linux-kernel - Re: [PATCH] x86/entry_32: Move CLEAR_CPU

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Date: Thu, 9 May 2024 17:20:31 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
 Robert Gill <rtgill82@...il.com>,
 "Linux regression tracking (Thorsten Leemhuis)" <regressions@...mhuis.info>,
 antonio.gomez.iglesias@...ux.intel.com, daniel.sneddon@...ux.intel.com
Subject: Re: [PATCH] x86/entry_32: Move CLEAR_CPU_BUFFERS before CR3 switch

On 4/26/24 16:48, Pawan Gupta wrote:
> Move the VERW before the CR3 switch for 32-bit kernels as a workaround.

I look at the 32-bit code so rarely, I seem to forget have to re-learn
this gunk every time I look at it.  Take a look at RESTORE_INT_REGS.  On
32-bit, we actually restore %ds:

	popl    %ds

So even doing this:

> +       CLEAR_CPU_BUFFERS
>         /* Restore user state */
>         RESTORE_REGS pop=4                      # skip orig_eax/error_code
> -       CLEAR_CPU_BUFFERS
>  .Lirq_return:

fixes the issue.  Moving it above the CR3 switch also works of course,
but I don't think this has anything to do with CR3.  It's just that
userspace sets a funky %ds value and CLEAR_CPU_BUFFERS uses ds:.

I don't think any of the segment registers can have secrets in them, can
they?  I mean, it's possible, but in practice I can't imagine.

So why not just do the CLEAR_CPU_BUFFERS in RESTORE_REGS but after
RESTORE_INT_REGS?  You might be able to do it universally, or you could
pass in a macro argument to do it conditionally.

P.S. Can we remove 32-bit support yet?  Please? :)