Re: [PATCH 2/5] evm: Implement per signature type decision in security_inode_copy_up_xattr

[Amir Goldstein]

On Tue, Jan 30, 2024 at 11:46 PM Stefan Berger <stefanb@xxxxxxxxxxxxx> wrote:
>
> To support portable and immutable signatures on otherwise unsupported
> filesystems, determine the EVM signature type by the content of a file's
> xattr. If the file has the appropriate signature then allow it to be
> copied up. All other signature types are discarded as before.
>
> Portable and immutable EVM signatures can be copied up by stacked file-
> system since the metadata their signature covers does not include file-
> system-specific data such as a file's inode number, generation, and UUID.
>
> Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> ---
>  security/integrity/evm/evm_main.c | 27 ++++++++++++++++++++++++---
>  1 file changed, 24 insertions(+), 3 deletions(-)
>
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 2555aa4501ae..22a5e26860ea 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -898,9 +898,30 @@ void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
>
>  int evm_inode_copy_up_xattr(struct dentry *src, const char *name)
>  {
> -       if (strcmp(name, XATTR_NAME_EVM) == 0)
> -               return 1; /* Discard */
> -       return -EOPNOTSUPP;
> +       struct evm_ima_xattr_data *xattr_data = NULL;
> +       int rc;
> +
> +       if (strcmp(name, XATTR_NAME_EVM) != 0)
> +               return -EOPNOTSUPP;
> +
> +       /* first need to know the sig type */
> +       rc = vfs_getxattr_alloc(&nop_mnt_idmap, src, XATTR_NAME_EVM,
> +                               (char **)&xattr_data, 0, GFP_NOFS);


See my suggestion for post-getxattr hook:
security_inode_copy_up_xattr(name, value, size)
to avoid using nop_mnt_idmap here.

Unless it is fine to use nop_mnt_idmap in this context? not sure.

Thanks,
Amir.