Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3

[Will Deacon]

On Tue, Aug 15, 2017 at 11:35:41PM +0100, Mark Rutland wrote:
> On Wed, Aug 16, 2017 at 09:14:41AM -0700, Andy Lutomirski wrote:
> > On Wed, Aug 16, 2017 at 5:57 AM, Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
> > > On Wed, 16 Aug, at 12:03:22PM, Mark Rutland wrote:
> > >>
> > >> I'd expect we'd abort at a higher level, not taking any sample. i.e.
> > >> we'd have the core overflow handler check in_funny_mm(), and if so, skip
> > >> the sample, as with the skid case.
> > >
> > > FYI, this is my preferred solution for x86 too.
> > 
> > One option for the "funny mm" flag would be literally the condition
> > current->mm != current->active_mm.  I *think* this gets all the cases
> > right as long as efi_switch_mm is careful with its ordering and that
> > the arch switch_mm() code can handle the resulting ordering.  (x86's
> > can now, I think, or at least will be able to in 4.14 -- not sure
> > about other arches).
> 
> For arm64 we'd have to rework things a bit to get the ordering right
> (especially when we flip to/from the idmap), but otherwise this sounds sane to
> me.
> 
> > That being said, there's a totally different solution: run EFI
> > callbacks in a kernel thread.  This has other benefits: we could run
> > those callbacks in user mode some day, and doing *that* in a user
> > thread seems like a mistake.
> 
> I think that wouldn't work for CPU-bound perf events (which are not
> ctx-switched with the task).
> 
> It might be desireable to do that anyway, though.

I'm still concerned that we're treating perf specially here -- are we
absolutely sure that nobody else is going to attempt user accesses off the
back of an interrupt? If not, then I'd much prefer a solution that catches
anybody doing that with the EFI page table installed, rather than trying
to play whack-a-mole like this.

Will