linux-kernel - Re: [PATCH v2 15/39] x86/mm: Check Shadow Stack page fault errors

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Date:   Fri, 14 Oct 2022 12:07:08 +0200
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>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com, rppt@...nel.org,
        jamorris@...ux.microsoft.com, dethoma@...rosoft.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 15/39] x86/mm: Check Shadow Stack page fault errors

On Thu, Sep 29, 2022 at 03:29:12PM -0700, Rick Edgecombe wrote:

> The architecture has concepts of both shadow stack reads and shadow stack
> writes. Any shadow stack access to non-shadow stack memory will generate
> a fault with the shadow stack error code bit set.
> 
> This means that, unlike normal write protection, the fault handler needs
> to create a type of memory that can be written to (with instructions that
> generate shadow stack writes), even to fulfill a read access. So in the
> case of COW memory, the COW needs to take place even with a shadow stack
> read. Otherwise the page will be left (shadow stack) writable in
> userspace. So to trigger the appropriate behavior, set FAULT_FLAG_WRITE
> for shadow stack accesses, even if the access was a shadow stack read.

That ^ should be moved into the comment below

>  - Clarify reasoning for FAULT_FLAG_WRITE for all shadow stack accesses

> @@ -1300,6 +1314,13 @@ void do_user_addr_fault(struct pt_regs *regs,
>  
>  	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>  
> +	/*
> +	 * In order to fullfull a shadow stack access, the page needs
> +	 * to be made (shadow stack) writable. So treat all shadow stack
> +	 * accesses as writes.
> +	 */

Because that's impenetrable.

> +	if (error_code & X86_PF_SHSTK)
> +		flags |= FAULT_FLAG_WRITE;
>  	if (error_code & X86_PF_WRITE)
>  		flags |= FAULT_FLAG_WRITE;
>  	if (error_code & X86_PF_INSTR)
> -- 
> 2.17.1
>