kasan: slab: always reset the tag in get_freepointer_safe()

With CONFIG_DEBUG_PAGEALLOC enabled, the kernel should also untag the
object pointer, as done in get_freepointer().

Failing to do so reportedly leads to SLUB freelist corruptions that
manifest as boot-time crashes.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alexander Potapenko <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Elliot Berman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/slub.c

@@ -301,6 +301,7 @@ static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
 	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);