Re: [PATCH v2 9/9] fs/fuse: allow idmapped mounts

[Aleksandr Mikhalitsyn]

On Wed, Aug 14, 2024 at 4:19 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> On Wed, Aug 14, 2024 at 01:40:34PM GMT, Alexander Mikhalitsyn wrote:
> > Now we have everything in place and we can allow idmapped mounts
> > by setting the FS_ALLOW_IDMAP flag. Notice that real availability
> > of idmapped mounts will depend on the fuse daemon. Fuse daemon
> > have to set FUSE_ALLOW_IDMAP flag in the FUSE_INIT reply.
> >
> > To discuss:
> > - we enable idmapped mounts support only if "default_permissions" mode is enabled,
> > because otherwise we would need to deal with UID/GID mappings in the userspace side OR
> > provide the userspace with idmapped req->in.h.uid/req->in.h.gid values which is not
> > something that we probably want to. Idmapped mounts phylosophy is not about faking
> > caller uid/gid.
> >
> > - We have a small offlist discussion with Christian around adding fs_type->allow_idmap
> > hook. Christian pointed that it would be nice to have a superblock flag instead like
> > SB_I_NOIDMAP and we can set this flag during mount time if we see that filesystem does not
> > support idmappings. But, unfortunately I didn't succeed here because the kernel will
> > know if the filesystem supports idmapping or not after FUSE_INIT request, but FUSE_INIT request
> > is being sent at the end of mounting process, so mount and superblock will exist and
> > visible by the userspace in that time. It seems like setting SB_I_NOIDMAP flag in this
> > case is too late as user may do the trick with creating a idmapped mount while it wasn't
> > restricted by SB_I_NOIDMAP. Alternatively, we can introduce a "positive" version SB_I_ALLOWIDMAP

Hi Christian,

>
> Hm, I'm confused why won't the following (uncompiled) work?

I believe that your way should work. Sorry about that. It's my bad that I
didn't consider setting SB_I_NOIDMAP in fill_super and unsetting it
later on once
we had enough information.

Huge thanks for pointing this out!

I'll drop -v3 soon and also add support for virtiofs in the same series.

Kind regards,
Alex

>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index ed4c2688047f..8ead1cacdd2f 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1346,10 +1346,12 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
>                         if (flags & FUSE_OWNER_UID_GID_EXT)
>                                 fc->owner_uid_gid_ext = 1;
>                         if (flags & FUSE_ALLOW_IDMAP) {
> -                               if (fc->owner_uid_gid_ext && fc->default_permissions)
> +                               if (fc->owner_uid_gid_ext && fc->default_permissions) {
>                                         fc->allow_idmap = 1;
> -                               else
> +                                       fm->sb->s_iflags &= ~SB_I_NOIDMAP;
> +                               } else {
>                                         ok = false;
> +                               }
>                         }
>                 } else {
>                         ra_pages = fc->max_read / PAGE_SIZE;
> @@ -1576,6 +1578,7 @@ static void fuse_sb_defaults(struct super_block *sb)
>         sb->s_time_gran = 1;
>         sb->s_export_op = &fuse_export_operations;
>         sb->s_iflags |= SB_I_IMA_UNVERIFIABLE_SIGNATURE;
> +       sb->s_iflags |= SB_I_NOIDMAP;
>         if (sb->s_user_ns != &init_user_ns)
>                 sb->s_iflags |= SB_I_UNTRUSTED_MOUNTER;
>         sb->s_flags &= ~(SB_NOSEC | SB_I_VERSION);
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 328087a4df8a..d1702285c915 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -4436,6 +4436,10 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
>         if (!(m->mnt_sb->s_type->fs_flags & FS_ALLOW_IDMAP))
>                 return -EINVAL;
>
> +       /* The filesystem has turned off idmapped mounts. */
> +       if (m->mnt_sb->s_iflags & SB_I_NOIDMAP)
> +               return -EINVAL;
> +
>         /* We're not controlling the superblock. */
>         if (!ns_capable(fs_userns, CAP_SYS_ADMIN))
>                 return -EPERM;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index fd34b5755c0b..185004c41a5e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1189,6 +1189,7 @@ extern int send_sigurg(struct fown_struct *fown);
>  #define SB_I_TS_EXPIRY_WARNED 0x00000400 /* warned about timestamp range expiry */
>  #define SB_I_RETIRED   0x00000800      /* superblock shouldn't be reused */
>  #define SB_I_NOUMASK   0x00001000      /* VFS does not apply umask */
> +#define SB_I_NOIDMAP   0x00002000      /* No idmapped mounts on this superblock */
>
>  /* Possible states of 'frozen' field */
>  enum {