linux-kernel - Re: [PATCH v28 25/32] x86/cet/shstk: Handle thread shadow stack

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Message-ID: <9b5eb1a8-fc75-d1e4-36c4-e83ab0ce682b@intel.com>
Date:   Thu, 22 Jul 2021 14:05:29 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Rick P Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH v28 25/32] x86/cet/shstk: Handle thread shadow stack

On 7/22/21 1:52 PM, Yu-cheng Yu wrote:
> +	if (!stack_size)
> +		stack_size = min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G);
> +
> +	if (!shstk->size)
> +		return 0;
> +
> +	/*
> +	 * For CLONE_VM, except vfork, the child needs a separate shadow
> +	 * stack.
> +	 */
> +	if ((clone_flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM)
> +		return 0;
> +
> +	/*
> +	 * This is in clone() syscall and fpu__copy() already copies xstates
> +	 * from the parent.  If get_xsave_addr() returns null, then XFEATURE_
> +	 * CET_USER is still in init state, which certainly is an error.
> +	 */
> +	state = get_xsave_addr(&tsk->thread.fpu.state.xsave, XFEATURE_CET_USER);
> +	if (!state)
> +		return -EINVAL;

I don't care much for that comment.

This code is meant to copy shadow stack config information into children
when it is already enabled.  We *just* checked for that above in the
"shstk->size" check.  The fact that this is called from clone() is
irrelevant.  The shadow stack enabling status *is*.

I think I'd rather this be more along the lines of:

	/*
	 * 'tsk' is configured with a shadow stack and the fpu.state is
	 * up to date since it was just copied from the parent.  There
	 * must be a valid non-init CET state location in the buffer.
	 */

There is also a strong enough assumption violation that I'd probably
WARN() in addition to returning -EINVAL.