Re: [RFC PATCH 4/4] VFS: allow filesystem to override mknod capability checks

[Serge E. Hallyn]

Quoting Miklos Szeredi (miklos@xxxxxxxxxx):
> > > From: Miklos Szeredi <mszeredi@xxxxxxx>
> > > 
> > > Add a new filesystem flag, that results in the VFS not checking if the
> > > current process has enough privileges to do an mknod().
> > > 
> > > This is needed on filesystems, where an unprivileged user may be able
> > > to create a device node, without causing security problems.
> > > 
> > > One such example is "mountlo" a loopback mount utility implemented
> > > with fuse and UML, which runs as an unprivileged userspace process.
> > > In this case the user does in fact have the right to create device
> > > nodes within the filesystem image, as long as the user has write
> > > access to the image.  Since the filesystem is mounted with "nodev",
> > > adding device nodes is not a security concern.
> > 
> > Could we enforce at do_new_mount() that if
> > type->fs_flags&FS_MKNOD_CHECKS_PERM then mnt_flags |= MS_NODEV?
> 
> Well, the problem with that is, there will be fuse filesystems which
> will want devices to work

Crud, sorry, I forgot all fuse filesystems will have the same fs_flags.

> and for those the capability checks will be
> reenabled inside ->mknod().  In fact, for backward compatibility all
> filesystems will have the mknod checks, except ones which explicitly
> request to turn it off.
> 
> Since unprivileged fuse mounts always have "nodev", the only way

Ah yes, I'd forgotten that we do if (!capable(mknod)) mnt_flags |= MNT_NODEV

No objections then anyway.  Thanks for indulging me :)

> security could be screwed up, is if a filesystem running with
> privileges disabled the mknod checks.
> 
> I will probably add some safety guards against that into the fuse
> library, but of course there's no way to stop a privileged user from
> screwing up security anyway.

Agreed.

> If for example there's a loop mount, where the disk image file is
> writable by a user, and root mounts it without "nodev", the user can
> still create device nodes (by modifying the image) even if the mknod
> checks are enabled.

thanks,
-serge

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