linux-kernel - Re: [PATCH v3 25/37] x86/shstk: Add user-mode shadow stack support

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Message-ID: <Y3OG31hoj/q4Bgh7@hirez.programming.kicks-ass.net>
Date:   Tue, 15 Nov 2022 13:32:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     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>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v3 25/37] x86/shstk: Add user-mode shadow stack support

On Fri, Nov 04, 2022 at 03:35:52PM -0700, Rick Edgecombe wrote:

> +static int shstk_setup(void)
> +{
> +	struct thread_shstk *shstk = &current->thread.shstk;
> +	unsigned long addr, size;
> +
> +	/* Already enabled */
> +	if (features_enabled(CET_SHSTK))
> +		return 0;
> +
> +	/* Also not supported for 32 bit and x32 */
> +	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) || in_32bit_syscall())
> +		return -EOPNOTSUPP;
> +
> +	size = adjust_shstk_size(0);
> +	addr = alloc_shstk(size);
> +	if (IS_ERR_VALUE(addr))
> +		return PTR_ERR((void *)addr);
> +
> +	fpregs_lock_and_load();
> +	wrmsrl(MSR_IA32_PL3_SSP, addr + size);
> +	wrmsrl(MSR_IA32_U_CET, CET_SHSTK_EN);

This..

> +	fpregs_unlock();
> +
> +	shstk->base = addr;
> +	shstk->size = size;
> +	features_set(CET_SHSTK);
> +
> +	return 0;
> +}

> +static int shstk_disable(void)
> +{
> +	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
> +		return -EOPNOTSUPP;
> +
> +	/* Already disabled? */
> +	if (!features_enabled(CET_SHSTK))
> +		return 0;
> +
> +	fpregs_lock_and_load();
> +	/* Disable WRSS too when disabling shadow stack */
> +	set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_SHSTK_EN);

And this... aren't very consistent in approach. Given there is no U_IBT
yet, why complicate matters like this?

> +	wrmsrl(MSR_IA32_PL3_SSP, 0);
> +	fpregs_unlock();
> +
> +	shstk_free(current);
> +	features_clr(CET_SHSTK);
> +
> +	return 0;
> +}