linux-kernel - [PATCH v3 mm 08/13] kasan, mm: optimize krealloc poisoning

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Message-Id: <9bef90327c9cb109d736c40115684fd32f49e6b0.1612546384.git.andreyknvl@google.com>
Date:   Fri,  5 Feb 2021 18:34:42 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>
Cc:     Will Deacon <will.deacon@....com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        kasan-dev@...glegroups.com, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: [PATCH v3 mm 08/13] kasan, mm: optimize krealloc poisoning

Currently, krealloc() always calls ksize(), which unpoisons the whole
object including the redzone. This is inefficient, as kasan_krealloc()
repoisons the redzone for objects that fit into the same buffer.

This patch changes krealloc() instrumentation to use uninstrumented
__ksize() that doesn't unpoison the memory. Instead, kasan_kreallos()
is changed to unpoison the memory excluding the redzone.

For objects that don't fit into the old allocation, this patch disables
KASAN accessibility checks when copying memory into a new object instead
of unpoisoning it.

Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
---
 mm/kasan/common.c | 12 ++++++++++--
 mm/slab_common.c  | 20 ++++++++++++++------
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 7ea643f7e69c..a8a67dca5e55 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -476,7 +476,7 @@ static void *____kasan_kmalloc(struct kmem_cache *cache, const void *object,
 
 	/*
 	 * The object has already been unpoisoned by kasan_slab_alloc() for
-	 * kmalloc() or by ksize() for krealloc().
+	 * kmalloc() or by kasan_krealloc() for krealloc().
 	 */
 
 	/*
@@ -526,7 +526,7 @@ void * __must_check __kasan_kmalloc_large(const void *ptr, size_t size,
 
 	/*
 	 * The object has already been unpoisoned by kasan_alloc_pages() for
-	 * alloc_pages() or by ksize() for krealloc().
+	 * alloc_pages() or by kasan_krealloc() for krealloc().
 	 */
 
 	/*
@@ -554,8 +554,16 @@ void * __must_check __kasan_krealloc(const void *object, size_t size, gfp_t flag
 	if (unlikely(object == ZERO_SIZE_PTR))
 		return (void *)object;
 
+	/*
+	 * Unpoison the object's data.
+	 * Part of it might already have been unpoisoned, but it's unknown
+	 * how big that part is.
+	 */
+	kasan_unpoison(object, size);
+
 	page = virt_to_head_page(object);
 
+	/* Piggy-back on kmalloc() instrumentation to poison the redzone. */
 	if (unlikely(!PageSlab(page)))
 		return __kasan_kmalloc_large(object, size, flags);
 	else
diff --git a/mm/slab_common.c b/mm/slab_common.c
index dad70239b54c..60a2f49df6ce 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1140,19 +1140,27 @@ static __always_inline void *__do_krealloc(const void *p, size_t new_size,
 	void *ret;
 	size_t ks;
 
-	if (likely(!ZERO_OR_NULL_PTR(p)) && !kasan_check_byte(p))
-		return NULL;
-
-	ks = ksize(p);
+	/* Don't use instrumented ksize to allow precise KASAN poisoning. */
+	if (likely(!ZERO_OR_NULL_PTR(p))) {
+		if (!kasan_check_byte(p))
+			return NULL;
+		ks = kfence_ksize(p) ?: __ksize(p);
+	} else
+		ks = 0;
 
+	/* If the object still fits, repoison it precisely. */
 	if (ks >= new_size) {
 		p = kasan_krealloc((void *)p, new_size, flags);
 		return (void *)p;
 	}
 
 	ret = kmalloc_track_caller(new_size, flags);
-	if (ret && p)
-		memcpy(ret, p, ks);
+	if (ret && p) {
+		/* Disable KASAN checks as the object's redzone is accessed. */
+		kasan_disable_current();
+		memcpy(ret, kasan_reset_tag(p), ks);
+		kasan_enable_current();
+	}
 
 	return ret;
 }
-- 
2.30.0.365.g02bc693789-goog