kfence: make compatible with kmemleak

Because memblock allocations are registered with kmemleak, the KFENCE
pool was seen by kmemleak as one large object.  Later allocations
through kfence_alloc() that were registered with kmemleak via
slab_post_alloc_hook() would then overlap and trigger a warning.
Therefore, once the pool is initialized, we can remove (free) it from
kmemleak again, since it should be treated as allocator-internal and be
seen as "free memory".

The second problem is that kmemleak is passed the rounded size, and not
the originally requested size, which is also the size of KFENCE objects.
To avoid kmemleak scanning past the end of an object and trigger a
KFENCE out-of-bounds error, fix the size if it is a KFENCE object.

For simplicity, to avoid a call to kfence_ksize() in
slab_post_alloc_hook() (and avoid new IS_ENABLED(CONFIG_DEBUG_KMEMLEAK)
guard), just call kfence_ksize() in mm/kmemleak.c:create_object().

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Marco Elver <[email protected]>
Reported-by: Luis Henriques <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Tested-by: Luis Henriques <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/kfence/core.c

@@ -12,6 +12,7 @@
 #include <linux/debugfs.h>
 #include <linux/kcsan-checks.h>
 #include <linux/kfence.h>
+#include <linux/kmemleak.h>
 #include <linux/list.h>
 #include <linux/lockdep.h>
 #include <linux/memblock.h>
@@ -480,6 +481,14 @@ static bool __init kfence_init_pool(void)
 		addr += 2 * PAGE_SIZE;
 	}
 
+	/*
+	 * The pool is live and will never be deallocated from this point on.
+	 * Remove the pool object from the kmemleak object tree, as it would
+	 * otherwise overlap with allocations returned by kfence_alloc(), which
+	 * are registered with kmemleak through the slab post-alloc hook.
+	 */
+	kmemleak_free(__kfence_pool);
+
 	return true;
 
 err:

mm/kmemleak.c

@@ -97,6 +97,7 @@
 #include <linux/atomic.h>
 
 #include <linux/kasan.h>
+#include <linux/kfence.h>
 #include <linux/kmemleak.h>
 #include <linux/memory_hotplug.h>
 
@@ -589,7 +590,7 @@ static struct kmemleak_object *create_object(unsigned long ptr, size_t size,
 	atomic_set(&object->use_count, 1);
 	object->flags = OBJECT_ALLOCATED;
 	object->pointer = ptr;
-	object->size = size;
+	object->size = kfence_ksize((void *)ptr) ?: size;
 	object->excess_ref = 0;
 	object->min_count = min_count;
 	object->count = 0;			/* white color initially */