Re: [PATCH 03/11] fs: Allow superblock owner to change ownership of inodes

[Serge E. Hallyn]

On Fri, Dec 22, 2017 at 03:32:27PM +0100, Dongsu Park wrote:
> From: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> 
> Allow users with CAP_SYS_CHOWN over the superblock of a filesystem to

Note it is CAP_CHOWN

> chown files.  Ordinarily the capable_wrt_inode_uidgid check is
> sufficient to allow access to files but when the underlying filesystem
> has uids or gids that don't map to the current user namespace it is
> not enough, so the chown permission checks need to be extended to
> allow this case.
> 
> Calling chown on filesystem nodes whose uid or gid don't map is
> necessary if those nodes are going to be modified as writing back
> inodes which contain uids or gids that don't map is likely to cause
> filesystem corruption of the uid or gid fields.
> 
> Once chown has been called the existing capable_wrt_inode_uidgid
> checks are sufficient, to allow the owner of a superblock to do anything
> the global root user can do with an appropriate set of capabilities.
> 
> For the proc filesystem this relaxation of permissions is not safe, as
> some files are owned by users (particularly GLOBAL_ROOT_UID) outside
> of the control of the mounter of the proc and that would be unsafe to
> grant chown access to.  So update setattr on proc to disallow changing
> files whose uids or gids are outside of proc's s_user_ns.
> 
> The original version of this patch was written by: Seth Forshee.  I
> have rewritten and rethought this patch enough so it's really not the
> same thing (certainly it needs a different description), but he
> deserves credit for getting out there and getting the conversation
> started, and finding the potential gotcha's and putting up with my
> semi-paranoid feedback.
> 
> Patch v4 is available: https://patchwork.kernel.org/patch/8944611/
> 
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Inspired-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
> Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> [saf: Resolve conflicts caused by s/inode_change_ok/setattr_prepare/]
> Signed-off-by: Dongsu Park <dongsu@xxxxxxxxxx>

Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>

> ---
>  fs/attr.c             | 34 ++++++++++++++++++++++++++--------
>  fs/proc/base.c        |  7 +++++++
>  fs/proc/generic.c     |  7 +++++++
>  fs/proc/proc_sysctl.c |  7 +++++++
>  4 files changed, 47 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/attr.c b/fs/attr.c
> index 12ffdb6f..bf8e94f3 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -18,6 +18,30 @@
>  #include <linux/evm.h>
>  #include <linux/ima.h>
>  
> +static bool chown_ok(const struct inode *inode, kuid_t uid)
> +{
> +	if (uid_eq(current_fsuid(), inode->i_uid) &&
> +	    uid_eq(uid, inode->i_uid))
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	if (ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +
> +static bool chgrp_ok(const struct inode *inode, kgid_t gid)
> +{
> +	if (uid_eq(current_fsuid(), inode->i_uid) &&
> +	    (in_group_p(gid) || gid_eq(gid, inode->i_gid)))
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	if (ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +
>  /**
>   * setattr_prepare - check if attribute changes to a dentry are allowed
>   * @dentry:	dentry to check
> @@ -52,17 +76,11 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
>  		goto kill_priv;
>  
>  	/* Make sure a caller can chown. */
> -	if ((ia_valid & ATTR_UID) &&
> -	    (!uid_eq(current_fsuid(), inode->i_uid) ||
> -	     !uid_eq(attr->ia_uid, inode->i_uid)) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid))
>  		return -EPERM;
>  
>  	/* Make sure caller can chgrp. */
> -	if ((ia_valid & ATTR_GID) &&
> -	    (!uid_eq(current_fsuid(), inode->i_uid) ||
> -	    (!in_group_p(attr->ia_gid) && !gid_eq(attr->ia_gid, inode->i_gid))) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if ((ia_valid & ATTR_GID) && !chgrp_ok(inode, attr->ia_gid))
>  		return -EPERM;
>  
>  	/* Make sure a caller can chmod. */
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 31934cb9..9d50ec92 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -665,10 +665,17 @@ int proc_setattr(struct dentry *dentry, struct iattr *attr)
>  {
>  	int error;
>  	struct inode *inode = d_inode(dentry);
> +	struct user_namespace *s_user_ns;
>  
>  	if (attr->ia_valid & ATTR_MODE)
>  		return -EPERM;
>  
> +	/* Don't let anyone mess with weird proc files */
> +	s_user_ns = inode->i_sb->s_user_ns;
> +	if (!kuid_has_mapping(s_user_ns, inode->i_uid) ||
> +	    !kgid_has_mapping(s_user_ns, inode->i_gid))
> +		return -EPERM;
> +
>  	error = setattr_prepare(dentry, attr);
>  	if (error)
>  		return error;
> diff --git a/fs/proc/generic.c b/fs/proc/generic.c
> index 793a6757..527d46c8 100644
> --- a/fs/proc/generic.c
> +++ b/fs/proc/generic.c
> @@ -106,8 +106,15 @@ static int proc_notify_change(struct dentry *dentry, struct iattr *iattr)
>  {
>  	struct inode *inode = d_inode(dentry);
>  	struct proc_dir_entry *de = PDE(inode);
> +	struct user_namespace *s_user_ns;
>  	int error;
>  
> +	/* Don't let anyone mess with weird proc files */
> +	s_user_ns = inode->i_sb->s_user_ns;
> +	if (!kuid_has_mapping(s_user_ns, inode->i_uid) ||
> +	    !kgid_has_mapping(s_user_ns, inode->i_gid))
> +		return -EPERM;
> +
>  	error = setattr_prepare(dentry, iattr);
>  	if (error)
>  		return error;
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index c5cbbdff..0f9562d1 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -802,11 +802,18 @@ static int proc_sys_permission(struct inode *inode, int mask)
>  static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr)
>  {
>  	struct inode *inode = d_inode(dentry);
> +	struct user_namespace *s_user_ns;
>  	int error;
>  
>  	if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
>  		return -EPERM;
>  
> +	/* Don't let anyone mess with weird proc files */
> +	s_user_ns = inode->i_sb->s_user_ns;
> +	if (!kuid_has_mapping(s_user_ns, inode->i_uid) ||
> +	    !kgid_has_mapping(s_user_ns, inode->i_gid))
> +		return -EPERM;
> +
>  	error = setattr_prepare(dentry, attr);
>  	if (error)
>  		return error;
> -- 
> 2.13.6