Re: [RCF] [PATCH] unprivileged mount/umount

Christoph Hellwig
2005-05-11T08:48:32Z

On Tue, May 03, 2005 at 04:31:35PM +0200, Miklos Szeredi wrote:
> This (lightly tested) patch against 2.6.12-rc* adds some
> infrastructure and basic functionality for unprivileged mount/umount
> system calls.

Thanks for doing this.

> Details:
>
> - new mnt_owner field in struct vfsmount
> - if mnt_owner is NULL, it's a privileged mount
> - global limit on unprivileged mounts in /proc/sys/fs/mount-max

I think the name should be different. user-mount-max?

Acutally the accounting in your patch is a little odd, we account for
all mounts, and after mount-max is reached user mounts are denied.
Shouldn't we account only for user mounts?

> - per user limit of mounts in rlimit
> - allow umount for the owner (except force flag)
> - allow unprivileged bind mount to files/directories writable by owner
> - add nosuid,nodev flags to unprivileged mounts
>
> Next step would be to add some policy for new mounts. I'm thinking of
> either something static: e.g. FS_SAFE flag for "safe" filesystems, or
> a more configurable approach through sysfs or something.
>
> Comments?

> --- a6d962c4f559f3644678574a66310084fd13d130/fs/namespace.c (mode:100644 sha1:3b93e5d750ebf8452ea1264251c5b55cc89f48f8)
> +++ uncommitted/fs/namespace.c (mode:100644)
> @@ -42,7 +42,7 @@
> static struct list_head *mount_hashtable;
> static int hash_mask, hash_bits;
> static kmem_cache_t *mnt_cache;
> -
> +struct mounts_stat_struct mounts_stat;
> static inline unsigned long hash(struct vfsmount *mnt, struct dentry *dentry)

minor nipick - please keep a empty line before the function here.
Also I wonder whether we should have struct mounts_stat_struct at all,
just having two variables seems a lot saner to me.

> - if (!capable(CAP_SYS_ADMIN))
> + if (!capable(CAP_SYS_ADMIN) && (nd.mnt->mnt_owner != current->user ||
> + (flags & MNT_FORCE)))
> goto dput_and_out;

although it won't have different results I'd reorder this to make reading
more easy:

if ((nd.mnt->mnt_owner != current->user || (flags & MNT_FORCE)) &&
!capable(CAP_SYS_ADMIN))

> -static int mount_is_safe(struct nameidata *nd)
> +static struct user_struct *mount_is_safe(struct nameidata *nd)
> {
> if (capable(CAP_SYS_ADMIN))
> - return 0;
> - return -EPERM;
> -#ifdef notyet
> - if (S_ISLNK(nd->dentry->d_inode->i_mode))
> - return -EPERM;
> + return NULL;
> +
> + if (!S_ISDIR(nd->dentry->d_inode->i_mode) &&
> + !S_ISREG(nd->dentry->d_inode->i_mode))
> + return ERR_PTR(-EPERM);
> if (nd->dentry->d_inode->i_mode & S_ISVTX) {
> - if (current->uid != nd->dentry->d_inode->i_uid)
> - return -EPERM;
> + if (current->fsuid != nd->dentry->d_inode->i_uid)
> + return ERR_PTR(-EPERM);
> }
> if (permission(nd->dentry->d_inode, MAY_WRITE, nd))
> - return -EPERM;
> - return 0;
> -#endif
> + return ERR_PTR(-EPERM);
> + return current->user;

Currently we do allow bind mounts over every type of file for the super
user. I think we should keep allowing that. Also I think this function
wants a really big comment explaining all the rules for user mounts.