linux-kernel - Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack

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Message-ID: <b3357c67-15b9-3218-8b32-caa335a5ad1d@oracle.com>
Date:   Wed, 17 Jan 2018 10:22:24 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>, aliguori@...zon.com,
        daniel.gruss@...k.tugraz.at, hughd@...gle.com, keescook@...gle.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, jroedel@...e.de
Subject: Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack

On 01/17/2018 09:04 AM, Andrew Cooper wrote:
> On 17/01/18 09:02, Joerg Roedel wrote:
>> Hi Boris,
>>
>> thanks for testing this :)
>>
>> On Tue, Jan 16, 2018 at 09:47:06PM -0500, Boris Ostrovsky wrote:
>>> On 01/16/2018 11:36 AM, Joerg Roedel wrote:
>>>> +.macro SWITCH_TO_KERNEL_STACK nr_regs=0 check_user=0
>>> This (and next patch's SWITCH_TO_ENTRY_STACK) need X86_FEATURE_PTI check.
>>>
>>> With those macros fixed I was able to boot 32-bit Xen PV guest.
>> Hmm, on bare metal the stack switch happens regardless of the
>> X86_FEATURE_PTI feature being set, because we always program tss.sp0
>> with the systenter stack. How is the kernel entry stack setup on xen-pv?
>> I think something is missing there instead.
> There is one single stack registered with Xen, on which you get a normal
> exception frame in all cases, even via the registered (virtual)
> syscall/sysenter/failsafe handlers.

And so the check should be at least against X86_FEATURE_XENPV, not
necessarily X86_FEATURE_PTI.

But I guess you can still check against X86_FEATURE_PTI since without it
there is not much reason to switch stacks?

-boris