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From:   Ilya Leoshkevich <iii@linux.ibm.com>
To:     Alexander Gordeev <agordeev@linux.ibm.com>,
        Alexander Potapenko <glider@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Marco Elver <elver@google.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Pekka Enberg <penberg@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>
Cc:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-s390@vger.kernel.org,
        linux-trace-kernel@vger.kernel.org,
        Mark Rutland <mark.rutland@arm.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Sven Schnelle <svens@linux.ibm.com>,
        Ilya Leoshkevich <iii@linux.ibm.com>
Subject: [PATCH v3 12/34] kmsan: Support SLAB_POISON
Date:   Thu, 14 Dec 2023 00:24:32 +0100
Message-ID: <20231213233605.661251-13-iii@linux.ibm.com>
In-Reply-To: <20231213233605.661251-1-iii@linux.ibm.com>
References: <20231213233605.661251-1-iii@linux.ibm.com>
MIME-Version: 1.0
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Avoid false KMSAN negatives with SLUB_DEBUG by allowing
kmsan_slab_free() to poison the freed memory, and by preventing
init_object() from unpoisoning new allocations by using __memset().

There are two alternatives to this approach. First, init_object()
can be marked with __no_sanitize_memory. This annotation should be used
with great care, because it drops all instrumentation from the
function, and any shadow writes will be lost. Even though this is not a
concern with the current init_object() implementation, this may change
in the future.

Second, kmsan_poison_memory() calls may be added after memset() calls.
The downside is that init_object() is called from
free_debug_processing(), in which case poisoning will erase the
distinction between simply uninitialized memory and UAF.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 mm/kmsan/hooks.c |  2 +-
 mm/slub.c        | 13 +++++++++----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c
index 3acf010c9814..21004eeee240 100644
--- a/mm/kmsan/hooks.c
+++ b/mm/kmsan/hooks.c
@@ -74,7 +74,7 @@ void kmsan_slab_free(struct kmem_cache *s, void *object)
 		return;
 
 	/* RCU slabs could be legally used after free within the RCU period */
-	if (unlikely(s->flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)))
+	if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
 		return;
 	/*
 	 * If there's a constructor, freed memory must remain in the same state
diff --git a/mm/slub.c b/mm/slub.c
index 63d281dfacdb..b111bc315e3f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1030,7 +1030,12 @@ static void init_object(struct kmem_cache *s, void *object, u8 val)
 	unsigned int poison_size = s->object_size;
 
 	if (s->flags & SLAB_RED_ZONE) {
-		memset(p - s->red_left_pad, val, s->red_left_pad);
+		/*
+		 * Use __memset() here and below in order to avoid overwriting
+		 * the KMSAN shadow. Keeping the shadow makes it possible to
+		 * distinguish uninit-value from use-after-free.
+		 */
+		__memset(p - s->red_left_pad, val, s->red_left_pad);
 
 		if (slub_debug_orig_size(s) && val == SLUB_RED_ACTIVE) {
 			/*
@@ -1043,12 +1048,12 @@ static void init_object(struct kmem_cache *s, void *object, u8 val)
 	}
 
 	if (s->flags & __OBJECT_POISON) {
-		memset(p, POISON_FREE, poison_size - 1);
-		p[poison_size - 1] = POISON_END;
+		__memset(p, POISON_FREE, poison_size - 1);
+		__memset(p + poison_size - 1, POISON_END, 1);
 	}
 
 	if (s->flags & SLAB_RED_ZONE)
-		memset(p + poison_size, val, s->inuse - poison_size);
+		__memset(p + poison_size, val, s->inuse - poison_size);
 }
 
 static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
-- 
2.43.0