Re: [PATCH v3 03/25] proc: add /proc/<pid>/fsgid_map

From: "Serge E. Hallyn" <serge@hallyn.com>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: "Stéphane Graber" <stgraber@ubuntu.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"Aleksa Sarai" <cyphar@cyphar.com>,
	"Jann Horn" <jannh@google.com>,
	smbarber@chromium.org,
	"Seth Forshee" <seth.forshee@canonical.com>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Alexey Dobriyan" <adobriyan@gmail.com>,
	"Serge Hallyn" <serge@hallyn.com>,
	"James Morris" <jmorris@namei.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Phil Estes" <estesp@gmail.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	containers@lists.linux-foundation.org,
	linux-security-module@vger.kernel.org, linux-api@vger.kernel.org
Subject: Re: [PATCH v3 03/25] proc: add /proc/<pid>/fsgid_map
Date: Tue, 18 Feb 2020 20:33:43 -0600	[thread overview]
Message-ID: <20200219023343.GC19144@mail.hallyn.com> (raw)
In-Reply-To: <20200218143411.2389182-4-christian.brauner@ubuntu.com>

On Tue, Feb 18, 2020 at 03:33:49PM +0100, Christian Brauner wrote:
> The /proc/<pid>/fsgid_map file can be written once to setup an fsgid mapping
> for a user namespace. Writing to this file has the same restrictions as writing
> to /proc/<pid>/fsgid_map.
> 
> root@e1-vm:/# cat /proc/13023/fsgid_map
>          0     300000     100000
> 
> Fsid mappings have always been around. They are currently always identical to
> the id mappings for a user namespace. This means, currently whenever an fsid
> needs to be looked up the kernel will use the id mapping of the user namespace.
> With the introduction of fsid mappings the kernel will now lookup fsids in the
> fsid mappings of the user namespace. If no fsid mapping exists the kernel will
> continue looking up fsids in the id mappings of the user namespace. Hence, if a
> system supports fsid mappings through /proc/<pid>/fs*id_map and a container
> runtime is not aware of fsid mappings it or does not use them it will it will
> continue to work just as before.
> 
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Acked-by: Serge Hallyn <serge@hallyn.com>

> ---
> /* v2 */
> unchanged
> 
> /* v3 */
> - Christian Brauner <christian.brauner@ubuntu.com>:
>   - Fix grammar in commit message.
> ---
>  fs/proc/base.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 5fb28004663e..1303cdd2e617 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2975,6 +2975,11 @@ static int proc_fsuid_map_open(struct inode *inode, struct file *file)
>  {
>  	return proc_id_map_open(inode, file, &proc_fsuid_seq_operations);
>  }
> +
> +static int proc_fsgid_map_open(struct inode *inode, struct file *file)
> +{
> +	return proc_id_map_open(inode, file, &proc_fsgid_seq_operations);
> +}
>  #endif
>  
>  static const struct file_operations proc_uid_map_operations = {
> @@ -3009,6 +3014,14 @@ static const struct file_operations proc_fsuid_map_operations = {
>  	.llseek		= seq_lseek,
>  	.release	= proc_id_map_release,
>  };
> +
> +static const struct file_operations proc_fsgid_map_operations = {
> +	.open		= proc_fsgid_map_open,
> +	.write		= proc_fsgid_map_write,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= proc_id_map_release,
> +};
>  #endif
>  
>  static int proc_setgroups_open(struct inode *inode, struct file *file)
> @@ -3195,6 +3208,7 @@ static const struct pid_entry tgid_base_stuff[] = {
>  #ifdef CONFIG_USER_NS
>  #ifdef CONFIG_USER_NS_FSID
>  	REG("fsuid_map",  S_IRUGO|S_IWUSR, proc_fsuid_map_operations),
> +	REG("fsgid_map",  S_IRUGO|S_IWUSR, proc_fsgid_map_operations),
>  #endif
>  	REG("uid_map",    S_IRUGO|S_IWUSR, proc_uid_map_operations),
>  	REG("gid_map",    S_IRUGO|S_IWUSR, proc_gid_map_operations),
> -- 
> 2.25.0