[kernel-hardening] Re: [RFC v3 17/22] cgroup: Add access check for cgroup_get_from_fd()

From: "Mickaël Salaün" <mic@digikod.net>
To: linux-kernel@vger.kernel.org
Cc: Alexei Starovoitov <ast@kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Arnd Bergmann <arnd@arndb.de>,
	Casey Schaufler <casey@schaufler-ca.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Daniel Mack <daniel@zonque.org>,
	David Drysdale <drysdale@google.com>,
	"David S . Miller" <davem@davemloft.net>,
	Elena Reshetova <elena.reshetova@intel.com>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	James Morris <james.l.morris@oracle.com>,
	Kees Cook <keescook@chromium.org>, Paul Moore <pmoore@redhat.com>,
	Sargun Dhillon <sargun@sargun.me>,
	"Serge E . Hallyn" <serge@hallyn.com>, Tejun Heo <tj@kernel.org>,
	Will Drewry <wad@chromium.org>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-security-module@vger.kernel.org, netdev@vger.kernel.org,
	cgroups@vger.kernel.org
Subject: [kernel-hardening] Re: [RFC v3 17/22] cgroup: Add access check for cgroup_get_from_fd()
Date: Thu, 15 Sep 2016 00:06:22 +0200
Message-ID: <57D9C9DE.3090501@digikod.net> (raw)
In-Reply-To: <20160914072415.26021-18-mic@digikod.net>

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On 14/09/2016 09:24, Mickaël Salaün wrote:
> Add security access check for cgroup backed FD. The "cgroup.procs" file
> of the corresponding cgroup must be readable to identify the cgroup, and
> writable to prove that the current process can manage this cgroup (e.g.
> through delegation). This is similar to the check done by
> cgroup_procs_write_permission().
> 
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Daniel Mack <daniel@zonque.org>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Tejun Heo <tj@kernel.org>
> ---
>  include/linux/cgroup.h |  2 +-
>  kernel/bpf/arraymap.c  |  2 +-
>  kernel/bpf/syscall.c   |  6 +++---
>  kernel/cgroup.c        | 16 +++++++++++++++-
>  4 files changed, 20 insertions(+), 6 deletions(-)
...
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 48b650a640a9..3bbaf3f02ed2 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -6241,17 +6241,20 @@ EXPORT_SYMBOL_GPL(cgroup_get_from_path);
>  /**
>   * cgroup_get_from_fd - get a cgroup pointer from a fd
>   * @fd: fd obtained by open(cgroup2_dir)
> + * @access_mask: contains the permission mask
>   *
>   * Find the cgroup from a fd which should be obtained
>   * by opening a cgroup directory.  Returns a pointer to the
>   * cgroup on success. ERR_PTR is returned if the cgroup
>   * cannot be found.
>   */
> -struct cgroup *cgroup_get_from_fd(int fd)
> +struct cgroup *cgroup_get_from_fd(int fd, int access_mask)
>  {
>  	struct cgroup_subsys_state *css;
>  	struct cgroup *cgrp;
>  	struct file *f;
> +	struct inode *inode;
> +	int ret;
>  
>  	f = fget_raw(fd);
>  	if (!f)
> @@ -6268,6 +6271,17 @@ struct cgroup *cgroup_get_from_fd(int fd)
>  		return ERR_PTR(-EBADF);
>  	}
>  
> +	ret = -ENOMEM;
> +	inode = kernfs_get_inode(f->f_path.dentry->d_sb, cgrp->procs_file.kn);

I forgot to properly move fput(f) after this line… This will be fixed.

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