linux-kernel - Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200930111612.GZ2628@hirez.programming.kicks-ass.net>
Date:   Wed, 30 Sep 2020 13:16:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        shuo.a.liu@...el.com, linux-kernel@...r.kernel.org, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yu Wang <yu1.wang@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Yakui Zhao <yakui.zhao@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fengwei Yin <fengwei.yin@...el.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        segher@...nel.crashing.org, ndesaulniers@...gle.com
Subject: Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

On Sun, Sep 27, 2020 at 08:38:03AM -0700, Dave Hansen wrote:
> On 9/27/20 3:51 AM, Greg Kroah-Hartman wrote:
> >> +static inline long acrn_hypercall0(unsigned long hcall_id)
> >> +{
> >> +	register long r8 asm("r8");
> >> +	long result;
> >> +
> >> +	/* Nothing can come between the r8 assignment and the asm: */
> >> +	r8 = hcall_id;
> >> +	asm volatile("vmcall\n\t"
> >> +		     : "=a" (result)
> >> +		     : "r" (r8)
> >> +		     : );
> > What keeps an interrupt from happening between the r8 assignment and the
> > asm: ?
> 
> It's probably better phrased something like: "No other C code can come
> between this r8 assignment and the inline asm".  An interrupt would
> actually be fine in there because interrupts save and restore all
> register state, including r8.
> 
> The problem (mentioned in the changelog) is that gcc does not let you
> place data directly into r8.  But, it does allow you to declare a
> register variable that you can assign to use r8.  There might be a
> problem if a function calls was in between and clobber the register,
> thus the "nothing can come between" comment.
> 
> The comment is really intended to scare away anyone from adding printk()'s.
> 
> More information about these register variables is here:
> 
> > https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
> 
> Any better ideas for comments would be greatly appreciated.  It has 4 or
> 5 copies so I wanted it to be succinct.

This is disguisting.. Segher, does this actually work? Nick, does clang
also support this?