[PATCH] kernfs: fix possibility of NULL pointer dereference.

[PATCH] kernfs: fix possibility of NULL pointer dereference.

[youngjun]

When dentry is negative, "kernfs_dentry_node" returns NULL.
In this case, "kernfs_root" dereferences NULL pointer.

Signed-off-by: youngjun <her0gyugyu@gmail.com>
---
 fs/kernfs/dir.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 9aec80b9d7c6..02fb5cc76e33 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1133,15 +1133,16 @@ static int kernfs_iop_mkdir(struct inode *dir, struct dentry *dentry,
 static int kernfs_iop_rmdir(struct inode *dir, struct dentry *dentry)
 {
 	struct kernfs_node *kn  = kernfs_dentry_node(dentry);
-	struct kernfs_syscall_ops *scops = kernfs_root(kn)->syscall_ops;
+	struct kernfs_syscall_ops *scops;
 	int ret;
 
-	if (!scops || !scops->rmdir)
-		return -EPERM;
-
 	if (!kernfs_get_active(kn))
 		return -ENODEV;
 
+	scops = kernfs_root(kn)->syscall_ops;
+	if (!scops || !scops->rmdir)
+		return -EPERM;
+
 	ret = scops->rmdir(kn);
 
 	kernfs_put_active(kn);
@@ -1154,18 +1155,19 @@ static int kernfs_iop_rename(struct inode *old_dir, struct dentry *old_dentry,
 {
 	struct kernfs_node *kn = kernfs_dentry_node(old_dentry);
 	struct kernfs_node *new_parent = new_dir->i_private;
-	struct kernfs_syscall_ops *scops = kernfs_root(kn)->syscall_ops;
+	struct kernfs_syscall_ops *scops;
 	int ret;
 
 	if (flags)
 		return -EINVAL;
 
-	if (!scops || !scops->rename)
-		return -EPERM;
-
 	if (!kernfs_get_active(kn))
 		return -ENODEV;
 
+	scops = kernfs_root(kn)->syscall_ops;
+	if (!scops || !scops->rename)
+		return -EPERM;
+
 	if (!kernfs_get_active(new_parent)) {
 		kernfs_put_active(kn);
 		return -ENODEV;
-- 
2.17.1

Re: [PATCH] kernfs: fix possibility of NULL pointer dereference.

[Greg Kroah-Hartman]

On Mon, Apr 27, 2020 at 09:48:36AM -0700, youngjun wrote:
> When dentry is negative, "kernfs_dentry_node" returns NULL.
> In this case, "kernfs_root" dereferences NULL pointer.
> 
> Signed-off-by: youngjun <her0gyugyu@gmail.com>
> ---
>  fs/kernfs/dir.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)

And how can this be triggered on a kernfs filesystem?

thanks,

greg k-h