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Date: Wed, 20 Apr 2022 09:07:30 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Josh Poimboeuf <jpoimboe@...hat.com> Cc: x86@...nel.org, brgerst@...il.com, jiangshanlai@...il.com, Andrew.Cooper3@...rix.com, linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org> Subject: Re: [PATCH 2/2] x86,entry: Use PUSH_AND_CLEAR_REGS for compat 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.
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