linux-kernel - Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack

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Message-ID: <20181109171740.GT3074@bombadil.infradead.org>
Date:   Fri, 9 Nov 2018 09:17:40 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        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>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@...el.com>
Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for
 shadow stack

On Fri, Nov 09, 2018 at 09:13:32AM -0800, Dave Hansen wrote:
> On 11/8/18 4:32 PM, Matthew Wilcox wrote:
> >> Now, looking at Yu-cheng's specific example, it doesn't matter.  We've
> >> got 64-bit types and natural 64-bit alignment.  Without __packed, we
> >> need to look out for natural alignment screwing us up.  With __packed,
> >> it just does what it *looks* like it does.
> > The question is whether Yu-cheng's struct is ever embedded in another
> > struct.  And if so, what does the hardware do?
> 
> It's not really.
> 
> +struct cet_user_state {
> +	u64 u_cet;	/* user control flow settings */
> +	u64 user_ssp;	/* user shadow stack pointer */
> +} __packed;
> 
> This ends up embedded in 'struct fpu'.  The hardware tells us what the
> sum of all the sizes of all the state components are, and also tells us
> the offsets inside the larger buffer.
> 
> We double-check that the structure sizes exactly match the sizes that
> the hardware tells us that the buffer pieces are via XCHECK_SZ().
> 
> But, later versions of the hardware have instructions that don't have
> static offsets for the state components (when the XSAVES/XSAVEC
> instructions are used).  So, for those, the structure embedding isn't
> used at *all* since some state might not be present.

But *when present*, this structure is always aligned on an 8-byte
boundary, right?