linux-kernel - Re: [PATCHv2 0/2] introduce get_task_exe_file and use it to fix audit_exe

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Date:   Tue, 30 Aug 2016 22:13:28 +0200
From:   Mateusz Guzik <mguzik@...hat.com>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        ebiederm@...ssion.com, oleg@...hat.com, sgrubb@...hat.com,
        pmoore@...hat.com, eparis@...hat.com, luto@...capital.net,
        linux-audit@...hat.com, linux-kernel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCHv2 0/2] introduce get_task_exe_file and use it to fix
 audit_exe_compare

On Tue, Aug 30, 2016 at 02:50:21PM -0400, Richard Guy Briggs wrote:
> On 2016-08-23 16:20, Mateusz Guzik wrote:
> > audit_exe_compare directly accesses mm->exe_file without making sure the
> > object is stable. Fixing it using current primitives results in
> > partially duplicating what proc_exe_link is doing.
> > 
> > As such, introduce a trivial helper which can be used in both places and
> > fix the func.
> > 
> > Changes since v1:
> > * removed an unused 'out' label which crept in
> > 
> > Mateusz Guzik (2):
> >   mm: introduce get_task_exe_file
> >   audit: fix exe_file access in audit_exe_compare
> 
> The task_lock affects a much bigger struct than the mm ref count.  Is
> this really necessary?  Is a spin-lock significantly lower cost than a
> refcount?  Other than that, this refactorization looks sensible.
> 

proc_exe_link was taking the lock anyway to guarantee a stable mm.
I think the helper cleans the code up a little bit and there is
microoptimisation to not play with the refcount.

If audit_exe_compare has guarantees the task wont reach exit_mm, it can
use get_mm_exe_file which means the atomic op would be only on the file
object.

I was under the impression this is the expected behaviour, but your
patch used the task lock to grab mm, so I mimicked it here.

-- 
Mateusz Guzik