Re: TLB flushes on fixmap changes

[Peter Zijlstra]

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.

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*