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

[Munehisa Kamata <kamatam@amazon.com>]

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@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@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
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