regmap: Fix possible double-free in regcache_rbtree_exit()

In regcache_rbtree_insert_to_block(), when 'present' realloc failed,
the 'blk' which is supposed to assign to 'rbnode->block' will be freed,
so 'rbnode->block' points a freed memory, in the error handling path of
regcache_rbtree_init(), 'rbnode->block' will be freed again in
regcache_rbtree_exit(), KASAN will report double-free as follows:

BUG: KASAN: double-free or invalid-free in kfree+0xce/0x390
Call Trace:
 slab_free_freelist_hook+0x10d/0x240
 kfree+0xce/0x390
 regcache_rbtree_exit+0x15d/0x1a0
 regcache_rbtree_init+0x224/0x2c0
 regcache_init+0x88d/0x1310
 __regmap_init+0x3151/0x4a80
 __devm_regmap_init+0x7d/0x100
 madera_spi_probe+0x10f/0x333 [madera_spi]
 spi_probe+0x183/0x210
 really_probe+0x285/0xc30

To fix this, moving up the assignment of rbnode->block to immediately after
the reallocation has succeeded so that the data structure stays valid even
if the second reallocation fails.
Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 3f4ff561 ("regmap: rbtree: Make cache_present bitmap per node")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20211012023735.1632786-1-yangyingliang@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regcache-rbtree.c

@@ -281,14 +281,14 @@ static int regcache_rbtree_insert_to_block(struct regmap *map,
 	if (!blk)
 		return -ENOMEM;
 
+	rbnode->block = blk;
+
 	if (BITS_TO_LONGS(blklen) > BITS_TO_LONGS(rbnode->blklen)) {
 		present = krealloc(rbnode->cache_present,
 				   BITS_TO_LONGS(blklen) * sizeof(*present),
 				   GFP_KERNEL);
-		if (!present) {
-			kfree(blk);
+		if (!present)
 			return -ENOMEM;
-		}
 
 		memset(present + BITS_TO_LONGS(rbnode->blklen), 0,
 		       (BITS_TO_LONGS(blklen) - BITS_TO_LONGS(rbnode->blklen))
@@ -305,7 +305,6 @@ static int regcache_rbtree_insert_to_block(struct regmap *map,
 	}
 
 	/* update the rbnode block, its size and the base register */
-	rbnode->block = blk;
 	rbnode->blklen = blklen;
 	rbnode->base_reg = base_reg;
 	rbnode->cache_present = present;