From 26a5ca7a73be31f76c291465680517cde37051ca Mon Sep 17 00:00:00 2001
From: Andrey Konovalov <andreyknvl@google.com>
Date: Thu, 25 Feb 2021 17:20:19 -0800
Subject: [PATCH] kasan, mm: fail krealloc on freed objects

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.

Link: https://lkml.kernel.org/r/cbcf7b02be0a1ca11de4f833f2ff0b3f2c9b00c8.1612546384.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Branislav Rankov <Branislav.Rankov@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 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 e1bd1d1096de..e5647d147b35 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 897c3a446b04..4aedb8455352 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1136,6 +1136,9 @@ 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) {