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Message-ID: <908d65db-6926-449e-8aca-5f9968f7fcd6@zytor.com>
Date: Wed, 17 Apr 2024 09:39:13 -0700
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Subject: Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED
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To: Xin Li <xin@zytor.com>, Nikolay Borisov <nik.borisov@suse.com>,
        linux-kernel@vger.kernel.org
Cc: luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org
References: <20240417063001.3773507-1-xin@zytor.com>
 <d0979bd9-fd12-4672-b451-23f23fc2353c@suse.com>
 <2d0e67dd-c3e3-4701-839a-68d73c61c170@zytor.com>
 <58272a50-fa74-4ae8-8ce2-aeb92a0f6ece@zytor.com>
From: "H. Peter Anvin" <hpa@zytor.com>
In-Reply-To: <58272a50-fa74-4ae8-8ce2-aeb92a0f6ece@zytor.com>
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On 4/17/24 08:55, Xin Li wrote:
> 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:

s/when/if it is ever 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.