Re: [PATCH 08/11] fuse: Support fuse filesystems outside of init_user_ns

[Alban Crequy <alban@kinvolk.io>]

[Adding Tejun, David, Tom for question about cuse]

On Fri, Dec 22, 2017 at 3:32 PM, Dongsu Park <dongsu@kinvolk.io> wrote:
> From: Seth Forshee <seth.forshee@canonical.com>
>
> In order to support mounts from namespaces other than
> init_user_ns, fuse must translate uids and gids to/from the
> userns of the process servicing requests on /dev/fuse. This
> patch does that, with a couple of restrictions on the namespace:
>
>  - The userns for the fuse connection is fixed to the namespace
>    from which /dev/fuse is opened.
>
>  - The namespace must be the same as s_user_ns.
>
> These restrictions simplify the implementation by avoiding the
> need to pass around userns references and by allowing fuse to
> rely on the checks in inode_change_ok for ownership changes.
> Either restriction could be relaxed in the future if needed.
>
> For cuse the namespace used for the connection is also simply
> current_user_ns() at the time /dev/cuse is opened.

Was a use case discussed for using cuse in a new unprivileged userns?

I ran some tests yesterday with cusexmp [1] and I could add a new char
device as an unprivileged user with:

$ unshare -U -r -m sh -c 'mount --bind /mnt/cuse /dev/cuse ; cusexmp
--maj=99 --min=30 --name=foo

where /mnt/cuse is previously mknod'ed correctly and chmod'ed 777.
Then, I could see the new device:

$ cat /proc/devices | grep foo
 99 foo

On normal distros, we don't have a /mnt/cuse chmod'ed 777 but still it
seems dangerous if the dev node can be provided otherwise and if we
don't have a use case for it.

Thoughts?

[1] https://github.com/fuse4x/fuse/blob/master/example/cusexmp.c#L9

Cheers,
Alban


> Patch v4 is available: https://patchwork.kernel.org/patch/8944661/
>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: Miklos Szeredi <mszeredi@redhat.com>
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> Signed-off-by: Dongsu Park <dongsu@kinvolk.io>
> ---
>  fs/fuse/cuse.c   |  3 ++-
>  fs/fuse/dev.c    | 11 ++++++++---
>  fs/fuse/dir.c    | 14 +++++++-------
>  fs/fuse/fuse_i.h |  6 +++++-
>  fs/fuse/inode.c  | 31 +++++++++++++++++++------------
>  5 files changed, 41 insertions(+), 24 deletions(-)
>
> diff --git a/fs/fuse/cuse.c b/fs/fuse/cuse.c
> index e9e97803..b1b83259 100644
> --- a/fs/fuse/cuse.c
> +++ b/fs/fuse/cuse.c
> @@ -48,6 +48,7 @@
>  #include <linux/stat.h>
>  #include <linux/module.h>
>  #include <linux/uio.h>
> +#include <linux/user_namespace.h>
>
>  #include "fuse_i.h"
>
> @@ -498,7 +499,7 @@ static int cuse_channel_open(struct inode *inode, struct file *file)
>         if (!cc)
>                 return -ENOMEM;
>
> -       fuse_conn_init(&cc->fc);
> +       fuse_conn_init(&cc->fc, current_user_ns());
>
>         fud = fuse_dev_alloc(&cc->fc);
>         if (!fud) {
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 17f0d05b..0f780e16 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -114,8 +114,8 @@ static void __fuse_put_request(struct fuse_req *req)
>
>  static void fuse_req_init_context(struct fuse_conn *fc, struct fuse_req *req)
>  {
> -       req->in.h.uid = from_kuid_munged(&init_user_ns, current_fsuid());
> -       req->in.h.gid = from_kgid_munged(&init_user_ns, current_fsgid());
> +       req->in.h.uid = from_kuid(fc->user_ns, current_fsuid());
> +       req->in.h.gid = from_kgid(fc->user_ns, current_fsgid());
>         req->in.h.pid = pid_nr_ns(task_pid(current), fc->pid_ns);
>  }
>
> @@ -167,6 +167,10 @@ static struct fuse_req *__fuse_get_req(struct fuse_conn *fc, unsigned npages,
>         __set_bit(FR_WAITING, &req->flags);
>         if (for_background)
>                 __set_bit(FR_BACKGROUND, &req->flags);
> +       if (req->in.h.uid == (uid_t)-1 || req->in.h.gid == (gid_t)-1) {
> +               fuse_put_request(fc, req);
> +               return ERR_PTR(-EOVERFLOW);
> +       }
>
>         return req;
>
> @@ -1260,7 +1264,8 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
>         in = &req->in;
>         reqsize = in->h.len;
>
> -       if (task_active_pid_ns(current) != fc->pid_ns) {
> +       if (task_active_pid_ns(current) != fc->pid_ns ||
> +           current_user_ns() != fc->user_ns) {
>                 rcu_read_lock();
>                 in->h.pid = pid_vnr(find_pid_ns(in->h.pid, fc->pid_ns));
>                 rcu_read_unlock();
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index 24967382..ad1cfac1 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -858,8 +858,8 @@ static void fuse_fillattr(struct inode *inode, struct fuse_attr *attr,
>         stat->ino = attr->ino;
>         stat->mode = (inode->i_mode & S_IFMT) | (attr->mode & 07777);
>         stat->nlink = attr->nlink;
> -       stat->uid = make_kuid(&init_user_ns, attr->uid);
> -       stat->gid = make_kgid(&init_user_ns, attr->gid);
> +       stat->uid = make_kuid(fc->user_ns, attr->uid);
> +       stat->gid = make_kgid(fc->user_ns, attr->gid);
>         stat->rdev = inode->i_rdev;
>         stat->atime.tv_sec = attr->atime;
>         stat->atime.tv_nsec = attr->atimensec;
> @@ -1475,17 +1475,17 @@ static bool update_mtime(unsigned ivalid, bool trust_local_mtime)
>         return true;
>  }
>
> -static void iattr_to_fattr(struct iattr *iattr, struct fuse_setattr_in *arg,
> -                          bool trust_local_cmtime)
> +static void iattr_to_fattr(struct fuse_conn *fc, struct iattr *iattr,
> +                          struct fuse_setattr_in *arg, bool trust_local_cmtime)
>  {
>         unsigned ivalid = iattr->ia_valid;
>
>         if (ivalid & ATTR_MODE)
>                 arg->valid |= FATTR_MODE,   arg->mode = iattr->ia_mode;
>         if (ivalid & ATTR_UID)
> -               arg->valid |= FATTR_UID,    arg->uid = from_kuid(&init_user_ns, iattr->ia_uid);
> +               arg->valid |= FATTR_UID,    arg->uid = from_kuid(fc->user_ns, iattr->ia_uid);
>         if (ivalid & ATTR_GID)
> -               arg->valid |= FATTR_GID,    arg->gid = from_kgid(&init_user_ns, iattr->ia_gid);
> +               arg->valid |= FATTR_GID,    arg->gid = from_kgid(fc->user_ns, iattr->ia_gid);
>         if (ivalid & ATTR_SIZE)
>                 arg->valid |= FATTR_SIZE,   arg->size = iattr->ia_size;
>         if (ivalid & ATTR_ATIME) {
> @@ -1646,7 +1646,7 @@ int fuse_do_setattr(struct dentry *dentry, struct iattr *attr,
>
>         memset(&inarg, 0, sizeof(inarg));
>         memset(&outarg, 0, sizeof(outarg));
> -       iattr_to_fattr(attr, &inarg, trust_local_cmtime);
> +       iattr_to_fattr(fc, attr, &inarg, trust_local_cmtime);
>         if (file) {
>                 struct fuse_file *ff = file->private_data;
>                 inarg.valid |= FATTR_FH;
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index d5773ca6..364e65c8 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -26,6 +26,7 @@
>  #include <linux/xattr.h>
>  #include <linux/pid_namespace.h>
>  #include <linux/refcount.h>
> +#include <linux/user_namespace.h>
>
>  /** Max number of pages that can be used in a single read request */
>  #define FUSE_MAX_PAGES_PER_REQ 32
> @@ -466,6 +467,9 @@ struct fuse_conn {
>         /** The pid namespace for this mount */
>         struct pid_namespace *pid_ns;
>
> +       /** The user namespace for this mount */
> +       struct user_namespace *user_ns;
> +
>         /** Maximum read size */
>         unsigned max_read;
>
> @@ -870,7 +874,7 @@ struct fuse_conn *fuse_conn_get(struct fuse_conn *fc);
>  /**
>   * Initialize fuse_conn
>   */
> -void fuse_conn_init(struct fuse_conn *fc);
> +void fuse_conn_init(struct fuse_conn *fc, struct user_namespace *user_ns);
>
>  /**
>   * Release reference to fuse_conn
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 2f504d61..7f6b2e55 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -171,8 +171,8 @@ void fuse_change_attributes_common(struct inode *inode, struct fuse_attr *attr,
>         inode->i_ino     = fuse_squash_ino(attr->ino);
>         inode->i_mode    = (inode->i_mode & S_IFMT) | (attr->mode & 07777);
>         set_nlink(inode, attr->nlink);
> -       inode->i_uid     = make_kuid(&init_user_ns, attr->uid);
> -       inode->i_gid     = make_kgid(&init_user_ns, attr->gid);
> +       inode->i_uid     = make_kuid(fc->user_ns, attr->uid);
> +       inode->i_gid     = make_kgid(fc->user_ns, attr->gid);
>         inode->i_blocks  = attr->blocks;
>         inode->i_atime.tv_sec   = attr->atime;
>         inode->i_atime.tv_nsec  = attr->atimensec;
> @@ -477,7 +477,8 @@ static int fuse_match_uint(substring_t *s, unsigned int *res)
>         return err;
>  }
>
> -static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev)
> +static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev,
> +                         struct user_namespace *user_ns)
>  {
>         char *p;
>         memset(d, 0, sizeof(struct fuse_mount_data));
> @@ -513,7 +514,7 @@ static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev)
>                 case OPT_USER_ID:
>                         if (fuse_match_uint(&args[0], &uv))
>                                 return 0;
> -                       d->user_id = make_kuid(current_user_ns(), uv);
> +                       d->user_id = make_kuid(user_ns, uv);
>                         if (!uid_valid(d->user_id))
>                                 return 0;
>                         d->user_id_present = 1;
> @@ -522,7 +523,7 @@ static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev)
>                 case OPT_GROUP_ID:
>                         if (fuse_match_uint(&args[0], &uv))
>                                 return 0;
> -                       d->group_id = make_kgid(current_user_ns(), uv);
> +                       d->group_id = make_kgid(user_ns, uv);
>                         if (!gid_valid(d->group_id))
>                                 return 0;
>                         d->group_id_present = 1;
> @@ -565,8 +566,8 @@ static int fuse_show_options(struct seq_file *m, struct dentry *root)
>         struct super_block *sb = root->d_sb;
>         struct fuse_conn *fc = get_fuse_conn_super(sb);
>
> -       seq_printf(m, ",user_id=%u", from_kuid_munged(&init_user_ns, fc->user_id));
> -       seq_printf(m, ",group_id=%u", from_kgid_munged(&init_user_ns, fc->group_id));
> +       seq_printf(m, ",user_id=%u", from_kuid_munged(fc->user_ns, fc->user_id));
> +       seq_printf(m, ",group_id=%u", from_kgid_munged(fc->user_ns, fc->group_id));
>         if (fc->default_permissions)
>                 seq_puts(m, ",default_permissions");
>         if (fc->allow_other)
> @@ -597,7 +598,7 @@ static void fuse_pqueue_init(struct fuse_pqueue *fpq)
>         fpq->connected = 1;
>  }
>
> -void fuse_conn_init(struct fuse_conn *fc)
> +void fuse_conn_init(struct fuse_conn *fc, struct user_namespace *user_ns)
>  {
>         memset(fc, 0, sizeof(*fc));
>         spin_lock_init(&fc->lock);
> @@ -621,6 +622,7 @@ void fuse_conn_init(struct fuse_conn *fc)
>         fc->attr_version = 1;
>         get_random_bytes(&fc->scramble_key, sizeof(fc->scramble_key));
>         fc->pid_ns = get_pid_ns(task_active_pid_ns(current));
> +       fc->user_ns = get_user_ns(user_ns);
>  }
>  EXPORT_SYMBOL_GPL(fuse_conn_init);
>
> @@ -630,6 +632,7 @@ void fuse_conn_put(struct fuse_conn *fc)
>                 if (fc->destroy_req)
>                         fuse_request_free(fc->destroy_req);
>                 put_pid_ns(fc->pid_ns);
> +               put_user_ns(fc->user_ns);
>                 fc->release(fc);
>         }
>  }
> @@ -1061,7 +1064,7 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
>
>         sb->s_flags &= ~(MS_NOSEC | SB_I_VERSION);
>
> -       if (!parse_fuse_opt(data, &d, is_bdev))
> +       if (!parse_fuse_opt(data, &d, is_bdev, sb->s_user_ns))
>                 goto err;
>
>         if (is_bdev) {
> @@ -1086,8 +1089,12 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
>         if (!file)
>                 goto err;
>
> -       if ((file->f_op != &fuse_dev_operations) ||
> -           (file->f_cred->user_ns != &init_user_ns))
> +       /*
> +        * Require mount to happen from the same user namespace which
> +        * opened /dev/fuse to prevent potential attacks.
> +        */
> +       if (file->f_op != &fuse_dev_operations ||
> +           file->f_cred->user_ns != sb->s_user_ns)
>                 goto err_fput;
>
>         fc = kmalloc(sizeof(*fc), GFP_KERNEL);
> @@ -1095,7 +1102,7 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
>         if (!fc)
>                 goto err_fput;
>
> -       fuse_conn_init(fc);
> +       fuse_conn_init(fc, sb->s_user_ns);
>         fc->release = fuse_free_conn;
>
>         fud = fuse_dev_alloc(fc);
> --
> 2.13.6
>