[PATCH v3 mm 07/13] kasan, mm: fail krealloc on freed objects

[Andrey Konovalov]

Currently, if krealloc() is called on a freed object with KASAN enabled,
it allocates and returns a new object, but doesn't copy any memory from
the old one as ksize() returns 0. This makes the caller believe that
krealloc() succeeded (KASAN report is printed though).

This patch adds an accessibility check into __do_krealloc(). If the check
fails, krealloc() returns NULL. This check duplicates the one in ksize();
this is fixed in the following patch.

This patch also adds a KASAN-KUnit test to check krealloc() behaviour
when it's called on a freed object.

Reviewed-by: Marco Elver <elver at google.com>
Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
---
 lib/test_kasan.c | 20 ++++++++++++++++++++
 mm/slab_common.c |  3 +++
 2 files changed, 23 insertions(+)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index ffebad2f0e6e..1328c468fdb5 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -353,6 +353,25 @@ static void krealloc_pagealloc_less_oob(struct kunit *test)
 					KMALLOC_MAX_CACHE_SIZE + 201);
 }
 
+/*
+ * Check that krealloc() detects a use-after-free, returns NULL,
+ * and doesn't unpoison the freed object.
+ */
+static void krealloc_uaf(struct kunit *test)
+{
+	char *ptr1, *ptr2;
+	int size1 = 201;
+	int size2 = 235;
+
+	ptr1 = kmalloc(size1, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
+	kfree(ptr1);
+
+	KUNIT_EXPECT_KASAN_FAIL(test, ptr2 = krealloc(ptr1, size2, GFP_KERNEL));
+	KUNIT_ASSERT_PTR_EQ(test, (void *)ptr2, NULL);
+	KUNIT_EXPECT_KASAN_FAIL(test, *(volatile char *)ptr1);
+}
+
 static void kmalloc_oob_16(struct kunit *test)
 {
 	struct {
@@ -1050,6 +1069,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(krealloc_less_oob),
 	KUNIT_CASE(krealloc_pagealloc_more_oob),
 	KUNIT_CASE(krealloc_pagealloc_less_oob),
+	KUNIT_CASE(krealloc_uaf),
 	KUNIT_CASE(kmalloc_oob_16),
 	KUNIT_CASE(kmalloc_uaf_16),
 	KUNIT_CASE(kmalloc_oob_in_memset),
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 39d1a8ff9bb8..dad70239b54c 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1140,6 +1140,9 @@ 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);
 
 	if (ks >= new_size) {
-- 
2.30.0.365.g02bc693789-goog