Re: [PATCH] x86/boot/64: Make level2_kernel_pgt pages invalid outside kernel area.

[Steve Wahl]

On Mon, Sep 16, 2019 at 07:25:48AM -0700, Dave Hansen wrote:
> On 9/16/19 2:00 AM, Kirill A. Shutemov wrote:
> >>> I think we also need to make it clear that this is workaround for a broken
> >>> hardware: speculative execution must not trigger a halt.
> >> I think the word broken is a bit loaded here.  According to the UEFI
> >> spec (version 2.8, page 167), "Regions that are backed by the physical
> >> hardware, but are not supposed to be accessed by the OS, must be
> >> returned as EfiReservedMemoryType."  Our interpretation is that
> >> includes speculative accesses.
> > +Dave.
> > 
> > I don't think it is. Speculative access is done by hardware, not OS.
> > 
> > BTW, isn't it a BIOS issue?
> > 
> > I believe it should have a way to hide a range of physical address space
> > from OS or force a caching mode on to exclude it from speculative
> > execution. Like setup MTRRs or something.
> 
> Ugh.  I bet that was a fun one to chase down.  Have the hardware
> engineers learned a lesson or are they hiding behind the EFI spec in an
> act of pure cowardice? ;)

Yes, it was fun.  My main BIOS contact has explained to me how they are
stuck between a rock and a hard place on any other options for this.

> The patch is small and fixes a real problem.  The changelog is OK,
> although I'd prefer some differentiation between "occupied by the
> kernel" and the kernel *image*.

OK, is the phrase "kernel image" generally understood to cover
everything from _text to _end, including the bss?  As long as that's
true, I will adopt this phrase.

> The code is also gloriously free of any comments about what it's
> doing or why.

I'm intending to add something like this in the next version:

/*
 * Only the region occupied by the kernel image has so far been checked against
 * the table of usable memory regions provided by the firmware, so
 * invalidate pages outside that region.  A page table entry that maps to
 * a reserved area of memory would allow processor speculation into that
 * area, and on some hardware (particularly the UV platform) speculation
 * into reserved areas can cause a system halt.
 */


> But, I'm left with lots of questions:
> 
> Why do PMD-level changes fix this?  Is it because we 2MB pad the kernel
> image?  Why can't we still get within 2MB of the memory address in
> question?

This fix works for our hardware because the problematic reserved
regions are 64M aligned, and going up to the next 2MB boundary from
_end is not going to cross the next 64M boundary.

One could argue the next step would be going into
boot/compressed/{kaslr.c, misc.c} and rounding the size of the kernel
up to the next 2MB boundary to ensure the chosen random location is
covered by usable RAM up to the next PMD-level boundary.  I did not go
there because for us it is not necessary.

> Is it in the lower 1MB, by chance?

No, this is a reserved range at the top physical addresses for each
NUMA node in a collection of them.  

> If this is all about avoiding EFI reserved ranges, why doesn't the
> patch *LOOK* At EFI reserved ranges?

Because the range the kernel image is located in is already checked
against them in boot/compressed/kaslr.c.  This will now be explained
in the comment I mention above, which you had not yet seen.

--> Steve Wahl


-- 
Steve Wahl, Hewlett Packard Enterprise