leaking path in android binder: set_nice

[Theodore Y. Ts'o]

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