linux-kernel - Re: [PATCH v5 22/34] x86/fred: FRED initialization code

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Message-ID: <3e76f5c2-8398-0be6-f8ac-c9a2b04f551d@citrix.com>
Date:   Wed, 22 Mar 2023 18:25:25 +0000
From:   "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>
To:     "Li, Xin3" <xin3.li@...el.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH v5 22/34] x86/fred: FRED initialization code

On 22/03/2023 2:22 am, Li, Xin3 wrote:
>> If there is no other concrete reason other than overflowing for assigning NMI and
>> #DB with a stack level > 0, #VE should also be assigned with a stack level > 0, and
>> #BP too. #VE can happen anytime and anywhere, so it is subject to overflowing too.
> With IDT, both #VE and #BP do not use IST, but NMI, #DB, #MC and #DF do.
>
> Let's keep this "secret" logic for now, i.e., not change the stack levels
> for #VE and #BP at this point. We can do "optimization", i.e., change them
> later :).

Fun fact.  #BP used to be IST, and used to share the same IST as #DF.

This was spoiled by CVE-2018-8897 and a MovSS-delayed breakpoint over
INT3, at which point hardware queued both a #BP and #DB on the same IST
stack and lost program state.

There's no need specific need for #BP to be IST to begin with, hence why
making it not-IST was the security fix.

~Andrew