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

Amir Goldstein
2023-05-20T09:15:50Z

On Fri, May 19, 2023 at 10:42 PM Mimi Zohar <zohar@linux.ibm.com> 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-
> >s_uuid);

That is not a possible solution Mimi.

The i_gneration copy *may* be acceptable in "all layers on same fs"
setup, but changing overlayfs s_uuid over and over is a non-starter.

If you explain the problem, I may be able to help you find a better solution.

Thanks,
Amir.