[PATCH v4 19/21] fuse: Support fuse filesystems outside of init_user_ns

[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.

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  | 33 +++++++++++++++++++++------------
 5 files changed, 43 insertions(+), 26 deletions(-)

diff --git a/fs/fuse/cuse.c b/fs/fuse/cuse.c
index c5b6b7165489..98ebd0f4fd4c 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 4e91b2ac25a7..8fa1ce934df3 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 4b855b65d457..ecba75bf6640 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 = 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;
@@ -1459,17 +1459,17 @@ 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) {
@@ -1629,7 +1629,7 @@ 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 9145445a759a..9f4c3c82edd6 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -24,6 +24,7 @@
 #include <linux/workqueue.h>
 #include <linux/kref.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
@@ -469,6 +470,9 @@ 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;
 
@@ -867,7 +871,7 @@ 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 eade0bfa4488..0a771145d853 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;
@@ -467,7 +467,8 @@ 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));
@@ -503,7 +504,7 @@ 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 +513,7 @@ 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 +556,10 @@ 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 +590,7 @@ 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 +614,7 @@ 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 +624,7 @@ 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 +1051,7 @@ 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 +1075,12 @@ 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 +1088,7 @@ 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);
-- 
2.7.4