linux-kernel - [PATCH 1/4] firmware: qemu_fw_cfg: fix NULL-pointer deref on duplicate entries

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Message-Id: <20211201132528.30025-2-johan@kernel.org>
Date:   Wed,  1 Dec 2021 14:25:25 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Gabriel Somlo <somlo@....edu>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>, qemu-devel@...gnu.org,
        linux-kernel@...r.kernel.org, Johan Hovold <johan@...nel.org>,
        stable@...r.kernel.org, Qiushi Wu <wu000273@....edu>
Subject: [PATCH 1/4] firmware: qemu_fw_cfg: fix NULL-pointer deref on duplicate entries

Commit fe3c60684377 ("firmware: Fix a reference count leak.") "fixed"
a kobject leak in the file registration helper by properly calling
kobject_put() for the entry in case registration of the object fails
(e.g. due to a name collision).

This would however result in a NULL pointer dereference when the
release function tries to remove the never added entry from the
fw_cfg_entry_cache list.

Fix this by moving the list-removal out of the release function.

Note that the offending commit was one of the benign looking umn.edu
fixes which was reviewed but not reverted. [1][2]

[1] https://lore.kernel.org/r/202105051005.49BFABCE@keescook
[2] https://lore.kernel.org/all/YIg7ZOZvS3a8LjSv@kroah.com

Fixes: fe3c60684377 ("firmware: Fix a reference count leak.")
Cc: stable@...r.kernel.org      # 5.8
Cc: Qiushi Wu <wu000273@....edu>
Cc: Kees Cook <keescook@...omium.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Johan Hovold <johan@...nel.org>
---
 drivers/firmware/qemu_fw_cfg.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
index 172c751a4f6c..a9c64ebfc49a 100644
--- a/drivers/firmware/qemu_fw_cfg.c
+++ b/drivers/firmware/qemu_fw_cfg.c
@@ -388,9 +388,7 @@ static void fw_cfg_sysfs_cache_cleanup(void)
 	struct fw_cfg_sysfs_entry *entry, *next;
 
 	list_for_each_entry_safe(entry, next, &fw_cfg_entry_cache, list) {
-		/* will end up invoking fw_cfg_sysfs_cache_delist()
-		 * via each object's release() method (i.e. destructor)
-		 */
+		fw_cfg_sysfs_cache_delist(entry);
 		kobject_put(&entry->kobj);
 	}
 }
@@ -448,7 +446,6 @@ static void fw_cfg_sysfs_release_entry(struct kobject *kobj)
 {
 	struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
 
-	fw_cfg_sysfs_cache_delist(entry);
 	kfree(entry);
 }
 
-- 
2.32.0