linux-kernel - Re: leaking path in android binder: set

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Date:   Tue, 25 Sep 2018 22:58:08 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Stephen Smalley <sds@...ho.nsa.gov>
Cc:     Tong Zhang <ztong@...edu>, gregkh@...uxfoundation.org,
        arve@...roid.com, tkjos@...roid.com, maco@...roid.com,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Wenbo Shen <shenwenbosmile@...il.com>
Subject: Re: leaking path in android binder: set_nice

On Tue, Sep 25, 2018 at 01:52:57PM -0400, Stephen Smalley wrote:
> On 09/25/2018 01:27 PM, Tong Zhang wrote:
> > Kernel Version: 4.18.5
> > 
> > Problem Description:
> > 
> > When setting nice value, it is checked by LSM function security_task_setnice().
> > see kernel/sched/core.c:3972 SYSCALL_DEFINE1(nice, int, increment)
> > 
> > We discovered a leaking path in android binder which allows using binder’s interface to change
> > a process’s nice value. This path is leaked from being monitored by LSM.
> > see drivers/android/binder.c:1107 binder_set_nice.
> 
> Not sure you want to invoke the LSM hook (or at least the same hook) when
> binder is performing priority inheritance.  There is a difference between a
> userspace process switching its own priority and the kernel binder driver
> performing it.  IIUC, the can_nice() check is more about honoring
> RLIMIT_NICE than anything else.

I agree with Stephen; it doesn't make sense to subject the binder PI
mechanism to the LSM hook.

					- Ted