Re: [PATCH] devpts: Make ptmx be owned by the userns owner instead of userns-local 0

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Quoting Andy Lutomirski (luto@kernel.org):
> We used to have ptmx be owned by the inner uid and gid 0.  Change
> this: if the owner and group are both mapped but are not both 0,
> then use the owner instead.
> 
> For container-style namespaces (LXC, etc), this should have no
> effect -- UID 0 is will either be the owner or will be unmapped.

This doesn't seem right - it's often the case that the owner is mapped
in as non-0 uid, safe or not.  The actual namespace root uid should be
the owner (so long as it exists).

Why not reverse the cases?  If 0 is not mapped, then check whether the
current_user_ns()->owner is mapped?

> The important behavior change is for sandboxes: many sandboxes
> intentionally do not create an inner uid 0.  Without this patch,
> mounting devpts in such a sandbox is awkward.  With this patch, it
> will just work and ptmx will be owned by the namespace owner.
> 
> 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>
> ---
>  fs/devpts/inode.c | 34 ++++++++++++++++++++++++++++++----
>  1 file changed, 30 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index 655f21f99160..d6fa2d1beee3 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
>  /*
> @@ -250,10 +251,35 @@ static int mknod_ptmx(struct super_block *sb)
>  	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 the creating user
> +	 * namespace's owner.  Usually, that will be 0 as seen by the
> +	 * user namespace, but for unprivileged sandbox namespaces,
> +	 * there may not be a uid 0 or gid 0 at all.
> +	 */
> +	root_uid = current_user_ns()->owner;
> +	root_gid = current_user_ns()->group;
> +
> +	if (!uid_valid(root_uid) || !gid_valid(root_gid)) {
> +		/*
> +		 * It's very unlikely for us to get here if the userns
> +		 * owner is not mapped, but it's possible -- we'd have
> +		 * to be running in the userns with capabilities granted
> +		 * by unshare or setns, since there is no inner
> +		 * privileged user.  Nonetheless, this could happen, and
> +		 * we don't want ptmx to be owned by an unmapped user or
> +		 * group.
> +		 *
> +		 * If this happens fall back to historical behavior:
> +		 * try to have ptmx be owned by 0:0.
> +		 */
> +		root_uid = make_kuid(current_user_ns(), 0);
> +		root_gid = make_kgid(current_user_ns(), 0);
> +
> +		/* If this still doesn't work, give up. */
> +		if (!uid_valid(root_uid) || !gid_valid(root_gid))
> +			return -EINVAL;
> +	}
>  
>  	inode_lock(d_inode(root));
>  
> -- 
> 2.5.0
> 
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