 headers.

Let's use a "struct gre_base_hdr *greh" pointer to make this code more readable.

p[1] becomes greh->protocol.
grhlen is the GRE header length.

+++ b/net/ipv6/ip6_gre.c
@@ -367,35 +367,37 @@ static void ip6gre_tunnel_uninit(struct net_device *dev)
 
 
 static void ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
-u8 type, u8 code, int offset, __be32 info)
+       u8 type, u8 code, int offset, __be32 info)
 {
-const struct ipv6hdr *ipv6h = (const struct ipv6hdr *)skb->data;
-__be16 *p = (__be16 *)(skb->data + offset);
-int grehlen = offset + 4;
+const struct gre_base_hdr *greh;
+const struct ipv6hdr *ipv6h;
+int grehlen = sizeof(*greh);
 struct ip6_tnl *t;
+int key_off = 0;
 __be16 flags;
+__be32 key;
 
-flags = p[0];
-if (flags&(GRE_CSUM|GRE_KEY|GRE_SEQ|GRE_ROUTING|GRE_VERSION)) {
-if (flags&(GRE_VERSION|GRE_ROUTING))
-return;
-if (flags&GRE_KEY) {
-grehlen += 4;
-if (flags&GRE_CSUM)
-grehlen += 4;
-}
+if (!pskb_may_pull(skb, offset + grehlen))
+return;
+greh = (const struct gre_base_hdr *)(skb->data + offset);
+flags = greh->flags;
+if (flags & (GRE_VERSION | GRE_ROUTING))
+return;
+if (flags & GRE_CSUM)
+grehlen += 4;
+if (flags & GRE_KEY) {
+key_off = grehlen + offset;
+grehlen += 4;
 }
 
-/* If only 8 bytes returned, keyed message will be dropped here */
-if (!pskb_may_pull(skb, grehlen))
+if (!pskb_may_pull(skb, offset + grehlen))
 return;
 ipv6h = (const struct ipv6hdr *)skb->data;
-p = (__be16 *)(skb->data + offset);
+greh = (const struct gre_base_hdr *)(skb->data + offset);
+key = key_off ? *(__be32 *)(skb->data + key_off) : 0;
 
 t = ip6gre_tunnel_lookup(skb->dev, &ipv6h->daddr, &ipv6h->saddr,
-flags & GRE_KEY ?
-*(((__be32 *)p) + (grehlen / 4) - 1) : 0,
-p[1]);
+ key, greh->protocol);
 if (!t)
 return;

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