[RFC PATCH 13/14] mm/slub: sanity-check freepointers

[Matteo Rizzo]

From: Jann Horn <jannh@xxxxxxxxxx>

Sanity-check that:
 - non-NULL freepointers point into the slab
 - freepointers look plausibly aligned

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Co-developed-by: Matteo Rizzo <matteorizzo@xxxxxxxxxx>
Signed-off-by: Matteo Rizzo <matteorizzo@xxxxxxxxxx>
---
 lib/slub_kunit.c |  4 ++++
 mm/slab.h        |  8 +++++++
 mm/slub.c        | 57 ++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+)

diff --git a/lib/slub_kunit.c b/lib/slub_kunit.c
index d4a3730b08fa..acf8600bd1fd 100644
--- a/lib/slub_kunit.c
+++ b/lib/slub_kunit.c
@@ -45,6 +45,10 @@ static void test_clobber_zone(struct kunit *test)
 #ifndef CONFIG_KASAN
 static void test_next_pointer(struct kunit *test)
 {
+	if (IS_ENABLED(CONFIG_SLAB_VIRTUAL))
+		kunit_skip(test,
+			"incompatible with freepointer corruption detection in CONFIG_SLAB_VIRTUAL");
+
 	struct kmem_cache *s = test_kmem_cache_create("TestSlub_next_ptr_free",
 							64, SLAB_POISON);
 	u8 *p = kmem_cache_alloc(s, GFP_KERNEL);
diff --git a/mm/slab.h b/mm/slab.h
index 460c802924bd..8d10a011bdf0 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -79,6 +79,14 @@ struct slab {
 
 	struct list_head flush_list_elem;
 
+	/*
+	 * Not in kmem_cache because it depends on whether the allocation is
+	 * normal order or fallback order.
+	 * an alternative might be to over-allocate virtual memory for
+	 * fallback-order pages.
+	 */
+	unsigned long align_mask;
+
 	/* Replaces the page lock */
 	spinlock_t slab_lock;
 
diff --git a/mm/slub.c b/mm/slub.c
index 0f7f5bf0b174..57474c8a6569 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -392,6 +392,44 @@ static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s,
 	return (freeptr_t){.v = encoded};
 }
 
+/*
+ * Does some validation of freelist pointers. Without SLAB_VIRTUAL this is
+ * currently a no-op.
+ */
+static inline bool freelist_pointer_corrupted(struct slab *slab, freeptr_t ptr,
+	void *decoded)
+{
+#ifdef CONFIG_SLAB_VIRTUAL
+	/*
+	 * If the freepointer decodes to 0, use 0 as the slab_base so that
+	 * the check below always passes (0 & slab->align_mask == 0).
+	 */
+	unsigned long slab_base = decoded ? (unsigned long)slab_to_virt(slab)
+		: 0;
+
+	/*
+	 * This verifies that the SLUB freepointer does not point outside the
+	 * slab. Since at that point we can basically do it for free, it also
+	 * checks that the pointer alignment looks vaguely sane.
+	 * However, we probably don't want the cost of a proper division here,
+	 * so instead we just do a cheap check whether the bottom bits that are
+	 * clear in the size are also clear in the pointer.
+	 * So for kmalloc-32, it does a perfect alignment check, but for
+	 * kmalloc-192, it just checks that the pointer is a multiple of 32.
+	 * This should probably be reconsidered - is this a good tradeoff, or
+	 * should that part be thrown out, or do we want a proper accurate
+	 * alignment check (and can we make it work with acceptable performance
+	 * cost compared to the security improvement - probably not)?
+	 */
+	return CHECK_DATA_CORRUPTION(
+		((unsigned long)decoded & slab->align_mask) != slab_base,
+		"bad freeptr (encoded %lx, ptr %p, base %lx, mask %lx",
+		ptr.v, decoded, slab_base, slab->align_mask);
+#else
+	return false;
+#endif
+}
+
 static inline void *freelist_ptr_decode(const struct kmem_cache *s,
 					freeptr_t ptr, unsigned long ptr_addr,
 					struct slab *slab)
@@ -403,6 +441,10 @@ static inline void *freelist_ptr_decode(const struct kmem_cache *s,
 #else
 	decoded = (void *)ptr.v;
 #endif
+
+	if (unlikely(freelist_pointer_corrupted(slab, ptr, decoded)))
+		return NULL;
+
 	return decoded;
 }
 
@@ -2122,6 +2164,21 @@ static struct slab *get_free_slab(struct kmem_cache *s,
 	if (slab == NULL)
 		return NULL;
 
+	/*
+	 * Bits that must be equal to start-of-slab address for all
+	 * objects inside the slab.
+	 * For compatibility with pointer tagging (like in HWASAN), this would
+	 * need to clear the pointer tag bits from the mask.
+	 */
+	slab->align_mask = ~((PAGE_SIZE << oo_order(oo)) - 1);
+
+	/*
+	 * Object alignment bits (must be zero, which is equal to the bits in
+	 * the start-of-slab address)
+	 */
+	if (s->red_left_pad == 0)
+		slab->align_mask |= (1 << (ffs(s->size) - 1)) - 1;
+
 	return slab;
 }
 
-- 
2.42.0.459.ge4e396fd5e-goog