Re: [PATCH v2 2/5] fs: create proper filename objects using getname_kernel()

[Richard Guy Briggs]

On 15/01/22, Paul Moore wrote:
> There are several areas in the kernel that create temporary filename
> objects using the following pattern:
> 
> 	int func(const char *name)
> 	{
> 		struct filename *file = { .name = name };
> 		...
> 		return 0;
> 	}
> 
> ... which for the most part works okay, but it causes havoc within the
> audit subsystem as the filename object does not persist beyond the
> lifetime of the function.  This patch converts all of these temporary
> filename objects into proper filename objects using getname_kernel()
> and putname() which ensure that the filename object persists until the
> audit subsystem is finished with it.
> 
> Also, a special thanks to Al Viro, Guenter Roeck, and Sabrina Dubroca
> for helping resolve a difficult kernel panic on boot related to a
> use-after-free problem in kern_path_create(); the thread can be seen
> at the link below:
> 
>  * https://lkml.org/lkml/2015/1/20/710
> 
> This patch includes code that was either based on, or directly written
> by Al in the above thread.
> 
> CC: viro@xxxxxxxxxxxxxxxxxx
> CC: linux@xxxxxxxxxxxx
> CC: sd@xxxxxxxxxxxxxxx
> CC: linux-fsdevel@xxxxxxxxxxxxxxx
> Signed-off-by: Paul Moore <pmoore@xxxxxxxxxx>

Noted change to use ERR_CAST() for consistency.

Reviewed-by: Richard Guy Briggs <rgb@xxxxxxxxxx>

> ---
>  fs/exec.c  |   11 +++++++-
>  fs/namei.c |   80 +++++++++++++++++++++++++++++++++++++++++++++++-------------
>  fs/open.c  |   11 +++++++-
>  3 files changed, 81 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index a3d33fe..d067771 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -789,8 +789,15 @@ exit:
>  
>  struct file *open_exec(const char *name)
>  {
> -	struct filename tmp = { .name = name };
> -	return do_open_exec(&tmp);
> +	struct file *file;
> +	struct filename *tmp;
> +
> +	tmp = getname_kernel(name);
> +	if (unlikely(IS_ERR(tmp)))
> +		return (void *)tmp;
> +	file = do_open_exec(tmp);
> +	putname(tmp);
> +	return file;
>  }
>  EXPORT_SYMBOL(open_exec);
>  
> diff --git a/fs/namei.c b/fs/namei.c
> index 63eaaf6..f793fe4 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2001,31 +2001,50 @@ static int filename_lookup(int dfd, struct filename *name,
>  static int do_path_lookup(int dfd, const char *name,
>  				unsigned int flags, struct nameidata *nd)
>  {
> -	struct filename filename = { .name = name };
> +	int retval;
> +	struct filename *filename;
>  
> -	return filename_lookup(dfd, &filename, flags, nd);
> +	filename = getname_kernel(name);
> +	if (unlikely(IS_ERR(filename)))
> +		return PTR_ERR(filename);
> +	retval = filename_lookup(dfd, filename, flags, nd);
> +	putname(filename);
> +	return retval;
>  }
>  
>  /* does lookup, returns the object with parent locked */
>  struct dentry *kern_path_locked(const char *name, struct path *path)
>  {
> +	struct filename *filename;
>  	struct nameidata nd;
>  	struct dentry *d;
> -	int err = do_path_lookup(AT_FDCWD, name, LOOKUP_PARENT, &nd);
> -	if (err)
> -		return ERR_PTR(err);
> +	int err;
> +
> +	filename = getname_kernel(name);
> +	if (IS_ERR(filename))
> +		return ERR_CAST(filename);
> +
> +	err = filename_lookup(AT_FDCWD, filename, LOOKUP_PARENT, &nd);
> +	if (err) {
> +		d = ERR_PTR(err);
> +		goto out;
> +	}
>  	if (nd.last_type != LAST_NORM) {
>  		path_put(&nd.path);
> -		return ERR_PTR(-EINVAL);
> +		d = ERR_PTR(-EINVAL);
> +		goto out;
>  	}
>  	mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
>  	d = __lookup_hash(&nd.last, nd.path.dentry, 0);
>  	if (IS_ERR(d)) {
>  		mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
>  		path_put(&nd.path);
> -		return d;
> +		goto out;
>  	}
>  	*path = nd.path;
> +
> +out:
> +	putname(filename);
>  	return d;
>  }
>  
> @@ -2368,8 +2387,15 @@ int
>  kern_path_mountpoint(int dfd, const char *name, struct path *path,
>  			unsigned int flags)
>  {
> -	struct filename s = {.name = name};
> -	return filename_mountpoint(dfd, &s, path, flags);
> +	int retval;
> +	struct filename *s;
> +
> +	s = getname_kernel(name);
> +	if (unlikely(IS_ERR(s)))
> +		return PTR_ERR(s);
> +	retval = filename_mountpoint(dfd, s, path, flags);
> +	putname(s);
> +	return retval;
>  }
>  EXPORT_SYMBOL(kern_path_mountpoint);
>  
> @@ -3259,7 +3285,7 @@ struct file *do_file_open_root(struct dentry *dentry, struct vfsmount *mnt,
>  {
>  	struct nameidata nd;
>  	struct file *file;
> -	struct filename filename = { .name = name };
> +	struct filename *filename;
>  	int flags = op->lookup_flags | LOOKUP_ROOT;
>  
>  	nd.root.mnt = mnt;
> @@ -3268,16 +3294,22 @@ struct file *do_file_open_root(struct dentry *dentry, struct vfsmount *mnt,
>  	if (d_is_symlink(dentry) && op->intent & LOOKUP_OPEN)
>  		return ERR_PTR(-ELOOP);
>  
> -	file = path_openat(-1, &filename, &nd, op, flags | LOOKUP_RCU);
> +	filename = getname_kernel(name);
> +	if (unlikely(IS_ERR(filename)))
> +		return ERR_CAST(filename);
> +
> +	file = path_openat(-1, filename, &nd, op, flags | LOOKUP_RCU);
>  	if (unlikely(file == ERR_PTR(-ECHILD)))
> -		file = path_openat(-1, &filename, &nd, op, flags);
> +		file = path_openat(-1, filename, &nd, op, flags);
>  	if (unlikely(file == ERR_PTR(-ESTALE)))
> -		file = path_openat(-1, &filename, &nd, op, flags | LOOKUP_REVAL);
> +		file = path_openat(-1, filename, &nd, op, flags | LOOKUP_REVAL);
> +	putname(filename);
>  	return file;
>  }
>  
> -struct dentry *kern_path_create(int dfd, const char *pathname,
> -				struct path *path, unsigned int lookup_flags)
> +static struct dentry *filename_create(int dfd, struct filename *name,
> +				      struct path *path,
> +				      unsigned int lookup_flags)
>  {
>  	struct dentry *dentry = ERR_PTR(-EEXIST);
>  	struct nameidata nd;
> @@ -3291,7 +3323,7 @@ struct dentry *kern_path_create(int dfd, const char *pathname,
>  	 */
>  	lookup_flags &= LOOKUP_REVAL;
>  
> -	error = do_path_lookup(dfd, pathname, LOOKUP_PARENT|lookup_flags, &nd);
> +	error = filename_lookup(dfd, name, LOOKUP_PARENT|lookup_flags, &nd);
>  	if (error)
>  		return ERR_PTR(error);
>  
> @@ -3345,6 +3377,20 @@ out:
>  	path_put(&nd.path);
>  	return dentry;
>  }
> +
> +struct dentry *kern_path_create(int dfd, const char *pathname,
> +				struct path *path, unsigned int lookup_flags)
> +{
> +	struct filename *filename;
> +	struct dentry *res;
> +
> +	filename = getname_kernel(pathname);
> +	if (IS_ERR(filename))
> +		return ERR_CAST(filename);
> +	res = filename_create(dfd, filename, path, lookup_flags);
> +	putname(filename);
> +	return res;
> +}
>  EXPORT_SYMBOL(kern_path_create);
>  
>  void done_path_create(struct path *path, struct dentry *dentry)
> @@ -3363,7 +3409,7 @@ struct dentry *user_path_create(int dfd, const char __user *pathname,
>  	struct dentry *res;
>  	if (IS_ERR(tmp))
>  		return ERR_CAST(tmp);
> -	res = kern_path_create(dfd, tmp->name, path, lookup_flags);
> +	res = filename_create(dfd, tmp, path, lookup_flags);
>  	putname(tmp);
>  	return res;
>  }
> diff --git a/fs/open.c b/fs/open.c
> index d6fd3ac..33839e1 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -940,8 +940,15 @@ struct file *file_open_name(struct filename *name, int flags, umode_t mode)
>   */
>  struct file *filp_open(const char *filename, int flags, umode_t mode)
>  {
> -	struct filename name = {.name = filename};
> -	return file_open_name(&name, flags, mode);
> +	struct file *file;
> +	struct filename *name;
> +
> +	name = getname_kernel(filename);
> +	if (unlikely(IS_ERR(name)))
> +		return ERR_CAST(name);
> +	file = file_open_name(name, flags, mode);
> +	putname(name);
> +	return file;
>  }
>  EXPORT_SYMBOL(filp_open);
>  
> 

- RGB

--
Richard Guy Briggs <rbriggs@xxxxxxxxxx>
Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635, Alt: +1.613.693.0684x3545
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