linux-kernel - Re: [PATCH v7 45/72] x86/entry/64: Add entry code for #VC handler

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Date:   Thu, 28 Jan 2021 14:18:18 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Lai Jiangshan <jiangshanlai+lkml@...il.com>
Cc:     X86 ML <x86@...nel.org>, Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v7 45/72] x86/entry/64: Add entry code for #VC handler

Hello Lai,

On Sun, Jan 24, 2021 at 10:11:14PM +0800, Lai Jiangshan wrote:
> > +
> > +       /*
> > +        * No need to switch back to the IST stack. The current stack is either
> > +        * identical to the stack in the IRET frame or the VC fall-back stack,
> > +        * so it is definitly mapped even with PTI enabled.
> > +        */
> > +       jmp     paranoid_exit
> > +
> >
> 
> Hello
> 
> I know we don't enable PTI on AMD, but the above comment doesn't align to the
> next code.
> 
> We assume PTI is enabled as the comments said "even with PTI enabled".
> 
> When #VC happens after entry_SYSCALL_64 but before it switches to the
> kernel CR3.  vc_switch_off_ist() will switch the stack to the kernel stack
> and paranoid_exit can't work when it switches to user CR3 on the kernel stack.
> 
> The comment above lost information that the current stack is possible to be
> the kernel stack which is mapped not user CR3.
> 
> Maybe I missed something.

You are right, the scenario above would cause problems for the current
#VC entry code. With SEV-ES an #VC exception can't happen in the early
syscall entry code, so I think its the best to update the comment
reflecting this.

In the future this might change and then the #VC entry code needs to
take care of this case too. Thanks for pointing it out.

Regards,

	Joerg