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

["Serge E. Hallyn" <serge hallyn com>]

Quoting Andy Lutomirski (luto 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 redhat com>
> Cc: mclasen redhat com
> Cc: "Eric W. Biederman" <ebiederm xmission com>
> Cc: Linux Containers <containers lists linux-foundation org>
> Signed-off-by: Andy Lutomirski <luto kernel org>

Thanks, looks good.

Acked-by: Serge Hallyn <serge hallyn 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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