How to temporary change 'current' (task)

How to temporary change 'current' (task)

[Vasiliy Kulikov]

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?  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.

Thanks,

-- 
Vasiliy

Re: How to temporary change 'current' (task)

[Peter Zijlstra]

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.

Re: How to temporary change 'current' (task)

[Vasiliy Kulikov]

On Fri, Jun 17, 2011 at 11:29 +0200, Peter Zijlstra wrote:
> On Fri, 2011-06-17 at 12:36 +0400, Vasiliy Kulikov wrote:
> > 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.

After a moment's thought, it would also break some synchronization that
relies on some structures are accesses only from the current task.

> >  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.

Well, yes, but it implies pushing explicit ptrace actor into LSM
modules.  OK, it should be not as noisy as I initially thought - almost
all current LSMs already use "current" as an explicit argument in
theirs internal functions.

Thank you,

-- 
Vasiliy