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

Mickaël Salaün
2024-07-23T13:16:12Z

On Sat, Jul 20, 2024 at 09:59:33AM +0800, 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
>
> And for all ld.so knows, some_program is actually an interpreter and
> will "check" something else. And the LSMs have absolutely no clue
> what's what. So I think for this to work right, the APIs need to be a
> lot more expressive and explicit:
>
> check_library(fd to libc.so); <-- does not transition or otherwise drop privs
> check_transition_main_program(fd to ~/.local/bin/some_program); <--
> may drop privs
>
> and if some_program is really an interpreter, then it will do:
>
> check_library(fd to some thing imported by the script);
> check_transition_main_program(fd to the actual script);
>
> And maybe that takes a parameter that gets run eval-style:
>
> check_unsafe_user_script("actual contents of snippet");
>
> The actual spelling of all this doesn't matter so much. But the user
> code and the kernel code need to be on the same page as to what the
> user program is doing and what it's asking the kernel program to do.

I agree. I'll remove any references to "role transition". This kind of
feature should come with something like getpeercon/setexeccon(3).

>
> --Andy
>