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

Miklos Szeredi
2016-03-09T11:29:23Z

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@canonical.com>
> ---
> 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
>