Re: [PATCH RESEND v2 16/18] fuse: Support fuse filesystems outside of init_user_ns

[Miklos Szeredi]

On Mon, Jan 04, 2016 at 12:03:55PM -0600, Seth Forshee wrote:
> 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.
> 
> Signed-off-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
> ---
>  fs/fuse/cuse.c   |  3 ++-
>  fs/fuse/dev.c    | 13 ++++++++-----
>  fs/fuse/dir.c    | 14 +++++++-------
>  fs/fuse/fuse_i.h |  6 +++++-
>  fs/fuse/inode.c  | 35 +++++++++++++++++++++++------------
>  5 files changed, 45 insertions(+), 26 deletions(-)
> 
> diff --git a/fs/fuse/cuse.c b/fs/fuse/cuse.c
> index eae2c11268bc..a10aca57bfe4 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 a4f6f30d6d86..11b4cb0a0e2f 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -127,8 +127,8 @@ 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);
>  }
>  
> @@ -186,7 +186,8 @@ 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.pid == 0) {
> +	if (req->in.h.pid == 0 || req->in.h.uid == (uid_t)-1 ||
> +	    req->in.h.gid == (gid_t)-1) {
>  		fuse_put_request(fc, req);
>  		return ERR_PTR(-EOVERFLOW);
>  	}
> @@ -1248,7 +1249,8 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
>  	struct fuse_in *in;
>  	unsigned reqsize;
>  
> -	if (task_active_pid_ns(current) != fc->pid_ns)
> +	if (task_active_pid_ns(current) != fc->pid_ns ||
> +	    current_user_ns() != fc->user_ns)
>  		return -EIO;
>  
>   restart:
> @@ -1880,7 +1882,8 @@ static ssize_t fuse_dev_do_write(struct fuse_dev *fud,
>  	struct fuse_req *req;
>  	struct fuse_out_header oh;
>  
> -	if (task_active_pid_ns(current) != fc->pid_ns)
> +	if (task_active_pid_ns(current) != fc->pid_ns ||
> +	    current_user_ns() != fc->user_ns)
>  		return -EIO;
>  
>  	if (nbytes < sizeof(struct fuse_out_header))
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index 5e2e08712d3b..8fd9fe4dcd43 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -841,8 +841,8 @@ 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 = inode->i_uid;
> +	stat->gid = inode->i_gid;

This breaks the attr_version logic in fuse_change_attributes().

So just use make_k[ug]id() here as well.

>  	stat->rdev = inode->i_rdev;
>  	stat->atime.tv_sec = attr->atime;
>  	stat->atime.tv_nsec = attr->atimensec;
> @@ -1455,17 +1455,17 @@ 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) {
> @@ -1625,7 +1625,7 @@ int fuse_do_setattr(struct inode *inode, 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 143b595197b6..5897805405ba 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -23,6 +23,7 @@
>  #include <linux/poll.h>
>  #include <linux/workqueue.h>
>  #include <linux/pid_namespace.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
> @@ -460,6 +461,9 @@ 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;
> +
>  	/** The fuse mount flags for this mount */
>  	unsigned flags;
>  
> @@ -855,7 +859,7 @@ 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 2f31874ea9db..b7bdfdac3521 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -167,8 +167,8 @@ 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;
> @@ -467,12 +467,15 @@ 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));
>  	d->max_read = ~0;
>  	d->blksize = FUSE_DEFAULT_BLKSIZE;
> +	d->user_id = make_kuid(user_ns, 0);
> +	d->group_id = make_kgid(user_ns, 0);

It is true that if "user_id=" or "group_id" options were omitted we used the
zero uid/gid values.  However, this isn't actually used by anybody AFAIK, and
generalizing it for userns doesn't seem to make much sense.

So I suggest we that we instead return an error if mounting from a userns AND
neither "allow_other" nor both "user_id" and "group_id" are specified.


>  
>  	while ((p = strsep(&opt, ",")) != NULL) {
>  		int token;
> @@ -503,7 +506,7 @@ 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;
> @@ -512,7 +515,7 @@ 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;
> @@ -555,8 +558,10 @@ 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->flags & FUSE_DEFAULT_PERMISSIONS)
>  		seq_puts(m, ",default_permissions");
>  	if (fc->flags & FUSE_ALLOW_OTHER)
> @@ -587,7 +592,7 @@ 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);
> @@ -611,6 +616,7 @@ 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);
>  
> @@ -620,6 +626,7 @@ 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);
>  	}
>  }
> @@ -1046,7 +1053,7 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
>  
>  	sb->s_flags &= ~(MS_NOSEC | MS_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) {
> @@ -1070,8 +1077,12 @@ 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);
> @@ -1079,7 +1090,7 @@ 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);
> -- 
> 1.9.1
>