Fw: [PATCH] proc: Update inode upon changing task security attribute

[Serge E. Hallyn]

On Sat, Dec 09, 2023 at 01:10:42AM +0000, Munehisa Kamata wrote:
> On Sat, 2023-12-09 00:24:42 +0000, Casey Schaufler wrote:
> >
> > On 12/8/2023 3:32 PM, Paul Moore wrote:
> > > On Fri, Dec 8, 2023 at 6:21 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> > >> On 12/8/2023 2:43 PM, Paul Moore wrote:
> > >>> On Thu, Dec 7, 2023 at 9:14 PM Munehisa Kamata <kamatam at amazon.com> wrote:
> > >>>> On Tue, 2023-12-05 14:21:51 -0800, Paul Moore wrote:
> > >>> ..
> > >>>
> > >>>>> I think my thoughts are neatly summarized by Andrew's "yuk!" comment
> > >>>>> at the top.  However, before we go too much further on this, can we
> > >>>>> get clarification that Casey was able to reproduce this on a stock
> > >>>>> upstream kernel?  Last I read in the other thread Casey wasn't seeing
> > >>>>> this problem on Linux v6.5.
> > >>>>>
> > >>>>> However, for the moment I'm going to assume this is a real problem, is
> > >>>>> there some reason why the existing pid_revalidate() code is not being
> > >>>>> called in the bind mount case?  From what I can see in the original
> > >>>>> problem report, the path walk seems to work okay when the file is
> > >>>>> accessed directly from /proc, but fails when done on the bind mount.
> > >>>>> Is there some problem with revalidating dentrys on bind mounts?
> > >>>> Hi Paul,
> > >>>>
> > >>>> https://lkml.kernel.org/linux-fsdevel/20090608201745.GO8633@ZenIV.linux.org.uk/
> > >>>>
> > >>>> After reading this thread, I have doubt about solving this in VFS.
> > >>>> Honestly, however, I'm not sure if it's entirely relevant today.
> > >>> Have you tried simply mounting proc a second time instead of using a bind mount?
> > >>>
> > >>>  % mount -t proc non /new/location/for/proc
> > >>>
> > >>> I ask because from your description it appears that proc does the
> > >>> right thing with respect to revalidation, it only becomes an issue
> > >>> when accessing proc through a bind mount.  Or did I misunderstand the
> > >>> problem?
> > >> It's not hard to make the problem go away by performing some simple
> > >> action. I was unable to reproduce the problem initially because I
> > >> checked the Smack label on the bind mounted proc entry before doing
> > >> the cat of it. The problem shows up if nothing happens to update the
> > >> inode.
> > > A good point.
> > >
> > > I'm kinda thinking we just leave things as-is, especially since the
> > > proposed fix isn't something anyone is really excited about.
> > 
> > "We have to compromise the performance of our sandboxing tool because of
> > a kernel bug that's known and for which a fix is available."
> > 
> > If this were just a curiosity that wasn't affecting real development I
> > might agree. But we've got a real world problem, and I don't see ignoring
> > it as a good approach. I can't see maintainers of other LSMs thinking so
> > if this were interfering with their users.
>  
> We do bind mount to make information exposed to the sandboxed task as little
> as possible. We also create a separate PID namespace for each sandbox, but

If not exposing information is the main motivation, then could you simply do:

mount -t proc proc dir
mount --bind dir/$$ dir

?

> still want to bind mount even with it to hide system-wide and pid 1
> information from the task. 
> 
> So, yeah, I see this as a real problem for our use case and want to seek an
> opinion about a possibly better fix.
> 
> 
> Thanks,
> Munehisa