[kernel-hardening] Re: How to temporary change 'current' (task)

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-06-17 at 12:36 +0400, Vasiliy Kulikov wrote:
> Hi,
> 
> I wonder whether there is a simple way to temporary switch 'current' to
> another task and then switch it back with minimum side effects? 

No.

>  I need
> it to call "reversed" ptrace_may_access() with swapped current and
> target task.  Introducing ptrace_task_may_access_me() would produce too
> much noise in LSM (it also needs reversed security_ptrace_access_check()),
> which is too loud for my needs.
> 
> Specifically, I need it to filter taskstats and proc connector requests
> for a restriction of getting other processes' information:
> 
> http://permalink.gmane.org/gmane.linux.kernel/1155354
> 
> As the check is handled in the context of the ptrace target process,
> ptrace_may_access() doesn't fit my needs.

looking at __ptrace_may_access() it doesn't look too hard to make it
take two task arguments and use __task_cred() twice instead of
current_cred().

It of course needs extending security_ptrace_access_check() as well, but
that comes with the territory.