Re: [PATCH v8 1/4] fs: add mode_strip_sgid() helper

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2022-04-26 at 19:11 +0800, Yang Xu wrote:
> Add a dedicated helper to handle the setgid bit when creating a new file
> in a setgid directory. This is a preparatory patch for moving setgid
> stripping into the vfs. The patch contains no functional changes.
> 
> Currently the setgid stripping logic is open-coded directly in
> inode_init_owner() and the individual filesystems are responsible for
> handling setgid inheritance. Since this has proven to be brittle as
> evidenced by old issues we uncovered over the last months (see [1] to
> [3] below) we will try to move this logic into the vfs.
> 
> Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [1]
> Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [2]
> Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [3]
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
> Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
> ---
>  fs/inode.c         | 37 +++++++++++++++++++++++++++++++++----
>  include/linux/fs.h |  2 ++
>  2 files changed, 35 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 9d9b422504d1..e9a5f2ec2f89 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2246,10 +2246,8 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
>  		/* Directories are special, and always inherit S_ISGID */
>  		if (S_ISDIR(mode))
>  			mode |= S_ISGID;
> -		else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
> -			 !in_group_p(i_gid_into_mnt(mnt_userns, dir)) &&
> -			 !capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
> -			mode &= ~S_ISGID;
> +		else
> +			mode = mode_strip_sgid(mnt_userns, dir, mode);
>  	} else
>  		inode_fsgid_set(inode, mnt_userns);
>  	inode->i_mode = mode;
> @@ -2405,3 +2403,34 @@ struct timespec64 current_time(struct inode *inode)
>  	return timestamp_truncate(now, inode);
>  }
>  EXPORT_SYMBOL(current_time);
> +
> +/**
> + * mode_strip_sgid - handle the sgid bit for non-directories
> + * @mnt_userns: User namespace of the mount the inode was created from
> + * @dir: parent directory inode
> + * @mode: mode of the file to be created in @dir
> + *
> + * If the @mode of the new file has both the S_ISGID and S_IXGRP bit
> + * raised and @dir has the S_ISGID bit raised ensure that the caller is
> + * either in the group of the parent directory or they have CAP_FSETID
> + * in their user namespace and are privileged over the parent directory.
> + * In all other cases, strip the S_ISGID bit from @mode.
> + *
> + * Return: the new mode to use for the file
> + */
> +umode_t mode_strip_sgid(struct user_namespace *mnt_userns,
> +			 const struct inode *dir, umode_t mode)
> +{
> +	if (S_ISDIR(mode) || !dir || !(dir->i_mode & S_ISGID))
> +		return mode;
> +	if ((mode & (S_ISGID | S_IXGRP)) != (S_ISGID | S_IXGRP))
> +		return mode;
> +	if (in_group_p(i_gid_into_mnt(mnt_userns, dir)))
> +		return mode;
> +	if (capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
> +		return mode;
> +
> +	mode &= ~S_ISGID;
> +	return mode;
> +}
> +EXPORT_SYMBOL(mode_strip_sgid);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index bbde95387a23..98b44a2732f5 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1897,6 +1897,8 @@ extern long compat_ptr_ioctl(struct file *file, unsigned int cmd,
>  void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
>  		      const struct inode *dir, umode_t mode);
>  extern bool may_open_dev(const struct path *path);
> +umode_t mode_strip_sgid(struct user_namespace *mnt_userns,
> +			 const struct inode *dir, umode_t mode);
>  
>  /*
>   * This is the "filldir" function type, used by readdir() to let

This series looks like a nice cleanup. I went ahead and added this pile
to another kernel I was testing with xfstests and it seemed to do fine.

You can add this (or some variant of it) to all 4 patches.

Reviewed-and-Tested-by: Jeff Layton <jlayton@kernel.org>