[PATCH v8 3/5] mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC

[Dominique Martinet]

Jeff Xu wrote on Wed, Jun 28, 2023 at 09:33:27PM -0700:
> > > BTW I find the current behaviour rather hard to use: setting this to 2
> > > should still set NOEXEC by default in my opinion, just refuse anything
> > > that explicitly requested EXEC.
> >
> > And I just noticed it's not possible to lower the value despite having
> > CAP_SYS_ADMIN: what the heck?! I have never seen such a sysctl and it
> > just forced me to reboot because I willy-nilly tested in the init pid
> > namespace, and quite a few applications that don't require exec broke
> > exactly as I described below.
> >
> > If the user has CAP_SYS_ADMIN there are more container escape methods
> > than I can count, this is basically free pass to root on main namespace
> > anyway, you're not protecting anything. Please let people set the sysctl
> > to what they want.
>
> Yama has a similar setting,  for example, 3 (YAMA_SCOPE_NO_ATTACH)
> will not allow downgrading at runtime.
> 
> Since this is a security feature, not allowing downgrading at run time
> is part of the security consideration. I hope you understand.

I didn't remember yama had this stuck bit; that still strikes me as
unusual, and if you require a custom LSM rule for memfd anyway I don't
see why it couldn't enforce that the sysctl is unchanged, but sure.

Please, though:
 - I have a hard time thinking of 1 as a security flag in general (even
if I do agree a sloppy LSM rule could require it); I would only lock 2
 - please make it clear, I don't see any entry in the sysctl
documentation[1] about memfd_noexec, there should be one and you can
copy the wording from yama's doc[2]: "Once set, this sysctl value cannot
be changed"
[1] Documentation/admin-guide/sysctl/vm.rst
[2] Documentation/admin-guide/LSM/Yama.rst


Either way as it stands I still don't think one can expect most
userspace applications to be converted until some libc wrapper takes
care of the retry logic and a couple of years, so I'll go look for
another way of filtering this (and eventually setting this to 1) as you
suggested.
I'll leave the follow-up up to you and won't bother you more.

Thanks,
-- 
Dominique Martinet | Asmadeus