linux-kernel - Re: [PATCH v9 04/11] x86/entry/64: Adapt assembly for PIE support

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Date:   Tue, 6 Aug 2019 10:30:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Garnier <thgarnie@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 04/11] x86/entry/64: Adapt assembly for PIE support

On Tue, Aug 06, 2019 at 07:08:51AM +0200, Borislav Petkov wrote:
> On Mon, Aug 05, 2019 at 10:50:30AM -0700, Thomas Garnier wrote:
> > I saw that %rdx was used for temporary usage and restored before the
> > end so I assumed that it was not an option.
> 
> PUSH_AND_CLEAR_REGS saves all regs earlier so I think you should be
> able to use others. Like SAVE_AND_SWITCH_TO_KERNEL_CR3/RESTORE_CR3, for
> example, uses %r15 and %r14.

AFAICT the CONFIG_DEBUG_ENTRY thing he's changing is before we setup
pt_regs.

Also consider the UNWIND hint that's in there, it states we only have
the IRET frame on stack, not a full regs set.