mm/slub: refactor freelist to use custom type

Currently the SLUB code represents encoded freelist entries as "void*".
That's misleading, those things are encoded under
CONFIG_SLAB_FREELIST_HARDENED so that they're not actually dereferencable.

Give them their own type, and split freelist_ptr() into one function per
direction (one for encoding, one for decoding).

Signed-off-by: Jann Horn <jannh@google.com>
Co-developed-by: Matteo Rizzo <matteorizzo@google.com>
Signed-off-by: Matteo Rizzo <matteorizzo@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/slub.c

@@ -360,13 +360,19 @@ static struct workqueue_struct *flushwq;
  * 			Core slab cache functions
  *******************************************************************/
 
+/*
+ * freeptr_t represents a SLUB freelist pointer, which might be encoded
+ * and not dereferenceable if CONFIG_SLAB_FREELIST_HARDENED is enabled.
+ */
+typedef struct { unsigned long v; } freeptr_t;
+
 /*
  * Returns freelist pointer (ptr). With hardening, this is obfuscated
  * with an XOR of the address where the pointer is held and a per-cache
  * random number.
  */
-static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
-				 unsigned long ptr_addr)
+static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s,
+					    void *ptr, unsigned long ptr_addr)
 {
 #ifdef CONFIG_SLAB_FREELIST_HARDENED
 	/*
@@ -379,25 +385,40 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
 	 * calls get_freepointer() with an untagged pointer, which causes the
 	 * freepointer to be restored incorrectly.
 	 */
-	return (void *)((unsigned long)ptr ^ s->random ^
-			swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
+	return (freeptr_t){.v = (unsigned long)ptr ^ s->random ^
+			swab((unsigned long)kasan_reset_tag((void *)ptr_addr))};
 #else
-	return ptr;
+	return (freeptr_t){.v = (unsigned long)ptr};
 #endif
 }
 
+static inline void *freelist_ptr_decode(const struct kmem_cache *s,
+					freeptr_t ptr, unsigned long ptr_addr)
+{
+	void *decoded;
+
+#ifdef CONFIG_SLAB_FREELIST_HARDENED
+	/* See the comment in freelist_ptr_encode */
+	decoded = (void *)(ptr.v ^ s->random ^
+		swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
+#else
+	decoded = (void *)ptr.v;
+#endif
+	return decoded;
+}
+
 /* Returns the freelist pointer recorded at location ptr_addr. */
 static inline void *freelist_dereference(const struct kmem_cache *s,
 					 void *ptr_addr)
 {
-	return freelist_ptr(s, (void *)*(unsigned long *)(ptr_addr),
+	return freelist_ptr_decode(s, *(freeptr_t *)(ptr_addr),
 			    (unsigned long)ptr_addr);
 }
 
 static inline void *get_freepointer(struct kmem_cache *s, void *object)
 {
 	object = kasan_reset_tag(object);
-	return freelist_dereference(s, object + s->offset);
+	return freelist_dereference(s, (freeptr_t *)(object + s->offset));
 }
 
 #ifndef CONFIG_SLUB_TINY
@@ -421,15 +442,15 @@ __no_kmsan_checks
 static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
 {
 	unsigned long freepointer_addr;
-	void *p;
+	freeptr_t p;
 
 	if (!debug_pagealloc_enabled_static())
 		return get_freepointer(s, object);
 
 	object = kasan_reset_tag(object);
 	freepointer_addr = (unsigned long)object + s->offset;
-	copy_from_kernel_nofault(&p, (void **)freepointer_addr, sizeof(p));
-	return freelist_ptr(s, p, freepointer_addr);
+	copy_from_kernel_nofault(&p, (freeptr_t *)freepointer_addr, sizeof(p));
+	return freelist_ptr_decode(s, p, freepointer_addr);
 }
 
 static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
@@ -441,7 +462,7 @@ static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
 #endif
 
 	freeptr_addr = (unsigned long)kasan_reset_tag((void *)freeptr_addr);
-	*(void **)freeptr_addr = freelist_ptr(s, fp, freeptr_addr);
+	*(freeptr_t *)freeptr_addr = freelist_ptr_encode(s, fp, freeptr_addr);
 }
 
 /* Loop over all objects in a slab */