Re: TLB flushes on fixmap changes

From: Jann Horn
Date: Mon Aug 27 2018 - 05:55:30 EST

On Mon, Aug 27, 2018 at 10:13 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Mon, Aug 27, 2018 at 12:03:05PM +0900, Masami Hiramatsu wrote:
> > On Sun, 26 Aug 2018 11:09:58 +0200
> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > FWIW, before text_poke_bp(), text_poke() would only be used from
> > > stop_machine, so all the other CPUs would be stuck busy-waiting with
> > > IRQs disabled. These days, yeah, that's lots more dodgy, but yes
> > > text_mutex should be serializing all that.
> >
> > I'm still not sure that speculative page-table walk can be done
> > over the mutex. Also, if the fixmap area is for aliasing
> > pages (which always mapped to memory), what kind of
> > security issue can happen?
> So suppose CPU-A is doing the text_poke (let's say through text_poke_bp,
> such that other CPUs get to continue with whatever they're doing).
> While at that point, CPU-B gets an interrupt, and the CPU's
> branch-trace-buffer for the IRET points to / near our fixmap. Then the
> CPU could do a speculative TLB fill based on the BTB value, either
> directly or indirectly (through speculative driven fault-ahead) of
> whatever is in te fixmap at the time.

Worse: The way academics have been defeating KASLR for a while is
based on TLB fills for kernel addresses, triggered from userspace.
Quoting :

| Additionally, even if a permission error occurs, this still allows to
| launch address translations and, hence, generate valid TLB entries
| by accessing privileged kernel space memory from user mode.

This was actually part of the original motivation for KAISER/KPTI.
Quoting :

| Modern operating system kernels employ address space layout
| randomization (ASLR) to prevent control-flow hijacking attacks and
| code-injection attacks. While kernel security relies fundamentally
on preventing
| access to address information, recent attacks have shown that the
| hardware directly leaks this information.

I believe that PTI probably prevents this way of directly triggering
TLB fills for now (under the assumption that hyperthreads with equal
CR3 don't share TLB entries), but I would still assume that an
attacker can probably trigger TLB fills for arbitrary addresses
anytime. And at some point in the future, I believe people would
probably like to be able to disable PTI again?

> Then CPU-A completes the text_poke and only does a local TLB invalidate
> on CPU-A, leaving CPU-B with an active translation.
> *FAIL*