Re: [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of poison pages

Borislav Petkov
2017-06-19T18:02:04Z

(drop stable from CC)

You could use git's --suppress-cc= option when sending.

On Fri, Jun 16, 2017 at 12:02:00PM -0700, Luck, Tony wrote:
> From: Tony Luck <tony.luck@intel.com>
>
> Speculative processor accesses may reference any memory that has a
> valid page table entry. While a speculative access won't generate
> a machine check, it will log the error in a machine check bank. That
> could cause escalation of a subsequent error since the overflow bit
> will be then set in the machine check bank status register.

...

> @@ -1056,6 +1057,40 @@ static int do_memory_failure(struct mce *m)
> return ret;
> }
>
> +#ifdef CONFIG_X86_64
> +
> +void arch_unmap_kpfn(unsigned long pfn)
> +{

I guess you can move the ifdeffery inside the function.

> + unsigned long decoy_addr;
> +
> + /*
> + * Unmap this page from the kernel 1:1 mappings to make sure
> + * we don't log more errors because of speculative access to
> + * the page.
> + * We would like to just call:
> + * set_memory_np((unsigned long)pfn_to_kaddr(pfn), 1);
> + * but doing that would radically increase the odds of a
> + * speculative access to the posion page because we'd have
> + * the virtual address of the kernel 1:1 mapping sitting
> + * around in registers.
> + * Instead we get tricky. We create a non-canonical address
> + * that looks just like the one we want, but has bit 63 flipped.
> + * This relies on set_memory_np() not checking whether we passed
> + * a legal address.
> + */
> +
> +#if PGDIR_SHIFT + 9 < 63 /* 9 because cpp doesn't grok ilog2(PTRS_PER_PGD) */

Please no side comments.

Also, explain why the build-time check. (Sign-extension going away for VA
space yadda yadda..., 5 2/3 level paging :-))

Also, I'm assuming this whole "workaround" of sorts should be Intel-only?

> + decoy_addr = (pfn << PAGE_SHIFT) + (PAGE_OFFSET ^ BIT(63));
> +#else
> +#error "no unused virtual bit available"
> +#endif
> +
> + if (set_memory_np(decoy_addr, 1))
> + pr_warn("Could not invalidate pfn=0x%lx from 1:1 map \n", pfn);

WARNING: unnecessary whitespace before a quoted newline
#107: FILE: arch/x86/kernel/cpu/mcheck/mce.c:1089:
+ pr_warn("Could not invalidate pfn=0x%lx from 1:1 map \n", pfn);

--
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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