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

Hello!

On Wed, Jun 28, 2023 at 12:31 PM Dominique Martinet
[off-list ref] wrote:

Dominique Martinet wrote on Wed, Jun 28, 2023 at 08:42:41PM +0900:

quoted

If flags already has either MFD_EXEC or MFD_NOEXEC_SEAL, you don't check
the sysctl at all.
[...repro snipped..]

What am I missing?

(Perhaps the intent is just to force people to use the flag so it is
easier to check for memfd_create in seccomp or other LSM?
But I don't see why such a check couldn't consider the absence of a flag
as well, so I don't see the point.)

Yes. There is consideration to motivate app devs to migrate their code
to use the new EXEC/NOEXEC_SEAL flag for memfd_create, if that answers
your question.

quoted

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.

--
Dominique Martinet | Asmadeus

Thanks!
-Jeff