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

[Jeff Xu <jeffxu@google.com>]

Hello

On Thu, Jun 29, 2023 at 3:30 AM Dominique Martinet
<asmadeus@codewreck.org> wrote:
>
> 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
>
Thanks for the suggestion.
Yes, it would be good to have some documentation.
I will send patch to update Documentation/admin-guide/sysctl/vm.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.
>
Not bothered at all! and thanks for reporting the bug to improve the
quality of memfd_noexec !

Much appreciated.
-Jeff


> Thanks,
> --
> Dominique Martinet | Asmadeus