leaking path in android binder: set_nice

[Stephen Smalley]

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.