Re: [PATCH 3/4] rseq: Make rseq work with protection keys

[Mathieu Desnoyers]

On 2025-02-18 10:10, Dmitry Vyukov wrote:
> On Tue, 18 Feb 2025 at 15:57, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> > On 2025-02-18 02:55, Dmitry Vyukov wrote:
> > > On Mon, 17 Feb 2025 at 21:21, Mathieu Desnoyers
> > [...]
> > > >     we still let this function read the rseq_cs.
> > > > > +      * It's unclear what benefits the resticted code gets by doing this
> > > >
> > > > restricted
> > > >
> > > > > +      * (it probably already hijacked control flow at this point), and
> > > > > +      * presumably any sane sandbox should prohibit restricted code
> > > > > +      * from accessing struct rseq, and this is still better than
> > > > > +      * terminating the app unconditionally (it always has a choice
> > > > > +      * of not using rseq and pkeys together).
> > > >
> > > > Note that because userspace can complete an rseq critical section
> > > > without clearing the rseq_cs pointer, this could happen simply because
> > > > the kernel is preempting the task after it has:
> > > >
> > > > 1) completed an rseq critical section, without clearing rseq_cs,
> > > > 2) changed pkey.
> > > >
> > > > So allowing this is important, and I would remove the comment about
> > > > hijacked control flow and such. This can happen with normal use of the
> > > > ABI.
> > >
> > > Thanks for the review!
> > >
> > > I've addressed all comments in the series in v2.
> > >
> > > I've reworded this paragraph to simplify sentences, but I still kept
> > > the note aboud malicious rseq_cs.
> > >
> > > If we would not be circumventing normal protection, then, yes, these
> > > cases would be the same. But since we are circumventing protection
> > > that otherwise exists, I think it's important to think about
> > > potentially malicious cases. In this context inaccessible rseq_cs
> > > values that resulted from normal execution are very different from
> > > malicious onces. Normal ones will point to a fixed set of real
> > > well-formed rseq_cs objects, while malicious ones may point to
> > > god-knows-where in an attempt of an attacker to do things we can't
> > > even imagine right now (e.g. rseq_cs overlapping with protected crypto
> > > keys).
> > >
> > > It's as if a particular instance of copy_to_user would allow
> > > user-space to write arbitrary kernel memory, and memory of other
> > > processes circumventing all normal protections. In that context we
> > > would need to be very careful regarding what we actually allow.
> >
> > I'm considering that we should clear the rseq_cs pointer whenever
> > userspace issues pkey_mprotect.
> >
> > This would ensure that no legitimate scenario can trigger a load
> > from a rseq_cs area which has the wrong pkey, and therefore we
> > could accept read/write from/to a struct rseq which has the wrong
> > pkey, but kill the process if trying to read/write from a
> > struct rseq_cs with the wrong key. This would prevent userspace
> > from making the kernel read/write from/to memory with the wrong
> > pkey through a pointer it controls (rseq_cs pointer).
> >
> > Thoughts ?
>
> I am not following.
>
> There are pkey_mprotect calls, then independently installs on rseq_cs
> pointers that happen concurrently and after pkey_mprotect, and
> independent set of pkey_set calls that happens concurrently and after
> the previous 2.
> I don't see how doing something at the pkey_mprotect call for the
> single thread avoids any scenarios.

Hrm. Sorry, I mixed up pkey_set() vs pkey_mprotect(). What I had in mind
was actually pkey_set(). And that would need to clear rseq_cs for all
threads belonging to the process, which may not be straightforward
because those could legitimately be inside a rseq critical section.

OK, let's try another approach: rather than kill the process if
read/write of the rseq_cs area with the wrong key fails, could we simply
clear the rseq_cs pointer in that case ? Technically there would be no
legitimate use of this except for the case where it is meant to be lazily
cleared.

Thanks,

Mathieu

> Moreover, pkey 0 is preinstalled for all pages, but access to it can
> be revoked in future.


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com