Linux Ethernet Bridging - Nov 2015 - [Bridge] user controllable usermodehelper in br_stp_if.c

Hi!

By spawning new network and user namesapces an unprivileged user
is able to execute /sbin/bridge-stp within the initial mount namespace
with global root rights.
While this cannot directly be used to break out of a container or gain
global root rights it could be used by exploit writers as valuable building
block.

e.g.
$ unshare -U -r -n /bin/sh
$ brctl addbr br0
$ brctl stp br0 on # this will execute /sbin/bridge-stp

As this mechanism clearly cannot work with containers and seems to be legacy
code
I suggest not calling call_usermodehelper() at all if we're not in the
initial user namespace.
What do you think?

Thanks,
//richard

On Sun, Nov 29, 2015 at 2:43 PM, Richard Weinberger <richard at nod.at>
wrote:
> Hi!
>
> By spawning new network and user namesapces an unprivileged user
> is able to execute /sbin/bridge-stp within the initial mount namespace
> with global root rights.
> While this cannot directly be used to break out of a container or gain
> global root rights it could be used by exploit writers as valuable building
block.
>
> e.g.
> $ unshare -U -r -n /bin/sh
> $ brctl addbr br0
> $ brctl stp br0 on # this will execute /sbin/bridge-stp
>
> As this mechanism clearly cannot work with containers and seems to be
legacy code
> I suggest not calling call_usermodehelper() at all if we're not in the
initial user namespace.
> What do you think?
I'm not familiar with how bridge-stp is expected to operate with a
network namespace, but if it's meaningless, then yeah, that seems like
a reasonable change. Can you send a patch? (Also, if it's legacy code,
maybe it could be turned off entirely, not just for containers?)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security

There is no defined mechanism to pass network namespace information
into /sbin/bridge-stp therefore don't even try to invoke it except
for bridge devices in the initial network namespace.

It is possible for unprivileged users to cause /sbin/bridge-stp to be
invoked for any network device name which if /sbin/bridge-stp does not
guard against unreasonable arguments or being invoked twice on the same
network device could cause problems.

Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
---
 net/bridge/br_stp_if.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/bridge/br_stp_if.c b/net/bridge/br_stp_if.c
index 5396ff08af32..742fa89528ab 100644
--- a/net/bridge/br_stp_if.c
+++ b/net/bridge/br_stp_if.c
@@ -142,7 +142,9 @@ static void br_stp_start(struct net_bridge *br)
 	char *envp[] = { NULL };
 	struct net_bridge_port *p;
 
-	r = call_usermodehelper(BR_STP_PROG, argv, envp, UMH_WAIT_PROC);
+	r = -ENOENT;
+	if (dev_net(br->dev) == &init_net)
+		r = call_usermodehelper(BR_STP_PROG, argv, envp, UMH_WAIT_PROC);
 
 	spin_lock_bh(&br->lock);
 
-- 
2.2.1

Am 30.11.2015 um 21:14 schrieb Kees Cook:
> On Sun, Nov 29, 2015 at 2:43 PM, Richard Weinberger <richard at
nod.at> wrote:
>> Hi!
>>
>> By spawning new network and user namesapces an unprivileged user
>> is able to execute /sbin/bridge-stp within the initial mount namespace
>> with global root rights.
>> While this cannot directly be used to break out of a container or gain
>> global root rights it could be used by exploit writers as valuable
building block.
>>
>> e.g.
>> $ unshare -U -r -n /bin/sh
>> $ brctl addbr br0
>> $ brctl stp br0 on # this will execute /sbin/bridge-stp
>>
>> As this mechanism clearly cannot work with containers and seems to be
legacy code
>> I suggest not calling call_usermodehelper() at all if we're not in
the initial user namespace.
>> What do you think?
> 
> I'm not familiar with how bridge-stp is expected to operate with a
> network namespace, but if it's meaningless, then yeah, that seems like
> a reasonable change. Can you send a patch? (Also, if it's legacy code,
> maybe it could be turned off entirely, not just for containers?)
Eric was faster than me. :-)

BTW: kernel.core_pattern is also worth a look.
If the pipe mode is used, "|/bin/core_tool", it will be executed in
the
initial namespace and any user/container can trigger it.
Shayan reported that some weeks ago: https://lkml.org/lkml/2015/10/24/134

Thanks,
//richard