firmware: qemu_fw_cfg: misc bug fixes

When searching for the latest umn.edu patches I stumbled upon a recently submitted fix for a bug purportedly introduced by umn.edu:

https://lore.kernel.org/r/[email protected]

The patch in question is obviously bogus, but a closer look at the function in question revealed a number of other bugs, including one that was indeed introduced by umn.edu but that was missed during the (re-)review.

Included is also a related clean up.

Johan

Johan Hovold (4):
firmware: qemu_fw_cfg: fix NULL-pointer deref on duplicate entries
firmware: qemu_fw_cfg: fix kobject leak in probe error path
firmware: qemu_fw_cfg: fix sysfs information leak
firmware: qemu_fw_cfg: remove sysfs entries explicitly

 drivers/firmware/qemu_fw_cfg.c | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

2.32.0

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/[email protected]

---
 drivers/firmware/qemu_fw_cfg.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

index 172c751a4f6c..a9c64ebfc49a 100644
@@ -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

An initialised kobject must be freed using kobject_put() to avoid leaking associated resources (e.g. the object name).

Commit fe3c60684377 ("firmware: Fix a reference count leak.") "fixed" the leak in the first error path of the file registration helper but left the second one unchanged. This "fix" would however result in a NULL pointer dereference due to the release function also removing the never added entry from the fw_cfg_entry_cache list. This has now been addressed.

Fix the remaining kobject leak by restoring the common error path and adding the missing kobject_put().

---
 drivers/firmware/qemu_fw_cfg.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

index a9c64ebfc49a..ccb7ed62452f 100644
@@ -603,15 +603,13 @@ static int fw_cfg_register_file(const struct fw_cfg_file *f)
 	/* register entry under "/sys/firmware/qemu_fw_cfg/by_key/" */
 	err = kobject_init_and_add(&entry->kobj, &fw_cfg_sysfs_entry_ktype,
 				   fw_cfg_sel_ko, "%d", entry->select);
-	if (err) {
-		kobject_put(&entry->kobj);
-		return err;
-	}
+	if (err)
+		goto err_put_entry;
 
 	/* add raw binary content access */
 	err = sysfs_create_bin_file(&entry->kobj, &fw_cfg_sysfs_attr_raw);
 	if (err)
-		goto err_add_raw;
+		goto err_del_entry;
 
 	/* try adding "/sys/firmware/qemu_fw_cfg/by_name/" symlink */
 	fw_cfg_build_symlink(fw_cfg_fname_kset, &entry->kobj, entry->name);

@@ -620,9 +618,10 @@ static int fw_cfg_register_file(const struct fw_cfg_file *f)
 	fw_cfg_sysfs_cache_enlist(entry);
 	return 0;
 
-err_add_raw:
+err_del_entry:
 	kobject_del(&entry->kobj);
-	kfree(entry);
+err_put_entry:
+	kobject_put(&entry->kobj);
 	return err;
 }
 

2.32.0

Make sure to always NUL-terminate file names retrieved from the firmware to avoid accessing data beyond the entry slab buffer and exposing it through sysfs in case the firmware data is corrupt.

---
 drivers/firmware/qemu_fw_cfg.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index ccb7ed62452f..f08e056ed0ae 100644
@@ -598,7 +598,7 @@ static int fw_cfg_register_file(const struct fw_cfg_file *f)
 	/* set file entry information */
 	entry->size = be32_to_cpu(f->size);
 	entry->select = be16_to_cpu(f->select);
-	memcpy(entry->name, f->name, FW_CFG_MAX_FILE_PATH);
+	strscpy(entry->name, f->name, FW_CFG_MAX_FILE_PATH);
 
 	/* register entry under "/sys/firmware/qemu_fw_cfg/by_key/" */
 	err = kobject_init_and_add(&entry->kobj, &fw_cfg_sysfs_entry_ktype,

2.32.0

Explicitly remove the file entries from sysfs before dropping the final reference for symmetry reasons and for consistency with the rest of the driver.

---
 drivers/firmware/qemu_fw_cfg.c | 1 +
 1 file changed, 1 insertion(+)

index f08e056ed0ae..b436342115af 100644
@@ -389,6 +389,7 @@ static void fw_cfg_sysfs_cache_cleanup(void)
 
 	list_for_each_entry_safe(entry, next, &fw_cfg_entry_cache, list) {
 		fw_cfg_sysfs_cache_delist(entry);
+		kobject_del(&entry->kobj);
 		kobject_put(&entry->kobj);
 	}
 }

2.32.0