linux-kernel - Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED

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Date: Wed, 17 Apr 2024 08:55:46 -0700
From: Xin Li <xin@...or.com>
To: "H. Peter Anvin" <hpa@...or.com>, Nikolay Borisov <nik.borisov@...e.com>,
        linux-kernel@...r.kernel.org
Cc: luto@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org
Subject: Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED

On 4/17/2024 8:07 AM, H. Peter Anvin wrote:
> On 4/17/24 04:02, Nikolay Borisov wrote:
>>
>> On 17.04.24 г. 9:30 ч., Xin Li (Intel) wrote:
>>> 4) int80_emulation() does a CLEAR_BRANCH_HISTORY, which is likely 
>>> >     overkill for new x86 CPU implementations that support FRED.
>>
>> Well, that's a bit of an overstatement/speculation, because 
>> clear_branch_history will only be effective if the machine is 
>> susceptible to the given bug and there isn't a better options (i.e 
>> using a hardware bit controlling the respective aspect of the CPU).
>>>
> 
> It would seem like a huge stretch to expect that a FRED-capable CPU 
> would not have such a facility. This is a matter of establishing a 
> baseline for FRED-capable hardware.
> 
> It would make more sense to me to add it if we turn out to need it; note 
> that FRED code is currently only enabled on demand, in order to defend 
> against bit rot until we have physical hardware.
> 
> Now, if this is still desired, it *probably* belongs better in either 
> fred_intx()/fred_other() or asm_fred_entrypoint_user, depending on if 
> this ought to be done for all entries from userspace or only system calls.

My bad that I didn't make this a good comment, even neglected that 
clear_branch_history is just a nop on machines w/o such security issues.


So how about?

int80_emulation() does a CLEAR_BRANCH_HISTORY, which is IDT-specific.
While FRED will likely take a different approach when it is needed:
it *probably* belongs in either fred_intx()/fred_other() or
asm_fred_entrypoint_user(), depending on if this ought to be done for 
all entries from userspace or only system calls.