Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

Jeff Xu
2024-07-18T01:51:51Z

On Wed, Jul 17, 2024 at 3:00 AM Mickaël Salaün <mic@digikod.net> wrote:
>
> On Wed, Jul 17, 2024 at 09:26:22AM +0100, Steve Dower wrote:
> > On 17/07/2024 07:33, Jeff Xu wrote:
> > > Consider those cases: I think:
> > > a> relying purely on userspace for enforcement does't seem to be
> > > effective, e.g. it is trivial to call open(), then mmap() it into
> > > executable memory.
> >
> > If there's a way to do this without running executable code that had to pass
> > a previous execveat() check, then yeah, it's not effective (e.g. a Python
> > interpreter that *doesn't* enforce execveat() is a trivial way to do it).
> >
> > Once arbitrary code is running, all bets are off. So long as all arbitrary
> > code is being checked itself, it's allowed to do things that would bypass
> > later checks (and it's up to whoever audited it in the first place to
> > prevent this by not giving it the special mark that allows it to pass the
> > check).
>
We will want to define what is considered as "arbitrary code is running"

Using an example of ROP, attackers change the return address in stack,
e.g. direct the execution flow to a gauge to call "ld.so /tmp/a.out",
do you consider "arbitrary code is running" when stack is overwritten
? or after execve() is called.
If it is later, this patch can prevent "ld.so /tmp/a.out".

> Exactly. As explained in the patches, one crucial prerequisite is that
> the executable code is trusted, and the system must provide integrity
> guarantees. We cannot do anything without that. This patches series is
> a building block to fix a blind spot on Linux systems to be able to
> fully control executability.

Even trusted executable can have a bug.

I'm thinking in the context of ChromeOS, where all its system services
are from trusted partitions, and legit code won't load .so from a
non-exec mount. But we want to sandbox those services, so even under
some kind of ROP attack, the service still won't be able to load .so
from /tmp. Of course, if an attacker can already write arbitrary
length of data into the stack, it is probably already a game over.