fs/super.c: Add NULL check for type in iterate_supers_type

[PATCH v1] fs/super.c: Add NULL check for type in iterate_supers_type

Posted by Xiaole He 10 months, 1 week ago

The first several lines of iterate_supers_type are below:

1 void iterate_supers_type(struct file_system_type *type,
2 	void (*f)(struct super_block *, void *), void *arg)
3 {
4 	struct super_block *sb, *p = NULL;
5
6 	spin_lock(&sb_lock);
7 	hlist_for_each_entry(sb, &type->fs_supers, s_instances) {
8 	...
9 }

The iterate_super_type is a exported symbol, and if iterate_supers_type
is called with type of NULL, then there will be a NULL pointer
dereference of argument type in line 7.

This patch fix above problem by adding NULL pointer check for argument
type.

Signed-off-by: Xiaole He <hexiaole1994@126.com>
---
 fs/super.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 5a7db4a556e3..105a275b8360 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -959,6 +959,8 @@ void iterate_supers_type(struct file_system_type *type,
 {
 	struct super_block *sb, *p = NULL;
 
+	if (unlikely(ZERO_OR_NULL_PTR(type)))
+		return;
 	spin_lock(&sb_lock);
 	hlist_for_each_entry(sb, &type->fs_supers, s_instances) {
 		bool locked;
-- 
2.43.0

Re: [PATCH v1] fs/super.c: Add NULL check for type in iterate_supers_type

Posted by Christian Brauner 10 months, 1 week ago

On Wed, Apr 02, 2025 at 11:45:29AM +0800, Xiaole He wrote:
> The first several lines of iterate_supers_type are below:
> 
> 1 void iterate_supers_type(struct file_system_type *type,
> 2 	void (*f)(struct super_block *, void *), void *arg)
> 3 {
> 4 	struct super_block *sb, *p = NULL;
> 5
> 6 	spin_lock(&sb_lock);
> 7 	hlist_for_each_entry(sb, &type->fs_supers, s_instances) {
> 8 	...
> 9 }
> 
> The iterate_super_type is a exported symbol, and if iterate_supers_type
> is called with type of NULL, then there will be a NULL pointer
> dereference of argument type in line 7.
> 
> This patch fix above problem by adding NULL pointer check for argument
> type.
> 
> Signed-off-by: Xiaole He <hexiaole1994@126.com>
> ---

Both Al and James already pointed out that this check is unnecessary and
ultimately misleading. So we're not going to accept this patch.

Re: [PATCH v1] fs/super.c: Add NULL check for type in iterate_supers_type

Posted by James Bottomley 10 months, 1 week ago

On Wed, 2025-04-02 at 11:45 +0800, Xiaole He wrote:
> The first several lines of iterate_supers_type are below:
> 
> 1 void iterate_supers_type(struct file_system_type *type,
> 2 	void (*f)(struct super_block *, void *), void *arg)
> 3 {
> 4 	struct super_block *sb, *p = NULL;
> 5
> 6 	spin_lock(&sb_lock);
> 7 	hlist_for_each_entry(sb, &type->fs_supers, s_instances) {
> 8 	...
> 9 }
> 
> The iterate_super_type is a exported symbol, and if
> iterate_supers_type is called with type of NULL, then there will be a
> NULL pointer dereference of argument type in line 7.

filesystem_type is an argument to alloc_super, which the filesystems
code always fills in.  If a filesystem passed a NULL type to the
context, the initialization code would crash on a NULL deref (iterating
type->fs_supers) which makes what you're checking for here an
impossible condition, doesn't it?

Regards,

James