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From:   andrey.konovalov@linux.dev
To:     Marco Elver <elver@google.com>
Cc:     Andrey Konovalov <andreyknvl@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        kasan-dev@googlegroups.com,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Arnd Bergmann <arnd@arndb.de>, stable@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>
Subject: [PATCH] kasan: fix type cast in memory_is_poisoned_n
Date:   Tue,  4 Jul 2023 02:52:05 +0200
Message-Id: <8c9e0251c2b8b81016255709d4ec42942dcaf018.1688431866.git.andreyknvl@google.com>
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From: Andrey Konovalov <andreyknvl@google.com>

Commit bb6e04a173f0 ("kasan: use internal prototypes matching gcc-13
builtins") introduced a bug into the memory_is_poisoned_n implementation:
it effectively removed the cast to a signed integer type after applying
KASAN_GRANULE_MASK.

As a result, KASAN started failing to properly check memset, memcpy,
and other similar functions.

Fix the bug by adding the cast back (through an additional signed integer
variable to make the code more readable).

Fixes: bb6e04a173f0 ("kasan: use internal prototypes matching gcc-13 builtins")
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 mm/kasan/generic.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index 5b4c97baa656..4d837ab83f08 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -130,9 +130,10 @@ static __always_inline bool memory_is_poisoned_n(const void *addr, size_t size)
 	if (unlikely(ret)) {
 		const void *last_byte = addr + size - 1;
 		s8 *last_shadow = (s8 *)kasan_mem_to_shadow(last_byte);
+		s8 last_accessible_byte = (unsigned long)last_byte & KASAN_GRANULE_MASK;
 
 		if (unlikely(ret != (unsigned long)last_shadow ||
-			(((long)last_byte & KASAN_GRANULE_MASK) >= *last_shadow)))
+			     last_accessible_byte >= *last_shadow))
 			return true;
 	}
 	return false;
-- 
2.25.1