linux-kernel - Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state

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Message-ID: <2020841.9MqWvG71rC@tjmaciei-mobl5>
Date:   Tue, 31 Aug 2021 15:39:02 -0700
From:   Thiago Macieira <thiago.macieira@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, Len Brown <lenb@...nel.org>
CC:     Borislav Petkov <bp@...en8.de>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state

On Tuesday, 31 August 2021 15:15:55 PDT Len Brown wrote:
> Indeed, I believe that there is universal agreement that a synchronous
> return code
> from a system call is a far superior programming model than decoding
> the location of a failure in a system call.  (no, the IP isn't random -- it
> is always the 1st instruction in that thread to touch a TMM register).

That instruction is actually likely going to be a memory load, probably an 
LDTILECFG. So the developer will see a crashing instruction with a pointer and 
will spend time trying to figure out why that pointer was wrong, when there 
was nothing wrong with it.

That's why I suggested (and Chang implemented) a SIGILL for when #NM is 
received and the arch_prctl() wasn't previously done. The OOM condition, if 
the extra state is dynamically allocated, was meant to stay a SIGSEGV, but 
maybe should change to SIGKILL.

On the other hand, if it it's allocated at the syscall, then the kernel can 
return -ENOMEM for it (which would allow for graceful degradation) or for a 
later clone() syscall starting a new thread (which I don't expect to ever 
gracefully degrade).

> decoding the location of the failure in a *signal hander*

That's a separate problem.

We can't be sure that the portion of the userspace doing the alt-stack crash 
handler is aware of the portion using AMX. There's no way to enforce this. The 
prctl() is a good indication, but I have no clue how high the correlation will 
be.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel DPG Cloud Engineering