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

Luck, Tony
2017-06-16T20:09:46Z

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