linux-kernel - Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS

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Message-ID: <20170425191809.uvdt4jimnbvqbyf2@pd.tnic>
Date:   Tue, 25 Apr 2017 21:18:09 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        x86@...nel.org, boris.ostrovsky@...cle.com, hpa@...or.com,
        tglx@...utronix.de, mingo@...hat.com
Subject: Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to
 zero

On Tue, Apr 25, 2017 at 08:34:34PM +0200, Juergen Gross wrote:
> And what happens when there is a scheduling event right here?
> __switch_to() will see X86_BUG_SYSRET_SS_ATTRS set and take a wrong
> path.

So the whole thing we're doing right now is wrong: set bit and then
clear bit.

We should not set the bit at all and there won't be any window to get it
wrong.

So can we do something like this instead:

	if (!cpu_has(c, X86_FEATURE_XENPV))
		set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);

or is XENPV the wrong thing to test?

-- 
Regards/Gruss,
    Boris.

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