Leaking path for set_task_comm

Leaking path for set_task_comm

[Tong Zhang]

Kernel Version: 4.18.5

Problem Description:

When using prctl(PR_SET_NAME) to set the thread name, it is checked by security_task_prctl.

We discovered a leaking path that can also use method implemented in 
fs/proc/base.c:1526 comm_write(), to do similar thing without asking LSM’s decision.

- Tong

Re: Leaking path for set_task_comm

[Cyrill Gorcunov]

On Tue, Sep 25, 2018 at 01:27:08PM -0400, Tong Zhang wrote:
> Kernel Version: 4.18.5
> 
> Problem Description:
> 
> When using prctl(PR_SET_NAME) to set the thread name, it is checked by security_task_prctl.
> 
> We discovered a leaking path that can also use method implemented in 
> fs/proc/base.c:1526 comm_write(), to do similar thing without asking LSM’s decision.

I don't understand how it is a problem. Could you please explain?
procfs/comm is created with S_IRUGO|S_IWUSR permissions. So
prctl and procfs are simply different interfaces.

Re: Leaking path for set_task_comm

[TongZhang]

Yes, this is exactly what I am saying.
A process can change its own name using prctl or /proc/self/comm.
prctl is protected by security_task_prctl, whereas /proc/self/comm is not protected by this LSM hook.

A system admin may expect to use security_task_prctl to block all attempt to change process name, however, it can still change name using /proc/self/comm.

> On Sep 25, 2018, at 2:39 PM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> 
> On Tue, Sep 25, 2018 at 01:27:08PM -0400, Tong Zhang wrote:
>> Kernel Version: 4.18.5
>> 
>> Problem Description:
>> 
>> When using prctl(PR_SET_NAME) to set the thread name, it is checked by security_task_prctl.
>> 
>> We discovered a leaking path that can also use method implemented in 
>> fs/proc/base.c:1526 comm_write(), to do similar thing without asking LSM’s decision.
> 
> I don't understand how it is a problem. Could you please explain?
> procfs/comm is created with S_IRUGO|S_IWUSR permissions. So
> prctl and procfs are simply different interfaces.

Re: Leaking path for set_task_comm

[Theodore Y. Ts'o]

On Tue, Sep 25, 2018 at 08:44:39PM -0400, TongZhang wrote:
> Yes, this is exactly what I am saying.
> A process can change its own name using prctl or /proc/self/comm.
> prctl is protected by security_task_prctl, whereas /proc/self/comm is not protected by this LSM hook.
> 
> A system admin may expect to use security_task_prctl to block all attempt to change process name, however, it can still change name using /proc/self/comm.

None of the in-tree LSM's try to affect PR_SET_NAME.  Looking at
security/commoncap.c, it's clear what is of interest is to checking
things relating to security sensitive things relating to capabilities, such as:

       PR_SET_SECUREBITS
       PR_CAPBSET_*
       PR_*_SECUREBITS
       PR_*_KEEPCAPS
       PR_CAP_AMBIENT

Trying to depend on task name for anything security sensitive is at
_really_ bad idea, so it seems unlikely that a LSM would want to
protect the process name.  (And if they did, the first thing I would
ask is "Why?  What are you trying to do?  Do you realize how many
*other* ways the process name can be spoofed or otherwise controlled
by a potentially malicious user?")

					- Ted

Re: Leaking path for set_task_comm

[Alan Cox]

> Trying to depend on task name for anything security sensitive is at
> _really_ bad idea, so it seems unlikely that a LSM would want to
> protect the process name.  (And if they did, the first thing I would
> ask is "Why?  What are you trying to do?  Do you realize how many
> *other* ways the process name can be spoofed or otherwise controlled
> by a potentially malicious user?")

Two processes that should not be able to otherwise communicate can keep
changing their name to a chunk of data, waiting for an ack flag name
change back.

Alan