Re: [PATCH v2 14/25] evm: add support for fscaps security hooks

[Roberto Sassu]

On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> Support the new fscaps security hooks by converting the vfs_caps to raw
> xattr data and then handling them the same as other xattrs.

I realized that you need to register hooks for IMA too.

This should be the content to add in ima_appraise.c:

int ima_inode_set_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
                         const struct vfs_caps *caps, int flags)
{
       if (evm_revalidate_status(XATTR_NAME_CAPS))
               ima_reset_appraise_flags(d_backing_inode(dentry), false);

       return 0;
}

int ima_inode_remove_fscaps(struct mnt_idmap *idmap, struct dentry *dentry)
{
       return ima_inode_set_fscaps(idmap, dentry, NULL, XATTR_REPLACE);
}

Thanks

Roberto

> Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@xxxxxxxxxx>
> ---
>  include/linux/evm.h               | 39 +++++++++++++++++++++++++
>  security/integrity/evm/evm_main.c | 60 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 99 insertions(+)
> 
> diff --git a/include/linux/evm.h b/include/linux/evm.h
> index 36ec884320d9..aeb9ff52ad22 100644
> --- a/include/linux/evm.h
> +++ b/include/linux/evm.h
> @@ -57,6 +57,20 @@ static inline void evm_inode_post_set_acl(struct dentry *dentry,
>  {
>  	return evm_inode_post_setxattr(dentry, acl_name, NULL, 0);
>  }
> +extern int evm_inode_set_fscaps(struct mnt_idmap *idmap,
> +				struct dentry *dentry,
> +				const struct vfs_caps *caps, int flags);
> +static inline int evm_inode_remove_fscaps(struct dentry *dentry)
> +{
> +	return evm_inode_set_fscaps(&nop_mnt_idmap, dentry, NULL, XATTR_REPLACE);
> +}
> +extern void evm_inode_post_set_fscaps(struct mnt_idmap *idmap,
> +				      struct dentry *dentry,
> +				      const struct vfs_caps *caps, int flags);
> +static inline void evm_inode_post_remove_fscaps(struct dentry *dentry)
> +{
> +	return evm_inode_post_set_fscaps(&nop_mnt_idmap, dentry, NULL, 0);
> +}
>  
>  int evm_inode_init_security(struct inode *inode, struct inode *dir,
>  			    const struct qstr *qstr, struct xattr *xattrs,
> @@ -164,6 +178,31 @@ static inline void evm_inode_post_set_acl(struct dentry *dentry,
>  	return;
>  }
>  
> +static inline int evm_inode_set_fscaps(struct mnt_idmap *idmap,
> +				       struct dentry *dentry,
> +				       const struct vfs_caps *caps, int flags)
> +{
> +	return 0;
> +}
> +
> +static inline int evm_inode_remove_fscaps(struct dentry *dentry)
> +{
> +	return 0;
> +}
> +
> +static inline void evm_inode_post_set_fscaps(struct mnt_idmap *idmap,
> +					     struct dentry *dentry,
> +					     const struct vfs_caps *caps,
> +					     int flags)
> +{
> +	return;
> +}
> +
> +static inline void evm_inode_post_remove_fscaps(struct dentry *dentry)
> +{
> +	return;
> +}
> +
>  static inline int evm_inode_init_security(struct inode *inode, struct inode *dir,
>  					  const struct qstr *qstr,
>  					  struct xattr *xattrs,
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index cc7956d7878b..ecf4634a921a 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -805,6 +805,66 @@ void evm_inode_post_removexattr(struct dentry *dentry, const char *xattr_name)
>  	evm_update_evmxattr(dentry, xattr_name, NULL, 0);
>  }
>  
> +int evm_inode_set_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +			 const struct vfs_caps *caps, int flags)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct vfs_ns_cap_data nscaps;
> +	const void *xattr_data = NULL;
> +	int size = 0;
> +
> +	/* Policy permits modification of the protected xattrs even though
> +	 * there's no HMAC key loaded
> +	 */
> +	if (evm_initialized & EVM_ALLOW_METADATA_WRITES)
> +		return 0;
> +
> +	if (caps) {
> +		size = vfs_caps_to_xattr(idmap, i_user_ns(inode), caps, &nscaps,
> +					 sizeof(nscaps));
> +		if (size < 0)
> +			return size;
> +		xattr_data = &nscaps;
> +	}
> +
> +	return evm_protect_xattr(idmap, dentry, XATTR_NAME_CAPS, xattr_data, size);
> +}
> +
> +void evm_inode_post_set_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +			       const struct vfs_caps *caps, int flags)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct vfs_ns_cap_data nscaps;
> +	const void *xattr_data = NULL;
> +	int size = 0;
> +
> +	if (!evm_revalidate_status(XATTR_NAME_CAPS))
> +		return;
> +
> +	evm_reset_status(dentry->d_inode);
> +
> +	if (!(evm_initialized & EVM_INIT_HMAC))
> +		return;
> +
> +	if (is_unsupported_fs(dentry))
> +		return;
> +
> +	if (caps) {
> +		size = vfs_caps_to_xattr(idmap, i_user_ns(inode), caps, &nscaps,
> +					 sizeof(nscaps));
> +		/*
> +		 * The fscaps here should have been converted to an xattr by
> +		 * evm_inode_set_fscaps() already, so a failure to convert
> +		 * here is a bug.
> +		 */
> +		if (WARN_ON_ONCE(size < 0))
> +			return;
> +		xattr_data = &nscaps;
> +	}
> +
> +	evm_update_evmxattr(dentry, XATTR_NAME_CAPS, xattr_data, size);
> +}
> +
>  static int evm_attr_change(struct mnt_idmap *idmap,
>  			   struct dentry *dentry, struct iattr *attr)
>  {
>