[PATCH] kasan: fix bug type classification for SW_TAGS mode

[Andrey Ryabinin <ryabinin.a.a@gmail.com>]

kasan_non_canonical_hook() derives orig_addr from kasan_shadow_to_mem(),
but the pointer tag may remain in the top byte. In SW_TAGS mode this
tagged address is compared against PAGE_SIZE and TASK_SIZE, which leads
to incorrect bug classification.

As a result, NULL pointer dereferences may be reported as
"wild-memory-access".

Strip the tag before performing these range checks and use the untagged
value when reporting addresses in these ranges.

Before:
  [ ] Unable to handle kernel paging request at virtual address ffef800000000000
  [ ] KASAN: maybe wild-memory-access in range [0xff00000000000000-0xff0000000000000f]

After:
  [ ] Unable to handle kernel paging request at virtual address ffef800000000000
  [ ] KASAN: null-ptr-deref in range [0x0000000000000000-0x000000000000000f]

Signed-off-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
---
 mm/kasan/report.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index 27efb78eb32d..e804b1e1f886 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -638,7 +638,7 @@ void kasan_report_async(void)
  */
 void kasan_non_canonical_hook(unsigned long addr)
 {
-	unsigned long orig_addr;
+	unsigned long orig_addr, user_orig_addr;
 	const char *bug_type;
 
 	/*
@@ -650,6 +650,9 @@ void kasan_non_canonical_hook(unsigned long addr)
 
 	orig_addr = (unsigned long)kasan_shadow_to_mem((void *)addr);
 
+	/* Strip pointer tag before comparing against userspace ranges */
+	user_orig_addr = (unsigned long)set_tag((void *)orig_addr, 0);
+
 	/*
 	 * For faults near the shadow address for NULL, we can be fairly certain
 	 * that this is a KASAN shadow memory access.
@@ -661,11 +664,13 @@ void kasan_non_canonical_hook(unsigned long addr)
 	 * address, but make it clear that this is not necessarily what's
 	 * actually going on.
 	 */
-	if (orig_addr < PAGE_SIZE)
+	if (user_orig_addr < PAGE_SIZE) {
 		bug_type = "null-ptr-deref";
-	else if (orig_addr < TASK_SIZE)
+		orig_addr = user_orig_addr;
+	} else if (user_orig_addr < TASK_SIZE) {
 		bug_type = "probably user-memory-access";
-	else if (addr_in_shadow((void *)addr))
+		orig_addr = user_orig_addr;
+	} else if (addr_in_shadow((void *)addr))
 		bug_type = "probably wild-memory-access";
 	else
 		bug_type = "maybe wild-memory-access";
-- 
2.52.0