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

[Amir Goldstein]

On Wed, Jan 31, 2024 at 4:40 PM Stefan Berger <stefanb at linux.ibm.com> wrote:
>
>
>
> On 1/31/24 08:16, Amir Goldstein wrote:
> > 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.
>
> I thought that we may have to go through vfs_getattr() but even better
> if we don't because we don't have the file *file anywhere 'near'.
>
> >
> > 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.
>
> What it needs are data structures with mode bits, uid, and gid that stat
> in userspace would show.
>
>

With or without metacopy enabled, an overlay inode st_uid st_gid st_mode
are always taken from the upper most inode which is what d_real_inode()
currently returns, so I do not understand what the problem is.

> >
> > 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.
>
> I always compare against what userspace sees with stat and that's what
> the EVM should also work with so it ends up in reasonable matching
> result in terms of hash calculation and then access permission/rejection.
>

I will need a lot more analysis information to be able to help you.
Exactly which setup, exactly which test, exactly which inode/dentry/file
objects are used and how they are accessed when things go wrong.

Thanks,
Amir.