mm, slub: avoid zeroing kmalloc redzone

With commit 946fa0d
("mm/slub: extend redzone check to extra allocated kmalloc space than requested"),
setting orig_size treats the wasted space (object_size - orig_size) as
redzones. But (in check_object()) when orig_size is set to zero, the entire
object is perceived as a redzone. To a valid allocated kmalloc space,
when init_on_free=1, the wasted space and the orig_size should
not be cleared to 0, otherwise there will be kernel dump:

[    0.000000] =============================================================================
[    0.000000] BUG kmalloc-8 (Not tainted): kmalloc Redzone overwritten
[    0.000000] -----------------------------------------------------------------------------
[    0.000000]
[    0.000000] 0xffff000010032858-0xffff00001003285f @offset=2136. First byte 0x0 instead of 0xcc
[    0.000000] FIX kmalloc-8: Restoring kmalloc Redzone 0xffff000010032858-0xffff00001003285f=0xcc
[    0.000000] Slab 0xfffffdffc0400c80 objects=36 used=23 fp=0xffff000010032a18 flags=0x3fffe0000000200(workingset|node=0|zone=0|lastcpupid=0x1ffff)
[    0.000000] Object 0xffff000010032858 @offset=2136 fp=0xffff0000100328c8
[    0.000000]
[    0.000000] Redzone  ffff000010032850: cc cc cc cc cc cc cc cc                          ........
[    0.000000] Object   ffff000010032858: cc cc cc cc cc cc cc cc                          ........
[    0.000000] Redzone  ffff000010032860: cc cc cc cc cc cc cc cc                          ........
[    0.000000] Padding  ffff0000100328b4: 00 00 00 00 00 00 00 00 00 00 00 00              ............
[    0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.11.0-rc3-next-20240814-00004-g61844c55c3f4 torvalds#144
[    0.000000] Hardware name: NXP i.MX95 19X19 board (DT)
[    0.000000] Call trace:
[    0.000000]  dump_backtrace+0x90/0xe8
[    0.000000]  show_stack+0x18/0x24
[    0.000000]  dump_stack_lvl+0x74/0x8c
[    0.000000]  dump_stack+0x18/0x24
[    0.000000]  print_trailer+0x150/0x218
[    0.000000]  check_object+0xe4/0x454
[    0.000000]  free_to_partial_list+0x2f8/0x5ec

To address the issue, use orig_size to clear the used area. And restore
the value of orig_size after clear the remaining area.

When CONFIG_SLUB_DEBUG not defined, (get_orig_size()' directly returns
s->object_size. So when using memset to init the area, the size can simply
be orig_size, as orig_size returns object_size when CONFIG_SLUB_DEBUG not
enabled. And orig_size can never be bigger than object_size.

Fixes: 946fa0d ("mm/slub: extend redzone check to extra allocated kmalloc space than requested")
Signed-off-by: Peng Fan <[email protected]>

mm/slub.c

@@ -1895,6 +1895,15 @@ static inline void inc_slabs_node(struct kmem_cache *s, int node,
 static inline void dec_slabs_node(struct kmem_cache *s, int node,
 							int objects) {}
 
+static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
+{
+	return s->object_size;
+}
+
+static inline void set_orig_size(struct kmem_cache *s, void *object,
+				 unsigned int orig_size)
+{}
+
 #ifndef CONFIG_SLUB_TINY
 static bool freelist_corrupted(struct kmem_cache *s, struct slab *slab,
 			       void **freelist, void *nextfree)
@@ -2286,14 +2295,21 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init,
 	 */
 	if (unlikely(init)) {
 		int rsize;
-		unsigned int inuse;
+		unsigned int inuse, orig_size;
 
 		inuse = get_info_end(s);
+		orig_size = get_orig_size(s, x);
 		if (!kasan_has_integrated_init())
-			memset(kasan_reset_tag(x), 0, s->object_size);
+			memset(kasan_reset_tag(x), 0, orig_size);
 		rsize = (s->flags & SLAB_RED_ZONE) ? s->red_left_pad : 0;
 		memset((char *)kasan_reset_tag(x) + inuse, 0,
 		       s->size - inuse - rsize);
+		/*
+		 * Restore orig_size, otherwize kmalloc redzone overwritten
+		 * would be reported
+		 */
+		set_orig_size(s, x, orig_size);
+
 	}
 	/* KASAN might put x into memory quarantine, delaying its reuse. */
 	return !kasan_slab_free(s, x, init, still_accessible);