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Subject[PATCH 3/7] kasan: accurately determine the type of the bad access
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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@google.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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