linux-kernel - Re: [RFC][PATCH v2 11/21] x86/pti: Extend PTI user mappings

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Message-ID: <87d009p385.fsf@nanos.tec.linutronix.de>
Date:   Thu, 19 Nov 2020 20:15:54 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        jan.setjeeilers@...cle.com, Junaid Shahid <junaids@...gle.com>,
        oweisse@...gle.com, Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Alexander Graf <graf@...zon.de>, mgross@...ux.intel.com,
        kuzuno@...il.com
Subject: Re: [RFC][PATCH v2 11/21] x86/pti: Extend PTI user mappings

On Tue, Nov 17 2020 at 09:42, Alexandre Chartre wrote:
> On 11/17/20 12:06 AM, Andy Lutomirski wrote:
> The PTI stack does have guard pages because it maps only a part of the task
> stack into the user page-table, so pages around the PTI stack are not mapped
> into the user-pagetable (the page below is the task stack guard, and the page
> above is part of the kernel-only stack so it's never mapped into the user
> page-table).
>
> + *   +-------------+
> + *   |             | ^                       ^
> + *   | kernel-only | | KERNEL_STACK_SIZE     |
> + *   |    stack    | |                       |
> + *   |             | V                       |
> + *   +-------------+ <- top of kernel stack  | THREAD_SIZE
> + *   |             | ^                       |
> + *   | kernel and  | | KERNEL_STACK_SIZE     |
> + *   | PTI stack   | |                       |
> + *   |             | V                       v
> + *   +-------------+ <- top of stack

Well, the PTI stack might have guard pages, but the kernel stack can now
underflow into the PTI stack. Not good.

Thanks,

        tglx