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[PATCH 1/2] x86/kasan: clear kasan_zero_page after TLB flush

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Currently we clear kasan_zero_page before __flush_tlb_all(). This
works with current implementation of native_flush_tlb[_global]()
because it doesn't cause do any writes to kasan shadow memory.
But any subtle change made in native_flush_tlb*() could break this.
Also current code seems doesn't work for paravirt guests (lguest).

Only after the TLB flush we can be sure that kasan_zero_page is not
used as early shadow anymore (instrumented code will not write to it).
So it should cleared it only after the TLB flush.

Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
---
 arch/x86/mm/kasan_init_64.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
index d470cf2..303e470 100644
--- a/arch/x86/mm/kasan_init_64.c
+++ b/arch/x86/mm/kasan_init_64.c
@@ -120,11 +120,16 @@ void __init kasan_init(void)
 	kasan_populate_zero_shadow(kasan_mem_to_shadow((void *)MODULES_END),
 			(void *)KASAN_SHADOW_END);
 
-	memset(kasan_zero_page, 0, PAGE_SIZE);
-
 	load_cr3(init_level4_pgt);
 	__flush_tlb_all();
-	init_task.kasan_depth = 0;
 
+	/*
+	 * kasan_zero_page has been used as early shadow memory, thus it may
+	 * contain some garbage. Now we can clear it, since after the TLB flush
+	 * no one should write to it.
+	 */
+	memset(kasan_zero_page, 0, PAGE_SIZE);
+
+	init_task.kasan_depth = 0;
 	pr_info("KernelAddressSanitizer initialized\n");
 }
-- 
2.4.10

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[RFC 01/13] x86/paravirt: Turn KASAN off for parvirt.o Andy Lutomirski <luto@kernel.org> - 2016-01-09 00:20 +0100
  Re: [RFC 01/13] x86/paravirt: Turn KASAN off for parvirt.o Borislav Petkov <bp@alien8.de> - 2016-01-10 20:10 +0100
    [PATCH 2/2] x86/kasan: write protect kasan zero shadow Andrey Ryabinin <aryabinin@virtuozzo.com> - 2016-01-11 14:00 +0100
    Re: [RFC 01/13] x86/paravirt: Turn KASAN off for parvirt.o Andrey Ryabinin <aryabinin@virtuozzo.com> - 2016-01-11 14:00 +0100
      [PATCH 1/2] x86/kasan: clear kasan_zero_page after TLB flush Andrey Ryabinin <aryabinin@virtuozzo.com> - 2016-01-11 14:00 +0100

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