linux-kernel - Re: [RFC PATCH] x86/mm/fault: Allow stack access below %rsp

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Message-ID: <08752b15-8771-266f-0155-3b721203e721@intel.com>
Date:   Mon, 5 Nov 2018 09:20:19 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Waiman Long <longman@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86/mm/fault: Allow stack access below %rsp


On 11/4/18 9:14 PM, Andy Lutomirski wrote:
> I should add: if this patch is *not* applied, then I think we'll
> need to replace the sw_error_code check with user_mode(regs) to avoid
> an info leak if CET is enabled.  Because, with CET, WRUSS will allow
> a *kernel* mode access (where regs->sp is the kernel stack pointer)
> with user permissions.

Are you saying that WRUSS, if it faults will set the "user" page fault
error code bit?  I seem to have some rough recollection about it being
that way, and the shadow-stack spec does say:

	paging access control checks will be treated as a user-mode
	shadow stack store

But the SDM says:

	For all instruction fetches and most data accesses, this
	distinction is determined by the current privilege level (CPL):
	accesses made while CPL < 3 are supervisor-mode accesses, while
	accesses made while CPL = 3 are user-mode accesses.

It would certainly be ideal if things affecting the core architecture
like this were in the SDM itself before we merged them.  It makes things
like this a lot easier to figure out.