Re: [PATCH v3] ptrace: reintroduce usage of subjective credentials in ptrace_has_cap()

[Kees Cook <keescook@chromium.org>]

On Fri, Jan 17, 2020 at 11:57:18AM +0100, Christian Brauner wrote:
> -static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
> +static int ptrace_has_cap(const struct cred *cred, struct user_namespace *ns,
> +			  unsigned int mode)
>  {
> -	if (mode & PTRACE_MODE_NOAUDIT)
> -		return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
> -	else
> -		return has_ns_capability(current, ns, CAP_SYS_PTRACE);
> +	return security_capable(cred, ns, CAP_SYS_PTRACE,
> +				(mode & PTRACE_MODE_NOAUDIT) ? CAP_OPT_NOAUDIT :
> +							       CAP_OPT_NONE);
>  }

Eek, no. I think this inverts the check.

Before:
bool has_ns_capability(struct task_struct *t,
                       struct user_namespace *ns, int cap)
{
	...
        ret = security_capable(__task_cred(t), ns, cap, CAP_OPT_NONE);
	...
        return (ret == 0);
}

static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
{
	...
                return has_ns_capability(current, ns, CAP_SYS_PTRACE);
}

After:
static int ptrace_has_cap(const struct cred *cred, struct user_namespace *ns,
                       unsigned int mode)
{
	return security_capable(cred, ns, CAP_SYS_PTRACE,
				(mode & PTRACE_MODE_NOAUDIT) ? CAP_OPT_NOAUDIT :
							       CAP_OPT_NONE);
}

Note lack of "== 0" on the security_capable() return value, but it's
needed. To avoid confusion, I think ptrace_has_cap() should likely
return bool too.

-Kees

-- 
Kees Cook