linux-kernel - [PATCH v3] bus: fsl-mc: fix invalid free in fsl_mc_device

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Message-Id: <20200709153119.5051-1-trix@redhat.com>
Date:   Thu,  9 Jul 2020 08:31:19 -0700
From:   trix@...hat.com
To:     stuyoder@...il.com, laurentiu.tudor@....com,
        gregkh@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org, Tom Rix <trix@...hat.com>
Subject: [PATCH v3] bus: fsl-mc: fix invalid free in fsl_mc_device_add

From: Tom Rix <trix@...hat.com>

clang static analysis flags this error

fsl-mc-bus.c:695:2: warning: Attempt to free released memory [unix.Malloc]
        kfree(mc_dev);
        ^~~~~~~~~~~~~

The problem block of code is

		mc_bus = kzalloc(sizeof(*mc_bus), GFP_KERNEL);
		if (!mc_bus)
			return -ENOMEM;

		mc_dev = &mc_bus->mc_dev;

mc_bus's structure contains a mc_dev element,
freeing it later is not appropriate.

So check if mc_bus was allocated before freeing mc_dev

This is a case where checkpatch

WARNING: kfree(NULL) is safe and this check is probably not required
+	if (mc_bus)
+		kfree(mc_bus);

is wrong.

Fixes: a042fbed0290 ("staging: fsl-mc: simplify couple of deallocations")

Signed-off-by: Tom Rix <trix@...hat.com>
---
 v1: add a comment to explain freeing uniqueness
 v2: add gregkh's suggestion to comment.
 
 drivers/bus/fsl-mc/fsl-mc-bus.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/bus/fsl-mc/fsl-mc-bus.c b/drivers/bus/fsl-mc/fsl-mc-bus.c
index 40526da5c6a6..839d96d03f0d 100644
--- a/drivers/bus/fsl-mc/fsl-mc-bus.c
+++ b/drivers/bus/fsl-mc/fsl-mc-bus.c
@@ -691,8 +691,16 @@ int fsl_mc_device_add(struct fsl_mc_obj_desc *obj_desc,
 
 error_cleanup_dev:
 	kfree(mc_dev->regions);
-	kfree(mc_bus);
-	kfree(mc_dev);
+	/*
+	 * mc_bus allocates a private version of mc_dev
+	 * it is not appropriate to free the private version.
+	 * Which means we have to check the pointer before freeing it.
+	 * Do not remove this check.
+	 */
+	if (mc_bus)
+		kfree(mc_bus);
+	else
+		kfree(mc_dev);
 
 	return error;
 }
-- 
2.18.1