[PATCH 4/5] evm: Use the real inode's metadata to calculate metadata hash

[Amir Goldstein]

On Wed, Jan 31, 2024 at 4:11 AM Stefan Berger <stefanb at linux.ibm.com> wrote:
>
>
>
> On 1/30/24 16:46, Stefan Berger wrote:
> > Changes to the file attribute (mode bits, uid, gid) on the lower layer
> > are not take into account when d_backing_inode() is used when a file is
> > accessed on the overlay layer and this file has not yet been copied up.
> > This is because d_backing_inode() does not return the real inode of the
> > lower layer but instead returns the backing inode which holds old file
> > attributes. When the old file attributes are used for calculating the
> > metadata hash then the expected hash is calculated and the file then
> > mistakenly passes signature verification. Therefore, use d_real_inode()
> > which returns the inode of the lower layer for as long as the file has
> > not been copied up and returns the upper layer's inode otherwise.
> >
> > Signed-off-by: Stefan Berger <stefanb at linux.ibm.com>
> > ---
> >   security/integrity/evm/evm_crypto.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> > index b1ffd4cc0b44..2e48fe54e899 100644
> > --- a/security/integrity/evm/evm_crypto.c
> > +++ b/security/integrity/evm/evm_crypto.c
> > @@ -223,7 +223,7 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
> >                                size_t req_xattr_value_len,
> >                                uint8_t type, struct evm_digest *data)
> >   {
> > -     struct inode *inode = d_backing_inode(dentry);
> > +     struct inode *inode = d_real_inode(dentry);
> >       struct xattr_list *xattr;
> >       struct shash_desc *desc;
> >       size_t xattr_size = 0;
>
> We need this patch when NOT activating CONFIG_OVERLAY_FS_METACOPY but
> when setting CONFIG_OVERLAY_FS_METACOPY=y it has to be reverted...  I am
> not sure what the solution is.

I think d_real_inode() does not work correctly for all its current users for
a metacopy file.

I think the solution is to change d_real_inode() to return the data inode
and add another helper to get the metadata inode if needed.
I will post some patches for it.

However, I must say that I do not know if evm_calc_hmac_or_hash()
needs the lower data inode, the upper metadata inode or both.

The last time you tried to fix ovl+IMA, I asked for documentation
of what data/metadata is protected with EVM and how are those
protections supposed to work across overlayfs copy up, when the
data and metadata are often split between 2 and myabe event 3
differnt inode.

>From the current patch set, I still don't understand what is the expected
behavior before and after copy up of data/metadata-only.

Thanks,
Amir.