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

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  [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of poison pages "Luck, Tony" <tony.luck@intel.com> - 2017-06-16 22:20 +0200
    Re: [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings  of poison pages Borislav Petkov <bp@suse.de> - 2017-06-19 20:10 +0200

#1668042 — [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of poison pages

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.

Code has to be double-plus-tricky to avoid mentioning the 1:1 virtual
address of the page we want to map out otherwise we may trigger the
very problem we are trying to avoid.  We use a non-canonical address
that passes through the usual Linux table walking code to get to the
same "pte".

Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: x86@kernel.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Tony Luck <tony.luck@intel.com>
---
Thanks to Dave Hansen for reviewing several iterations of this.

 arch/x86/include/asm/page_64.h   |  4 ++++
 arch/x86/kernel/cpu/mcheck/mce.c | 35 +++++++++++++++++++++++++++++++++++
 include/linux/mm_inline.h        |  6 ++++++
 mm/memory-failure.c              |  2 ++
 4 files changed, 47 insertions(+)

diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
index b4a0d43248cf..b50df06ad251 100644
--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -51,6 +51,10 @@ static inline void clear_page(void *page)
 
 void copy_page(void *to, void *from);
 
+#ifdef CONFIG_X86_MCE
+#define arch_unmap_kpfn arch_unmap_kpfn
+#endif
+
 #endif	/* !__ASSEMBLY__ */
 
 #ifdef CONFIG_X86_VSYSCALL_EMULATION
diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index 5cfbaeb6529a..56563db0b2be 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -51,6 +51,7 @@
 #include <asm/mce.h>
 #include <asm/msr.h>
 #include <asm/reboot.h>
+#include <asm/set_memory.h>
 
 #include "mce-internal.h"
 
@@ -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)
+{
+	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) */
+	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);
+
+}
+#endif
+
 /*
  * The actual machine check handler. This only handles real
  * exceptions when something got corrupted coming in through int 18.
diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index e030a68ead7e..25438b2b6f22 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -126,4 +126,10 @@ static __always_inline enum lru_list page_lru(struct page *page)
 
 #define lru_to_page(head) (list_entry((head)->prev, struct page, lru))
 
+#ifdef arch_unmap_kpfn
+extern void arch_unmap_kpfn(unsigned long pfn);
+#else
+static __always_inline void arch_unmap_kpfn(unsigned long pfn) { }
+#endif
+
 #endif
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 342fac9ba89b..9479e190dcbd 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1071,6 +1071,8 @@ int memory_failure(unsigned long pfn, int trapno, int flags)
 		return 0;
 	}
 
+	arch_unmap_kpfn(pfn);
+
 	/*
 	 * Currently errors on hugetlbfs pages are measured in hugepage units,
 	 * so nr_pages should be 1 << compound_order.  OTOH when errors are on
-- 
2.11.0

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#1669564 — Re: [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of poison pages

(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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