Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

Mickaël Salaün
2024-07-23T13:16:47Z

On Sat, Jul 20, 2024 at 02:43:41PM +0300, Jarkko Sakkinen wrote:
> On Sat Jul 20, 2024 at 4:59 AM EEST, Andy Lutomirski wrote:
> > > On Jul 18, 2024, at 8:22 PM, Mickaël Salaün <mic@digikod.net> wrote:
> > >
> > > ﻿On Thu, Jul 18, 2024 at 09:02:56AM +0800, Andy Lutomirski wrote:
> > >>>> On Jul 17, 2024, at 6:01 PM, Mickaël Salaün <mic@digikod.net> wrote:
> > >>>
> > >>> On Wed, Jul 17, 2024 at 09:26:22AM +0100, Steve Dower wrote:
> > >>>>> On 17/07/2024 07:33, Jeff Xu wrote:
> > >>>>> Consider those cases: I think:
> > >>>>> a> relying purely on userspace for enforcement does't seem to be
> > >>>>> effective, e.g. it is trivial to call open(), then mmap() it into
> > >>>>> executable memory.
> > >>>>
> > >>>> If there's a way to do this without running executable code that had to pass
> > >>>> a previous execveat() check, then yeah, it's not effective (e.g. a Python
> > >>>> interpreter that *doesn't* enforce execveat() is a trivial way to do it).
> > >>>>
> > >>>> Once arbitrary code is running, all bets are off. So long as all arbitrary
> > >>>> code is being checked itself, it's allowed to do things that would bypass
> > >>>> later checks (and it's up to whoever audited it in the first place to
> > >>>> prevent this by not giving it the special mark that allows it to pass the
> > >>>> check).
> > >>>
> > >>> Exactly. As explained in the patches, one crucial prerequisite is that
> > >>> the executable code is trusted, and the system must provide integrity
> > >>> guarantees. We cannot do anything without that. This patches series is
> > >>> a building block to fix a blind spot on Linux systems to be able to
> > >>> fully control executability.
> > >>
> > >> Circling back to my previous comment (did that ever get noticed?), I
> > >
> > > Yes, I replied to your comments. Did I miss something?
> >
> > I missed that email in the pile, sorry. I’ll reply separately.
> >
> > >
> > >> don’t think this is quite right:
> > >>
> > >> https://lore.kernel.org/all/CALCETrWYu=PYJSgyJ-vaa+3BGAry8Jo8xErZLiGR3U5h6+U0tA@mail.gmail.com/
> > >>
> > >> On a basic system configuration, a given path either may or may not be
> > >> executed. And maybe that path has some integrity check (dm-verity,
> > >> etc). So the kernel should tell the interpreter/loader whether the
> > >> target may be executed. All fine.
> > >>
> > >> But I think the more complex cases are more interesting, and the
> > >> “execute a program” process IS NOT BINARY. An attempt to execute can
> > >> be rejected outright, or it can be allowed *with a change to creds or
> > >> security context*. It would be entirely reasonable to have a policy
> > >> that allows execution of non-integrity-checked files but in a very
> > >> locked down context only.
> > >
> > > I guess you mean to transition to a sandbox when executing an untrusted
> > > file. This is a good idea. I talked about role transition in the
> > > patch's description:
> > >
> > > With the information that a script interpreter is about to interpret a
> > > script, an LSM security policy can adjust caller's access rights or log
> > > execution request as for native script execution (e.g. role transition).
> > > This is possible thanks to the call to security_bprm_creds_for_exec().
> >
> > …
> >
> > > This patch series brings the minimal building blocks to have a
> > > consistent execution environment. Role transitions for script execution
> > > are left to LSMs. For instance, we could extend Landlock to
> > > automatically sandbox untrusted scripts.
> >
> > I’m not really convinced. There’s more to building an API that
> > enables LSM hooks than merely sticking the hook somewhere in kernel
> > code. It needs to be a defined API. If you call an operation “check”,
> > then people will expect it to check, not to change the caller’s
> > credentials. And people will mess it up in both directions (e.g.
> > callers will call it and then open try to load some library that they
> > should have loaded first, or callers will call it and forget to close
> > fds first.
> >
> > And there should probably be some interaction with dumpable as well.
> > If I “check” a file for executability, that should not suddenly allow
> > someone to ptrace me?
> >
> > And callers need to know to exit on failure, not carry on.
> >
> >
> > More concretely, a runtime that fully opts in to this may well "check"
> > multiple things. For example, if I do:
> >
> > $ ld.so ~/.local/bin/some_program (i.e. I literally execve ld.so)
> >
> > then ld.so will load several things:
> >
> > ~/.local/bin/some_program
> > libc.so
> > other random DSOs, some of which may well be in my home directory
>
> What would really help to comprehend this patch set would be a set of
> test scripts, preferably something that you can run easily with
> BuildRoot or similar.
>
> Scripts would demonstrate the use cases for the patch set. Then it
> would be easier to develop scripts that would underline the corner
> cases. I would keep all this out of kselftest shenanigans for now.

I'll include a toy script interpreter with the next patch series. This
one was an RFC.

>
> I feel that the patch set is hovering in abstractions with examples
> that you cannot execute.
>
> I added the patches to standard test CI hack:
>
> https://codeberg.org/jarkko/linux-tpmdd-test
>
> But after I booted up a kernel I had no idea what to do with it. And
> all this lenghty discussion makes it even more confusing.

You can run the tests in the CI.

>
> Please find some connection to the real world before sending any new
> version of this (e.g. via test scripts). I think this should not be
> pulled before almost anyone doing kernel dev can comprehend the "gist"
> at least in some reasonable level.

You'll find in this patch series (cover letter, patch description, and
comments) connection to the real world. :)
The next patch series should take into account the current discussions.

>
> BR, Jarkko