Re: [PATCH v23 05/22] vfs: Add permission flags for setting file attributes

[Jeff Layton]

On Thu, 2016-06-30 at 15:46 +0200, Andreas Gruenbacher wrote:
> Richacls support permissions that allow to take ownership of a file,
> change the file permissions, and set the file timestamps.ÂÂSupport that
> by introducing new permission mask flags and by checking for those mask
> flags in inode_change_ok().
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
> Reviewed-by: J. Bruce Fields <bfields@xxxxxxxxxx>
> Reviewed-by: Steve French <steve.french@xxxxxxxxxxxxxxx>
> ---
> Âfs/attr.cÂÂÂÂÂÂÂÂÂÂ| 79 +++++++++++++++++++++++++++++++++++++++++++++---------
> Âinclude/linux/fs.h |ÂÂ3 +++
> Â2 files changed, 70 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/attr.c b/fs/attr.c
> index 7ca7fa0..2a8c49c 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -17,6 +17,65 @@
> Â#include 
> Â
> Â/**
> + * inode_extended_permissionÂÂ-ÂÂpermissions beyond read/write/execute
> + *
> + * Check for permissions that only richacls can currently grant.
> + */
> +static int inode_extended_permission(struct inode *inode, int mask)
> +{
> +	if (!IS_RICHACL(inode))
> +		return -EPERM;
> +	return inode_permission(inode, mask);
> +}
> +
> +static bool inode_uid_change_ok(struct inode *inode, kuid_t ia_uid)
> +{
> +	if (uid_eq(current_fsuid(), inode->i_uid) &&
> +	ÂÂÂÂuid_eq(ia_uid, inode->i_uid))
> +		return true;
> +	if (uid_eq(current_fsuid(), ia_uid) &&
> +	ÂÂÂÂinode_extended_permission(inode, MAY_TAKE_OWNERSHIP) == 0)
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +


> +static bool inode_gid_change_ok(struct inode *inode, kgid_t ia_gid)
> +{
> +	int in_group = in_group_p(ia_gid);
> +	if (uid_eq(current_fsuid(), inode->i_uid) &&
> +	ÂÂÂÂ(in_group || gid_eq(ia_gid, inode->i_gid)))
> +		return true;
> +	if (in_group && inode_extended_permission(inode, MAY_TAKE_OWNERSHIP) == 0)
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +
> +/**
> + * inode_owner_permitted_or_capable
> + *
> + * Check for permissions implicitly granted to the owner, like MAY_CHMOD or
> + * MAY_SET_TIMES.ÂÂEquivalent to inode_owner_or_capable for file systems
> + * without support for those permissions.
> + */
> +static bool inode_owner_permitted_or_capable(struct inode *inode, int mask)
> +{
> +	struct user_namespace *ns;
> +
> +	if (uid_eq(current_fsuid(), inode->i_uid))
> +		return true;
> +	if (inode_extended_permission(inode, mask) == 0)
> +		return true;
> +	ns = current_user_ns();
> +	if (ns_capable(ns, CAP_FOWNER) && kuid_has_mapping(ns, inode->i_uid))
> +		return true;
> +	return false;
> +}
> +
> +/**
> Â * inode_change_ok - check if attribute changes to an inode are allowed
> Â * @inode:	inode to check
> Â * @attr:	attributes to change
> @@ -47,22 +106,18 @@ int inode_change_ok(struct inode *inode, struct iattr *attr)
> Â		return 0;
> Â
> Â	/* Make sure a caller can chown. */
> -	if ((ia_valid & ATTR_UID) &&
> -	ÂÂÂÂ(!uid_eq(current_fsuid(), inode->i_uid) ||
> -	ÂÂÂÂÂ!uid_eq(attr->ia_uid, inode->i_uid)) &&
> -	ÂÂÂÂ!capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> -		return -EPERM;
> +	if (ia_valid & ATTR_UID)
> +		if (!inode_uid_change_ok(inode, attr->ia_uid))
> +			return -EPERM;
> Â
> Â	/* Make sure caller can chgrp. */
> -	if ((ia_valid & ATTR_GID) &&
> -	ÂÂÂÂ(!uid_eq(current_fsuid(), inode->i_uid) ||
> -	ÂÂÂÂ(!in_group_p(attr->ia_gid) && !gid_eq(attr->ia_gid, inode->i_gid))) &&
> -	ÂÂÂÂ!capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> -		return -EPERM;
> +	if (ia_valid & ATTR_GID)
> +		if (!inode_gid_change_ok(inode, attr->ia_gid))
> +			return -EPERM;
> Â
> Â	/* Make sure a caller can chmod. */
> Â	if (ia_valid & ATTR_MODE) {
> -		if (!inode_owner_or_capable(inode))
> +		if (!inode_owner_permitted_or_capable(inode, MAY_CHMOD))
> Â			return -EPERM;
> Â		/* Also check the setgid bit! */
> Â		if (!in_group_p((ia_valid & ATTR_GID) ? attr->ia_gid :
> @@ -73,7 +128,7 @@ int inode_change_ok(struct inode *inode, struct iattr *attr)
> Â
> Â	/* Check for setting the inode time. */
> Â	if (ia_valid & (ATTR_MTIME_SET | ATTR_ATIME_SET | ATTR_TIMES_SET)) {
> -		if (!inode_owner_or_capable(inode))
> +		if (!inode_owner_permitted_or_capable(inode, MAY_SET_TIMES))
> Â			return -EPERM;
> Â	}
> Â
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 208f73b..bb36561 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -88,6 +88,9 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
> Â#define MAY_CREATE_DIR		0x00000200
> Â#define MAY_DELETE_CHILD	0x00000400
> Â#define MAY_DELETE_SELF		0x00000800
> +#define MAY_TAKE_OWNERSHIP	0x00001000
> +#define MAY_CHMOD		0x00002000
> +#define MAY_SET_TIMES		0x00004000
> Â
> Â/*
> Â * flags in file.f_mode.ÂÂNote that FMODE_READ and FMODE_WRITE must correspond


Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>