linux-kernel - [tip:x86/mm] x86/kasan: Clear kasan_zero

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Date:	Tue, 9 Feb 2016 08:06:41 -0800
From:	tip-bot for Andrey Ryabinin <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	oleg@...hat.com, hpa@...or.com, peterz@...radead.org,
	luto@...nel.org, tglx@...utronix.de, dave.hansen@...ux.intel.com,
	dvlasenk@...hat.com, bp@...en8.de, mcgrof@...e.com,
	brgerst@...il.com, bp@...e.de, mingo@...nel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	luto@...capital.net, toshi.kani@...com, aryabinin@...tuozzo.com,
	torvalds@...ux-foundation.org
Subject: [tip:x86/mm] x86/kasan: Clear kasan_zero_page after TLB flush

Commit-ID:  69e0210fd01ff157d332102219aaf5c26ca8069b
Gitweb:     http://git.kernel.org/tip/69e0210fd01ff157d332102219aaf5c26ca8069b
Author:     Andrey Ryabinin <aryabinin@...tuozzo.com>
AuthorDate: Mon, 11 Jan 2016 15:51:18 +0300
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 9 Feb 2016 13:33:14 +0100

x86/kasan: Clear kasan_zero_page after TLB flush

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@...tuozzo.com>
Reviewed-by: Borislav Petkov <bp@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@...e.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Toshi Kani <toshi.kani@...com>
Cc: linux-mm@...ck.org
Link: http://lkml.kernel.org/r/1452516679-32040-2-git-send-email-aryabinin@virtuozzo.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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");
 }