Jeff Xu wrote on Tue, Aug 15, 2023 at 10:13:18PM -0700:

quoted

Given that it is possible for CAP_SYS_ADMIN users to create executable
binaries without memfd_create(2) and without touching the host
filesystem (not to mention the many other things a CAP_SYS_ADMIN process
would be able to do that would be equivalent or worse), it seems strange
to cause a fair amount of headache to admins when there doesn't appear
to be an actual security benefit to blocking this. There appear to be
concerns about confused-deputy-esque attacks[2] but a confused deputy that
can write to arbitrary sysctls is a bigger security issue than
executable memfds.

Something to point out: The demo code might be enough to prove your
case in other distributions, however, in ChromeOS, you can't run this
code. The executable in ChromeOS are all from known sources and
verified at boot.
If an attacker could run this code in ChromeOS, that means the
attacker already acquired arbitrary code execution through other ways,
at that point, the attacker no longer needs to create/find an
executable memfd, they already have the vehicle. You can't use an
example of an attacker already running arbitrary code to prove that
disable downgrading is useless.
I agree it is a big problem that an attacker already can modify a
sysctl.  Assuming this can happen by controlling arguments passed into
sysctl, at the time, the attacker might not have full arbitrary code
execution yet, that is the reason the original design is so
restrictive.

I don't understand how you can say an attacker cannot run arbitrary code
within a process here, yet assert that they'd somehow run memfd_create +
execveat on it if this sysctl is lowered -- the two look equivalent to
me?

CAP_SYS_ADMIN is a kludge of a capability that pretty much gives root as
soon as you can run arbitrary code (just have a look at the various
container escape example when the capability is given); I see little
point in trying to harden just this here.
It'd make more sense to limit all sysctl modifications in the context
you're thinking of through e.g. selinux or another LSM.

(in the context of users making their own containers, my suggestion is
always to never use CAP_SYS_ADMIN, or if they must give it to a separate
minimal container where they can limit user interaction)


FWIW, I also think the proposed =2 behaviour makes more sense, but this
is something we already discussed last month so I won't come back to it
as not really involved here.

-- 
Dominique Martinet | Asmadeus