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

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Date: Wed, 31 Jan 2024 19:23:03 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Stefan Berger <stefanb@...ux.ibm.com>
Cc: linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org, 
	linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com, zohar@...ux.ibm.com, 
	roberto.sassu@...wei.com, miklos@...redi.hu
Subject: Re: [PATCH 4/5] evm: Use the real inode's metadata to calculate
 metadata hash

On Wed, Jan 31, 2024 at 5:54 PM Amir Goldstein <amir73il@...il.com> wrote:
>
> On Wed, Jan 31, 2024 at 4:40 PM Stefan Berger <stefanb@...ux.ibm.com> wrote:
> >
> >
> >
> > On 1/31/24 08:16, Amir Goldstein wrote:
> > > On Wed, Jan 31, 2024 at 4:11 AM Stefan Berger <stefanb@...ux.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@...ux.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.
>

No, I was wrong. It is the other way around.
d_real_inode() always returns the real data inode and you need the
upper most real inode.

You can try this:

 -     struct inode *inode = d_backing_inode(dentry);
+     struct inode *inode = d_inode(d_real(dentry, false));

With the changes in:

https://github.com/amir73il/linux/commits/overlayfs-devel/

Not thoroughly tested...

Thanks,
Amir.