Re: [kernel-hardening] [PATCH v5 07/10] arm64/mm: Don't flush the data cache if the page is unmapped by XPFO

Mark Rutland
2017-08-12T11:57:44Z

On Wed, Aug 09, 2017 at 02:07:52PM -0600, Tycho Andersen wrote:
> From: Juerg Haefliger <juerg.haefliger@hpe.com>
>
> If the page is unmapped by XPFO, a data cache flush results in a fatal
> page fault. So don't flush in that case.

Do you have an example callchain where that happens? We might need to shuffle
things around to cater for that case.

> @@ -30,7 +31,9 @@ void sync_icache_aliases(void *kaddr, unsigned long len)
> unsigned long addr = (unsigned long)kaddr;
>
> if (icache_is_aliasing()) {
> - __clean_dcache_area_pou(kaddr, len);
> + /* Don't flush if the page is unmapped by XPFO */
> + if (!xpfo_page_is_unmapped(virt_to_page(kaddr)))
> + __clean_dcache_area_pou(kaddr, len);
> __flush_icache_all();
> } else {
> flush_icache_range(addr, addr + len);

I don't think this patch is correct. If data cache maintenance is required in
the absence of XPFO, I don't see why it wouldn't be required in the presence of
XPFO.

I'm not immediately sure why the non-aliasing case misses data cache
maintenance. I couldn't spot where that happens otherwise.

On a more general note, in future it would be good to Cc the arm64 maintainers
and the linux-arm-kernel mailing list for patches affecting arm64.

Thanks,
Mark.