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Message-ID: <20080114225826.GF6704@sergelap.austin.ibm.com>
Date: Mon, 14 Jan 2008 16:58:26 -0600
From: "Serge E. Hallyn" <serue@...ibm.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: akpm@...ux-foundation.org, hch@...radead.org, serue@...ibm.com,
viro@....linux.org.uk, ebiederm@...ssion.com, kzak@...hat.com,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
containers@...ts.osdl.org, util-linux-ng@...r.kernel.org
Subject: Re: [patch 6/9] unprivileged mounts: allow unprivileged mounts
Quoting Miklos Szeredi (miklos@...redi.hu):
> From: Miklos Szeredi <mszeredi@...e.cz>
>
> Define a new fs flag FS_SAFE, which denotes, that unprivileged mounting of
> this filesystem may not constitute a security problem.
>
> Since most filesystems haven't been designed with unprivileged mounting in
> mind, a thorough audit is needed before setting this flag.
>
> For "safe" filesystems also allow unprivileged forced unmounting.
>
> Move subtype handling from do_kern_mount() into do_new_mount(). All
> other callers are kernel-internal and do not need subtype support.
>
> Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
This patch itself doesn't assign FS_SAFE to any filesystems, so
presuming that there is such a thing as an fs safe for users to
mount, and/or users sign their systems away through a sysctl,
this patch in itself appears right.
Acked-by: Serge Hallyn <serue@...ibm.com>
> ---
>
> Index: linux/fs/namespace.c
> ===================================================================
> --- linux.orig/fs/namespace.c 2008-01-03 21:20:11.000000000 +0100
> +++ linux/fs/namespace.c 2008-01-03 21:21:06.000000000 +0100
> @@ -960,14 +960,16 @@ static bool is_mount_owner(struct vfsmou
> /*
> * umount is permitted for
> * - sysadmin
> - * - mount owner, if not forced umount
> + * - mount owner
> + * o if not forced umount,
> + * o if forced umount, and filesystem is "safe"
> */
> static bool permit_umount(struct vfsmount *mnt, int flags)
> {
> if (capable(CAP_SYS_ADMIN))
> return true;
>
> - if (flags & MNT_FORCE)
> + if ((flags & MNT_FORCE) && !(mnt->mnt_sb->s_type->fs_flags & FS_SAFE))
> return false;
>
> return is_mount_owner(mnt, current->fsuid);
> @@ -1025,13 +1027,17 @@ asmlinkage long sys_oldumount(char __use
> * - mountpoint is not a symlink
> * - mountpoint is in a mount owned by the user
> */
> -static bool permit_mount(struct nameidata *nd, int *flags)
> +static bool permit_mount(struct nameidata *nd, struct file_system_type *type,
> + int *flags)
> {
> struct inode *inode = nd->path.dentry->d_inode;
>
> if (capable(CAP_SYS_ADMIN))
> return true;
>
> + if (type && !(type->fs_flags & FS_SAFE))
> + return false;
> +
> if (S_ISLNK(inode->i_mode))
> return false;
>
> @@ -1285,7 +1291,7 @@ static int do_loopback(struct nameidata
> struct vfsmount *mnt = NULL;
> int err;
>
> - if (!permit_mount(nd, &flags))
> + if (!permit_mount(nd, NULL, &flags))
> return -EPERM;
> if (!old_name || !*old_name)
> return -EINVAL;
> @@ -1466,30 +1472,76 @@ out:
> return err;
> }
>
> +static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
> +{
> + int err;
> + const char *subtype = strchr(fstype, '.');
> + if (subtype) {
> + subtype++;
> + err = -EINVAL;
> + if (!subtype[0])
> + goto err;
> + } else
> + subtype = "";
> +
> + mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
> + err = -ENOMEM;
> + if (!mnt->mnt_sb->s_subtype)
> + goto err;
> + return mnt;
> +
> + err:
> + mntput(mnt);
> + return ERR_PTR(err);
> +}
> +
> /*
> * create a new mount for userspace and request it to be added into the
> * namespace's tree
> */
> -static int do_new_mount(struct nameidata *nd, char *type, int flags,
> +static int do_new_mount(struct nameidata *nd, char *fstype, int flags,
> int mnt_flags, char *name, void *data)
> {
> + int err;
> struct vfsmount *mnt;
> + struct file_system_type *type;
>
> - if (!type || !memchr(type, 0, PAGE_SIZE))
> + if (!fstype || !memchr(fstype, 0, PAGE_SIZE))
> return -EINVAL;
>
> - /* we need capabilities... */
> - if (!capable(CAP_SYS_ADMIN))
> - return -EPERM;
> -
> - mnt = do_kern_mount(type, flags & ~MS_SETUSER, name, data);
> - if (IS_ERR(mnt))
> + type = get_fs_type(fstype);
> + if (!type)
> + return -ENODEV;
> +
> + err = -EPERM;
> + if (!permit_mount(nd, type, &flags))
> + goto out_put_filesystem;
> +
> + if (flags & MS_SETUSER) {
> + err = reserve_user_mount();
> + if (err)
> + goto out_put_filesystem;
> + }
> +
> + mnt = vfs_kern_mount(type, flags & ~MS_SETUSER, name, data);
> + if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
> + !mnt->mnt_sb->s_subtype)
> + mnt = fs_set_subtype(mnt, fstype);
> + put_filesystem(type);
> + if (IS_ERR(mnt)) {
> + if (flags & MS_SETUSER)
> + dec_nr_user_mounts();
> return PTR_ERR(mnt);
> + }
>
> if (flags & MS_SETUSER)
> - set_mnt_user(mnt);
> + __set_mnt_user(mnt);
>
> return do_add_mount(mnt, nd, mnt_flags, NULL);
> +
> + out_put_filesystem:
> + put_filesystem(type);
> + return err;
> }
>
> /*
> @@ -1520,7 +1572,7 @@ int do_add_mount(struct vfsmount *newmnt
> if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
> goto unlock;
>
> - /* MNT_USER was set earlier */
> + /* some flags may have been set earlier */
> newmnt->mnt_flags |= mnt_flags;
> if ((err = graft_tree(newmnt, nd)))
> goto unlock;
> Index: linux/include/linux/fs.h
> ===================================================================
> --- linux.orig/include/linux/fs.h 2008-01-03 21:15:35.000000000 +0100
> +++ linux/include/linux/fs.h 2008-01-03 21:21:06.000000000 +0100
> @@ -96,6 +96,7 @@ extern int dir_notify_enable;
> #define FS_REQUIRES_DEV 1
> #define FS_BINARY_MOUNTDATA 2
> #define FS_HAS_SUBTYPE 4
> +#define FS_SAFE 8 /* Safe to mount by unprivileged users */
> #define FS_REVAL_DOT 16384 /* Check the paths ".", ".." for staleness */
> #define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move()
> * during rename() internally.
> Index: linux/fs/super.c
> ===================================================================
> --- linux.orig/fs/super.c 2008-01-02 21:42:10.000000000 +0100
> +++ linux/fs/super.c 2008-01-03 21:21:06.000000000 +0100
> @@ -906,29 +906,6 @@ out:
>
> EXPORT_SYMBOL_GPL(vfs_kern_mount);
>
> -static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
> -{
> - int err;
> - const char *subtype = strchr(fstype, '.');
> - if (subtype) {
> - subtype++;
> - err = -EINVAL;
> - if (!subtype[0])
> - goto err;
> - } else
> - subtype = "";
> -
> - mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
> - err = -ENOMEM;
> - if (!mnt->mnt_sb->s_subtype)
> - goto err;
> - return mnt;
> -
> - err:
> - mntput(mnt);
> - return ERR_PTR(err);
> -}
> -
> struct vfsmount *
> do_kern_mount(const char *fstype, int flags, const char *name, void *data)
> {
> @@ -937,9 +914,6 @@ do_kern_mount(const char *fstype, int fl
> if (!type)
> return ERR_PTR(-ENODEV);
> mnt = vfs_kern_mount(type, flags, name, data);
> - if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
> - !mnt->mnt_sb->s_subtype)
> - mnt = fs_set_subtype(mnt, fstype);
> put_filesystem(type);
> return mnt;
> }
>
> --
--
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