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

[Roberto Sassu]

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.

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.

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.