Re: [RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and SHOULD_EXEC_RESTRICT securebits

Mickaël Salaün
2024-07-18T16:21:01Z

On Thu, Jul 18, 2024 at 04:16:45PM +0200, Roberto Sassu wrote:
> On Sat, 2024-07-06 at 16:56 +0200, Mickaël Salaün wrote:
> > On Fri, Jul 05, 2024 at 02:44:03PM -0700, Kees Cook wrote:
> > > On Fri, Jul 05, 2024 at 07:54:16PM +0200, Mickaël Salaün wrote:
> > > > On Thu, Jul 04, 2024 at 05:18:04PM -0700, Kees Cook wrote:
> > > > > On Thu, Jul 04, 2024 at 09:01:34PM +0200, Mickaël Salaün wrote:
> > > > > > Such a secure environment can be achieved with an appropriate access
> > > > > > control policy (e.g. mount's noexec option, file access rights, LSM
> > > > > > configuration) and an enlighten ld.so checking that libraries are
> > > > > > allowed for execution e.g., to protect against illegitimate use of
> > > > > > LD_PRELOAD.
> > > > > >
> > > > > > Scripts may need some changes to deal with untrusted data (e.g. stdin,
> > > > > > environment variables), but that is outside the scope of the kernel.
> > > > >
> > > > > If the threat model includes an attacker sitting at a shell prompt, we
> > > > > need to be very careful about how process perform enforcement. E.g. even
> > > > > on a locked down system, if an attacker has access to LD_PRELOAD or a
> > > >
> > > > LD_PRELOAD should be OK once ld.so will be patched to check the
> > > > libraries. We can still imagine a debug library used to bypass security
> > > > checks, but in this case the issue would be that this library is
> > > > executable in the first place.
> > >
> > > Ah yes, that's fair: the shell would discover the malicious library
> > > while using AT_CHECK during resolution of the LD_PRELOAD.
> >
> > That's the idea, but it would be checked by ld.so, not the shell.
> >
> > >
> > > > > seccomp wrapper (which you both mention here), it would be possible to
> > > > > run commands where the resulting process is tricked into thinking it
> > > > > doesn't have the bits set.
> > > >
> > > > As explained in the UAPI comments, all parent processes need to be
> > > > trusted. This meeans that their code is trusted, their seccomp filters
> > > > are trusted, and that they are patched, if needed, to check file
> > > > executability.
> > >
> > > But we have launchers that apply arbitrary seccomp policy, e.g. minijail
> > > on Chrome OS, or even systemd on regular distros. In theory, this should
> > > be handled via other ACLs.
> >
> > Processes running with untrusted seccomp filter should be considered
> > untrusted. It would then make sense for these seccomp filters/programs
> > to be considered executable code, and then for minijail and systemd to
> > check them with AT_CHECK (according to the securebits policy).
> >
> > >
> > > > > But this would be exactly true for calling execveat(): LD_PRELOAD or
> > > > > seccomp policy could have it just return 0.
> > > >
> > > > If an attacker is allowed/able to load an arbitrary seccomp filter on a
> > > > process, we cannot trust this process.
> > > >
> > > > >
> > > > > While I like AT_CHECK, I do wonder if it's better to do the checks via
> > > > > open(), as was originally designed with O_MAYEXEC. Because then
> > > > > enforcement is gated by the kernel -- the process does not get a file
> > > > > descriptor _at all_, no matter what LD_PRELOAD or seccomp tricks it into
> > > > > doing.
> > > >
> > > > Being able to check a path name or a file descriptor (with the same
> > > > syscall) is more flexible and cover more use cases.
> > >
> > > If flexibility costs us reliability, I think that flexibility is not
> > > a benefit.
> >
> > Well, it's a matter of letting user space do what they think is best,
> > and I think there are legitimate and safe uses of path names, even if I
> > agree that this should not be used in most use cases. Would we want
> > faccessat2(2) to only take file descriptor as argument and not file
> > path? I don't think so but I'd defer to the VFS maintainers.
> >
> > Christian, Al, Linus?
> >
> > Steve, could you share a use case with file paths?
> >
> > >
> > > > The execveat(2)
> > > > interface, including current and future flags, is dedicated to file
> > > > execution. I then think that using execveat(2) for this kind of check
> > > > makes more sense, and will easily evolve with this syscall.
> > >
> > > Yeah, I do recognize that is feels much more natural, but I remain
> > > unhappy about how difficult it will become to audit a system for safety
> > > when the check is strictly per-process opt-in, and not enforced by the
> > > kernel for a given process tree. But, I think this may have always been
> > > a fiction in my mind. :)
> >
> > Hmm, I'm not sure to follow. Securebits are inherited, so process tree.
> > And we need the parent processes to be trusted anyway.
> >
> > >
> > > > > And this thinking also applies to faccessat() too: if a process can be
> > > > > tricked into thinking the access check passed, it'll happily interpret
> > > > > whatever. :( But not being able to open the fd _at all_ when O_MAYEXEC
> > > > > is being checked seems substantially safer to me...
> > > >
> > > > If attackers can filter execveat(2), they can also filter open(2) and
> > > > any other syscalls. In all cases, that would mean an issue in the
> > > > security policy.
> > >
> > > Hm, as in, make a separate call to open(2) without O_MAYEXEC, and pass
> > > that fd back to the filtered open(2) that did have O_MAYEXEC. Yes, true.
> > >
> > > I guess it does become morally equivalent.
> > >
> > > Okay. Well, let me ask about usability. Right now, a process will need
> > > to do:
> > >
> > > - should I use AT_CHECK? (check secbit)
> > > - if yes: perform execveat(AT_CHECK)
> > >
> > > Why not leave the secbit test up to the kernel, and then the program can
> > > just unconditionally call execveat(AT_CHECK)?
> >
> > That was kind of the approach of the previous patch series and Linus
> > wanted the new interface to follow the kernel semantic. Enforcing this
> > kind of restriction will always be the duty of user space anyway, so I
> > think it's simpler (i.e. no mix of policy definition, access check, and
> > policy enforcement, but a standalone execveat feature), more flexible,
> > and it fully delegates the policy enforcement to user space instead of
> > trying to enforce some part in the kernel which would only give the
> > illusion of security/policy enforcement.
>
> A problem could be that from IMA perspective there is no indication on
> whether the interpreter executed or not execveat(). Sure, we can detect
> that the binary supports it, but if the enforcement was
> enabled/disabled that it is not recorded.

We should assume that if the interpreter call execveat+AT_CHECK, it will
enforce restrictions according to its securebits.

>
> Maybe, setting the process flags should be influenced by the kernel,
> for example not allowing changes and enforcing when there is an IMA
> policy loaded requiring to measure/appraise scripts.

LSMs can set the required securebits per task/interpreter according to
their policies.

>
> Roberto
>
> > >
> > > Though perhaps the issue here is that an execveat() EINVAL doesn't
> > > tell the program if AT_CHECK is unimplemented or if something else
> > > went wrong, and the secbit prctl() will give the correct signal about
> > > AT_CHECK availability?
> >
> > This kind of check could indeed help to identify the issue.
>
>