[PATCH] evm: Don't update hmacs in user ns mounts

[Mimi Zohar]

On Wed, 2018-05-02 at 16:49 -0500, Eric W. Biederman wrote:
> From: Seth Forshee <seth.forshee at canonical.com>
> Date: Fri, 22 Dec 2017 15:32:35 +0100
> 
> The kernel should not calculate new hmacs for mounts done by
> non-root users. Update evm_calc_hmac_or_hash() to refuse to
> calculate new hmacs for mounts for non-init user namespaces.
> 
> Cc: linux-integrity at vger.kernel.org
> Cc: linux-security-module at vger.kernel.org
> Cc: linux-kernel at vger.kernel.org
> Cc: James Morris <james.l.morris at oracle.com>
> Cc: Mimi Zohar <zohar at linux.vnet.ibm.com>
> Cc: "Serge E. Hallyn" <serge at hallyn.com>
> Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> Signed-off-by: Dongsu Park <dongsu at kinvolk.io>
> Signed-off-by: Eric W. Biederman <ebiederm at xmission.com>
> ---
> 
> Mimi this patch has been floating around for a while and it appears to
> be the only piece missing from the vfs to make unprivileged mounts safe
> (at least semantically).  Do you want to merge this through your integrity
> tree or should merge this through my userns tree?

Matthew's EVM patches don't conflict with this change, so either way
is fine.

Mimi

> 
>  security/integrity/evm/evm_crypto.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> 
> diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> index a46fba322340..facf9cdd577d 100644
> --- a/security/integrity/evm/evm_crypto.c
> +++ b/security/integrity/evm/evm_crypto.c
> @@ -200,7 +200,8 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
>  	int size;
>  	bool ima_present = false;
> 
> -	if (!(inode->i_opflags & IOP_XATTR))
> +	if (!(inode->i_opflags & IOP_XATTR) ||
> +	    inode->i_sb->s_user_ns != &init_user_ns)
>  		return -EOPNOTSUPP;
> 
>  	desc = init_desc(type);

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