Re: [PATCH] overlayfs: Trigger file re-evaluation by IMA / EVM after writes

[Christian Brauner <brauner@kernel.org>]

On Fri, May 19, 2023 at 03:42:38PM -0400, Mimi Zohar wrote:
> On Fri, 2023-04-07 at 10:31 +0200, Christian Brauner wrote:
> > So, I think we want both; we want the ovl_copyattr() and the
> > vfs_getattr_nosec() change:
> > 
> > (1) overlayfs should copy up the inode version in ovl_copyattr(). That
> >     is in line what we do with all other inode attributes. IOW, the
> >     overlayfs inode's i_version counter should aim to mirror the
> >     relevant layer's i_version counter. I wouldn't know why that
> >     shouldn't be the case. Asking the other way around there doesn't
> >     seem to be any use for overlayfs inodes to have an i_version that
> >     isn't just mirroring the relevant layer's i_version.
> > (2) Jeff's changes for ima to make it rely on vfs_getattr_nosec().
> >     Currently, ima assumes that it will get the correct i_version from
> >     an inode but that just doesn't hold for stacking filesystem.
> > 
> > While (1) would likely just fix the immediate bug (2) is correct and
> > _robust_. If we change how attributes are handled vfs_*() helpers will
> > get updated and ima with it. Poking at raw inodes without using
> > appropriate helpers is much more likely to get ima into trouble.
> 
> In addition to properly setting the i_version for IMA, EVM has a
> similar issue with i_generation and s_uuid. Adding them to
> ovl_copyattr() seems to resolve it.   Does that make sense?
> 
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 923d66d131c1..cd0aeb828868 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -1118,5 +1118,8 @@ void ovl_copyattr(struct inode *inode)
>  	inode->i_atime = realinode->i_atime;
>  	inode->i_mtime = realinode->i_mtime;
>  	inode->i_ctime = realinode->i_ctime;
> +	inode->i_generation = realinode->i_generation;
> +	if (inode->i_sb)
> +		uuid_copy(&inode->i_sb->s_uuid, &realinode->i_sb-

Overlayfs can consist of multiple lower layers and each of those lower
layers may have a different uuid. So everytime you trigger a
ovl_copyattr() on a different layer this patch would alter the uuid of
the overlayfs superblock.

In addition the uuid should be set when the filesystem is mounted.
Unless the filesystem implements a dedicated ioctl() - like ext4 - to
change the uuid.