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Subject: Re: x86/clearregs: Register sanitizing at kernel entry for speculation hygiene
From: Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20180110010328.22163-1-andi@firstfloor.org>
Date: Tue, 9 Jan 2018 17:16:04 -0800
Cc: tglx@linutronix.de, x86@kernel.org, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, dwmw@amazon.co.uk, pjt@google.com,
        luto@kernel.org, peterz@infradead.org, thomas.lendacky@amd.com,
        tim.c.chen@linux.intel.com, gregkh@linux-foundation.org,
        dave.hansen@intel.com, jikos@kernel.org
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Message-Id: <50B38CB9-546E-4419-9162-D6A2AC2D80C3@amacapital.net>
References: <20180110010328.22163-1-andi@firstfloor.org>
To: Andi Kleen <andi@firstfloor.org>
Sender: linux-kernel-owner@vger.kernel.org


> On Jan 9, 2018, at 5:03 PM, Andi Kleen <andi@firstfloor.org> wrote:
> 
> This patch kit implements clearing of all unused registers on kernel entries,
> including system calls and all exceptions and interrupt.
> 
> This doesn't fix any known issue, but will make it harder in general
> to exploit the kernel with speculation because it will be harder
> to get user controlled values into kernel code.

I don't like this at all.  Once upon a time, Linux syscalls were supposed to be fast.  Then we learned about the Meltdown screwup, so we mostly fixed it for real upstream and the distroa seriously half-arsed their own fixes [1].  This came with a big performance cost, but it can be turned off on non-busted hardware.  So be it.

But now we're proposing to throw out the whole fast path because it might make it a bit harder to do the most obvious attack.  Not very hard, mind you, but a little bit harder.  And there's no off switch for less-leaky hardware.  No thanks.

Meanwhile we're doing nothing whatsoever to mitigate cross-process attacks because we can't do anything about it short of turning IBRS on systemwide.