mtd: docg3: fix use-after-free in docg3_release()

In docg3_release(), the docg3 pointer is obtained from
cascade->floors[0]->priv before the loop that calls
doc_release_device() on each floor. doc_release_device() frees the
docg3 struct via kfree(docg3) at line 1881. After the loop,
docg3->cascade->bch dereferences the already-freed pointer.

Fix this by accessing cascade->bch directly, which is equivalent
since docg3->cascade points back to the same cascade struct, and
is already available as a local variable. This also removes the
now-unused docg3 local variable.

Fixes: c8ae3f7 ("lib/bch: Rework a little bit the exported function names")
Cc: stable@vger.kernel.org
Signed-off-by: James Kim <james010kim@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

Author: james010kim

drivers/mtd/devices/docg3.c

@@ -2049,15 +2049,14 @@ static int __init docg3_probe(struct platform_device *pdev)
 static void docg3_release(struct platform_device *pdev)
 {
 	struct docg3_cascade *cascade = platform_get_drvdata(pdev);
-	struct docg3 *docg3 = cascade->floors[0]->priv;
 	int floor;
 
 	doc_unregister_sysfs(pdev, cascade);
 	for (floor = 0; floor < DOC_MAX_NBFLOORS; floor++)
 		if (cascade->floors[floor])
 			doc_release_device(cascade->floors[floor]);
 
-	bch_free(docg3->cascade->bch);
+	bch_free(cascade->bch);
 }
 
 #ifdef CONFIG_OF