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

Mimi Zohar
2023-05-22T12:22:29Z

On Sat, 2023-05-20 at 12:15 +0300, Amir Goldstein wrote:
> 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.

EVM calculates an HMAC of the file metadata (security xattrs, i_ino,
i_generation, i_uid, i_gid, i_mode, s_uuid) and stores it as
security.evm. Notrmally this would be used for mutable files, which
cannot be signed. The i_generation and s_uuid on the lower layer and
the overlay are not the same, causing the EVM HMAC verification to
fail.

--
thanks,

Mimi