linux-kernel - [PATCH 3/7] kasan: accurately determine the type of the bad access

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Message-Id: <4bcaa6fdf682a746c0a58de2884aeee13dd2805f.1441290220.git.andreyknvl@google.com>
Date:	Thu,  3 Sep 2015 16:47:38 +0200
From:	Andrey Konovalov <andreyknvl@...gle.com>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc:	dvyukov@...gle.com, glider@...gle.com, kcc@...gle.com,
	Andrey Konovalov <andreyknvl@...gle.com>
Subject: [PATCH 3/7] kasan: accurately determine the type of the bad access

Makes KASAN accurately determine the type of the bad access. If the shadow
byte value is in the [0, KASAN_SHADOW_SCALE_SIZE) range we can look at
the next shadow byte to determine the type of the access.

Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
---
 mm/kasan/report.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index a30ca44..6126272 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -49,15 +49,26 @@ static const void *find_first_bad_addr(const void *addr, size_t size)
 static void print_error_description(struct kasan_access_info *info)
 {
 	const char *bug_type = "unknown-crash";
-	u8 shadow_val;
+	u8 *shadow_addr;
 
 	info->first_bad_addr = find_first_bad_addr(info->access_addr,
 						info->access_size);
 
-	shadow_val = *(u8 *)kasan_mem_to_shadow(info->first_bad_addr);
+	shadow_addr = (u8 *)kasan_mem_to_shadow(info->first_bad_addr);
 
-	switch (shadow_val) {
+	/*
+	 * If shadow byte value is in [0, KASAN_SHADOW_SCALE_SIZE) we can look
+	 * at the next shadow byte to determine the type of the bad access.
+	 */
+	if (*shadow_addr > 0 && *shadow_addr <= KASAN_SHADOW_SCALE_SIZE - 1)
+		shadow_addr++;
+
+	switch (*shadow_addr) {
 	case 0 ... KASAN_SHADOW_SCALE_SIZE - 1:
+		/*
+		 * In theory it's still possible to see these shadow values
+		 * due to a data race in the kernel code.
+		 */
 		bug_type = "out-of-bounds";
 		break;
 	case KASAN_PAGE_REDZONE:
-- 
2.5.0.457.gab17608

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