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Date:   Wed, 20 Apr 2022 09:07:30 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     x86@kernel.org, brgerst@gmail.com, jiangshanlai@gmail.com,
        Andrew.Cooper3@citrix.com, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH 2/2] x86,entry: Use PUSH_AND_CLEAR_REGS for compat
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On Tue, Apr 19, 2022 at 08:21:23PM -0700, Josh Poimboeuf wrote:
> On Tue, Apr 19, 2022 at 10:41:11PM +0200, Peter Zijlstra wrote:
> > Since the upper regs don't exist for ia32 code, preserving them
> > doesn't hurt and it simplifies the code.
> 
> But an attacker can still control those registers, so clearing them on
> the stack is better, as it reduces user control over the kernel stack.
> 
> 64-bit syscalls *do* have to save those registers to the stack, so
> whether it truly matters if compat mode is made equally insecure, I
> can't say.  But without evidence to the contrary, my feeling is that we
> should err on the side of caution.

Right, so earlier Brian said simpler might be better, and I figured I'd
try to see if I could make that stick, because I too like simpler ;-)

Also, since int80 already has to do this, attackers already have their
attack surface.