Re: KASAN: stack-out-of-bounds Read in xfrm_state_find (3)

[Steffen Klassert]

On Wed, Nov 22, 2017 at 08:05:00AM -0800, syzbot wrote:
> syzkaller has found reproducer for the following crash on
> 0c86a6bd85ff0629cd2c5141027fc1c8bb6cde9c
> git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/master
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached
> Raw console output is attached.
> C reproducer is attached
> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> for information about syzkaller reproducers
> 
> 
> BUG: KASAN: stack-out-of-bounds in xfrm_state_find+0x30fc/0x3230
> net/xfrm/xfrm_state.c:1051
> Read of size 4 at addr ffff8801ccaa7af8 by task syzkaller231684/3045

The patch below should fix this. I plan to apply it to the ipsec tree
after some advanced testing.

Subject: [PATCH RFC] xfrm: Fix stack-out-of-bounds with misconfigured transport
 mode policies.

On policies with a transport mode template, we pass the addresses
from the flowi to xfrm_state_find(), assuming that the IP addresses
(and address family) don't change during transformation.

Unfortunately our policy template validation is not strict enough.
It is possible to configure policies with transport mode template
where the address family of the template does not match the selectors
address family. This lead to stack-out-of-bound reads because
we compare arddesses of the wrong family. Fix this by refusing
such a configuration, address family can not change on transport
mode.

We use the assumption that, on transport mode, the first templates
address family must match the address family of the policy selector.
Subsequent transport mode templates must mach the address family of
the previous template.

Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Signed-off-by: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
---
 net/xfrm/xfrm_user.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index 983b0233767b..57ad016ae675 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -1419,11 +1419,14 @@ static void copy_templates(struct xfrm_policy *xp, struct xfrm_user_tmpl *ut,
 
 static int validate_tmpl(int nr, struct xfrm_user_tmpl *ut, u16 family)
 {
+	u16 prev_family;
 	int i;
 
 	if (nr > XFRM_MAX_DEPTH)
 		return -EINVAL;
 
+	prev_family = family;
+
 	for (i = 0; i < nr; i++) {
 		/* We never validated the ut->family value, so many
 		 * applications simply leave it at zero.  The check was
@@ -1435,6 +1438,12 @@ static int validate_tmpl(int nr, struct xfrm_user_tmpl *ut, u16 family)
 		if (!ut[i].family)
 			ut[i].family = family;
 
+		if ((ut[i].mode == XFRM_MODE_TRANSPORT) &&
+		    (ut[i].family != prev_family))
+			return -EINVAL;
+
+		prev_family = ut[i].family;
+
 		switch (ut[i].family) {
 		case AF_INET:
 			break;
-- 
2.14.1