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From: Amir Goldstein <amir73il@gmail.com>
Date: Wed, 31 Jan 2024 17:54:37 +0200
Message-ID: <CAOQ4uxi6Te8izWpXROthknRaXrVA9jho5nbc+mkuQDrcTLY44Q@mail.gmail.com>
Subject: Re: [PATCH 4/5] evm: Use the real inode's metadata to calculate
 metadata hash
To: Stefan Berger <stefanb@linux.ibm.com>
Cc: linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, 
	linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org, 
	paul@paul-moore.com, jmorris@namei.org, serge@hallyn.com, zohar@linux.ibm.com, 
	roberto.sassu@huawei.com, miklos@szeredi.hu
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On Wed, Jan 31, 2024 at 4:40=E2=80=AFPM Stefan Berger <stefanb@linux.ibm.co=
m> wrote:
>
>
>
> On 1/31/24 08:16, Amir Goldstein wrote:
> > On Wed, Jan 31, 2024 at 4:11=E2=80=AFAM Stefan Berger <stefanb@linux.ib=
m.com> wrote:
> >>
> >>
> >>
> >> On 1/30/24 16:46, Stefan Berger wrote:
> >>> Changes to the file attribute (mode bits, uid, gid) on the lower laye=
r
> >>> 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 u=
p.
> >>> This is because d_backing_inode() does not return the real inode of t=
he
> >>> lower layer but instead returns the backing inode which holds old fil=
e
> >>> 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 ha=
s
> >>> not been copied up and returns the upper layer's inode otherwise.
> >>>
> >>> Signed-off-by: Stefan Berger <stefanb@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 *d=
entry,
> >>>                                 size_t req_xattr_value_len,
> >>>                                 uint8_t type, struct evm_digest *data=
)
> >>>    {
> >>> -     struct inode *inode =3D d_backing_inode(dentry);
> >>> +     struct inode *inode =3D d_real_inode(dentry);
> >>>        struct xattr_list *xattr;
> >>>        struct shash_desc *desc;
> >>>        size_t xattr_size =3D 0;
> >>
> >> We need this patch when NOT activating CONFIG_OVERLAY_FS_METACOPY but
> >> when setting CONFIG_OVERLAY_FS_METACOPY=3Dy 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 user=
s for
> > a metacopy file.
> >
> > I think the solution is to change d_real_inode() to return the data ino=
de
> > 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.