[RFC PATCH v4 1/2] fuse: introduce new fs_type flag FS_IMA_NO_CACHE

[Mimi Zohar]

Hi Miklos,

On Tue, 2018-01-30 at 19:06 +0100, Dongsu Park wrote:
> From: Alban Crequy <alban at kinvolk.io>
> 
> This new fs_type flag FS_IMA_NO_CACHE means files should be re-measured,
> re-appraised and re-audited each time. Cached integrity results should
> not be used.
> 
> It is useful in FUSE because the userspace FUSE process can change the
> underlying files at any time without notifying the kernel.

Both IMA-measurement and IMA-appraisal cache the integrity results and
are dependent on the kernel to detect when a file changes in order to
clear the cached info and force the file to be re-evaluated.  This
detection was dependent on i_version changing.  For filesystems that
do not support i_version, remote or fuse filesystems, where the kernel
does not detect the file change, the file was measured and the
signature evaluated just once.

With commit a2a2c3c8580a ("ima: Use i_version only when filesystem
supports it"), which is being upstreamed in this open window,
i_version is considered an optimization.  If i_version is not enabled,
either because the local filesystem does not support it or the
filesystem wasn't mounted with i_version, the file will now always be
re-evaluated.

That patch does not address FUSE or remote filesystems, as the kernel
does not detect the change.  Further, even if the kernel could detect
the change, FUSE filesystems by definition are untrusted.

The original patches addressed FUSE filesystems, by defining a new IMA
policy option, forcing the file to be re-evaluated based on the
filesystem magic number.  All of the changes were in the IMA
subsystem.  These patches are the result of Christoph's comment on the
original patches saying, "ima has no business looking at either the
name _or_ the magic number."

Your help in resolving this problem is much appreciated!

Mimi

> 
> Cc: linux-kernel at vger.kernel.org
> Cc: linux-integrity at vger.kernel.org
> Cc: linux-security-module at vger.kernel.org
> Cc: linux-fsdevel at vger.kernel.org
> Cc: Miklos Szeredi <miklos at szeredi.hu>
> Cc: Alexander Viro <viro at zeniv.linux.org.uk>
> Cc: Mimi Zohar <zohar at linux.vnet.ibm.com>
> Cc: Dmitry Kasatkin <dmitry.kasatkin at gmail.com>
> Cc: James Morris <jmorris at namei.org>
> Cc: Christoph Hellwig <hch at infradead.org>
> Acked-by: "Serge E. Hallyn" <serge at hallyn.com>
> Acked-by: Seth Forshee <seth.forshee at canonical.com>
> Tested-by: Dongsu Park <dongsu at kinvolk.io>
> Signed-off-by: Alban Crequy <alban at kinvolk.io>
> ---
>  fs/fuse/inode.c    | 2 +-
>  include/linux/fs.h | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 624f18bb..0a9e5164 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1205,7 +1205,7 @@ static void fuse_kill_sb_anon(struct super_block *sb)
>  static struct file_system_type fuse_fs_type = {
>  	.owner		= THIS_MODULE,
>  	.name		= "fuse",
> -	.fs_flags	= FS_HAS_SUBTYPE,
> +	.fs_flags	= FS_HAS_SUBTYPE | FS_IMA_NO_CACHE,
>  	.mount		= fuse_mount,
>  	.kill_sb	= fuse_kill_sb_anon,
>  };
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 511fbaab..ced841ba 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2075,6 +2075,7 @@ struct file_system_type {
>  #define FS_BINARY_MOUNTDATA	2
>  #define FS_HAS_SUBTYPE		4
>  #define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
> +#define FS_IMA_NO_CACHE		16	/* Force IMA to re-measure, re-appraise, re-audit files */
>  #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
>  	struct dentry *(*mount) (struct file_system_type *, int,
>  		       const char *, void *);

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