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

[Kirill A. Shutemov]

On Tue, Sep 10, 2019 at 09:28:10AM -0500, Steve Wahl wrote:
> On Mon, Sep 09, 2019 at 11:14:14AM +0300, Kirill A. Shutemov wrote:
> > On Fri, Sep 06, 2019 at 04:29:50PM -0500, Steve Wahl wrote:
> > > ...
> > > The answer is to invalidate the pages of this table outside the
> > > address range occupied by the kernel before the page table is
> > > activated.  This patch has been validated to fix this problem on our
> > > hardware.
> > 
> > If the goal is to avoid *any* mapping of the reserved region to stop
> > speculation, I don't think this patch will do the job. We still (likely)
> > have the same memory mapped as part of the identity mapping. And it
> > happens at least in two places: here and before on decompression stage.
> 
> I imagine you are likely correct, ideally you would not map any
> reserved pages in these spaces.
> 
> I've been reading the code to try to understand what you say above.
> For identity mappings in the kernel, I see level2_ident_pgt mapping
> the first 1G.

This is for XEN case. Not sure how relevant it is for you.

> And I see early_dyanmic_pgts being set up with an identity mapping of
> the kernel that seems to be pretty well restricted to the range _text
> through _end.

Right, but rounded to 2M around the place kernel was decompressed to.
Some of reserved areas from the listing below are smaller then 2M or not
aligned to 2M.

> Within the decompression code, I see an identity mapping of the first
> 4G set up within the 32 bit code.  I believe we go past that to the
> startup_64 entry point.  (I don't know how common that path is, but I
> don't have a way to test it without figuring out how to force it.)

Kernel can start in 64-bit mode directly and in this case we inherit page
tables from bootloader/BIOS. They trusted to provide identity mapping to
cover at least kernel (plus some more essential stuff), but it's free to
map more.

> From a pragmatic standpoint, the guy who can verify this for me is on
> vacation, but I believe our BIOS won't ever place the halt-causing
> ranges in a space below 4GiB.  Which explains why this patch works for
> our hardware.  (We do have reserved regions below 4G, just not the
> ones that hardware causes a halt for accessing.)
> 
> In case it helps you picture the situation, our hardware takes a small
> portion of RAM from the end of each NUMA node (or it might be pairs or
> quads of NUMA nodes, I'm not entirely clear on this at the moment) for
> its own purposes.  Here's a section of our e820 table:
> 
> [    0.000000] BIOS-e820: [mem 0x000000007c000000-0x000000008fffffff] reserved
> [    0.000000] BIOS-e820: [mem 0x00000000f8000000-0x00000000fbffffff] reserved
> [    0.000000] BIOS-e820: [mem 0x00000000fe000000-0x00000000fe010fff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000000100000000-0x0000002f7fffffff] usable
> [    0.000000] BIOS-e820: [mem 0x0000002f80000000-0x000000303fffffff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000003040000000-0x0000005f7bffffff] usable
> [    0.000000] BIOS-e820: [mem 0x0000005f7c000000-0x000000603fffffff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000006040000000-0x0000008f7bffffff] usable
> [    0.000000] BIOS-e820: [mem 0x0000008f7c000000-0x000000903fffffff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000009040000000-0x000000bf7bffffff] usable
> [    0.000000] BIOS-e820: [mem 0x000000bf7c000000-0x000000c03fffffff] reserved
> [    0.000000] BIOS-e820: [mem 0x000000c040000000-0x000000ef7bffffff] usable
> [    0.000000] BIOS-e820: [mem 0x000000ef7c000000-0x000000f03fffffff] reserved
> [    0.000000] BIOS-e820: [mem 0x000000f040000000-0x0000011f7bffffff] usable
> [    0.000000] BIOS-e820: [mem 0x0000011f7c000000-0x000001203fffffff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000012040000000-0x0000014f7bffffff] usable
> [    0.000000] BIOS-e820: [mem 0x0000014f7c000000-0x000001503fffffff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000015040000000-0x0000017f7bffffff] usable
> [    0.000000] BIOS-e820: [mem 0x0000017f7c000000-0x000001803fffffff] reserved

It would be interesting to know which of them are problematic.

> Our problem occurs when KASLR (or kexec) places the kernel close
> enough to the end of one of the usable sections, and the 1G of 1:1
> mapped space includes a portion of the following reserved section, and
> speculation touches the reserved area.

Are you sure that it's speculative access to blame? Speculative access
must not cause change in architectural state.

-- 
 Kirill A. Shutemov