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

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Date: Tue, 30 Jan 2024 21:10:47 -0500
From: Stefan Berger <stefanb@...ux.ibm.com>
To: linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-unionfs@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, paul@...l-moore.com, jmorris@...ei.org,
        serge@...lyn.com, zohar@...ux.ibm.com, roberto.sassu@...wei.com,
        amir73il@...il.com, miklos@...redi.hu
Subject: Re: [PATCH 4/5] evm: Use the real inode's metadata to calculate
 metadata hash



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.