[PATCH 17/19] fuse: Support fuse filesystems outside of init_user_ns

[Serge E. Hallyn]

Quoting Seth Forshee (seth.forshee at canonical.com):
> Update fuse to translate uids and gids to/from the user namspace
> of the process servicing requests on /dev/fuse. Any ids which do
> not map into the namespace will result in errors. inodes will
> also be marked bad when unmappable ids are received from the
> userspace fuse process.
> 
> Currently no use cases are known for letting the userspace fuse
> daemon switch namespaces after opening /dev/fuse. Given this
> fact, and in order to keep the implementation as simple as
> possible and ease security auditing, the user namespace from
> which /dev/fuse is opened is used for all id translations. This
> is required to be the same namespace as s_user_ns to maintain
> behavior consistent with other filesystems which can be mounted
> in user namespaces.
> 
> 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 at canonical.com>
> ---
>  fs/fuse/cuse.c   |  3 +-
>  fs/fuse/dev.c    | 13 +++++----
>  fs/fuse/dir.c    | 81 ++++++++++++++++++++++++++++++++++-------------------
>  fs/fuse/fuse_i.h | 14 ++++++----
>  fs/fuse/inode.c  | 85 ++++++++++++++++++++++++++++++++++++++++++--------------
>  5 files changed, 136 insertions(+), 60 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)

Do you think this should be using current_in_user_ns(fc->user_ns) ?

Opening a file, forking (maybe unsharing) then acting on the file is
pretty standard fare which this would break.

(same for pidns, i guess, as well as for write below)

>  		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))