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

[Kees Cook]

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
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.

But this would be exactly true for calling execveat(): LD_PRELOAD or
seccomp policy could have it just return 0.

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.

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...

-- 
Kees Cook