[PATCH v2] devpts: Make ptmx be owned by the userns owner as a fallback

[Serge E. Hallyn]

Quoting Andy Lutomirski (luto at kernel.org):
> New devpts instances have ptmx owned by the inner uid and gid 0.
> 
> For container-style namespaces (LXC, etc), this should have no
> effect, this is fine.
> 
> For sandbox-style namespaces (xdg-app and similar), this is
> problematic -- there may not be an inner 0:0.  If that happens,
> devpts mounts will fail.
> 
> Fix it by adding a fallback: if 0:0 is not mapped but the userns
> owner and group are mapped, then ptmx will be owned by the namespace
> owner.
> 
> This won't change behavior except in cases where mount would
> currently return -EINVAL.
> 
> Cc: Alexander Larsson <alexl at redhat.com>
> Cc: mclasen at redhat.com
> Cc: "Eric W. Biederman" <ebiederm at xmission.com>
> Cc: Linux Containers <containers at lists.linux-foundation.org>
> Signed-off-by: Andy Lutomirski <luto at kernel.org>

Thanks, looks good.

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

> ---
> 
> Changes from v1:
>  - Reversed the preference order (Serge)
>  - Fixed misuse of uid_valid on userns->owner
> 
> fs/devpts/inode.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index 655f21f99160..42b1e04d8334 100644
> --- a/fs/devpts/inode.c
> +++ b/fs/devpts/inode.c
> @@ -27,6 +27,7 @@
>  #include <linux/parser.h>
>  #include <linux/fsnotify.h>
>  #include <linux/seq_file.h>
> +#include <linux/user_namespace.h>
>  
>  #define DEVPTS_DEFAULT_MODE 0600
>  /*
> @@ -247,13 +248,33 @@ static int mknod_ptmx(struct super_block *sb)
>  	struct dentry *root = sb->s_root;
>  	struct pts_fs_info *fsi = DEVPTS_SB(sb);
>  	struct pts_mount_opts *opts = &fsi->mount_opts;
> +	struct user_namespace *userns = current_user_ns();
>  	kuid_t root_uid;
>  	kgid_t root_gid;
>  
> -	root_uid = make_kuid(current_user_ns(), 0);
> -	root_gid = make_kgid(current_user_ns(), 0);
> -	if (!uid_valid(root_uid) || !gid_valid(root_gid))
> -		return -EINVAL;
> +	/*
> +	 * For a new devpts instance, ptmx is owned by 0:0 if that uid
> +	 * and gid are mapped in the creating namespace.
> +	 */
> +	root_uid = make_kuid(userns, 0);
> +	root_gid = make_kgid(userns, 0);
> +
> +	if (!uid_valid(root_uid) || !gid_valid(root_gid)) {
> +		/*
> +		 * If the creating namespace does not have 0:0 mapped
> +		 * but does have the owner mapped (this is rare in
> +		 * container-style namespaces but common in
> +		 * sandbox-style namespaces), then let ptmx be owned by
> +		 * the namespace owner.
> +		 */
> +		root_uid = userns->owner;
> +		root_gid = userns->group;
> +
> +		/* If this still doesn't work, give up. */
> +		if (!kuid_has_mapping(userns, root_uid) ||
> +		    !kgid_has_mapping(userns, root_gid))
> +			return -EINVAL;
> +	}
>  
>  	inode_lock(d_inode(root));
>  
> -- 
> 2.5.0
> 
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