linux-kernel - Re: [PATCH v3 3/4] x86/signal/64: Re-add support for SS in the 64-bit signal context

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Date:	Fri, 12 Feb 2016 14:56:57 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Stas Sergeev <stsp@...t.ru>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3 3/4] x86/signal/64: Re-add support for SS in the
 64-bit signal context

On Thu, Feb 11, 2016 at 05:01:39PM -0800, Andy Lutomirski wrote:
> On Thu, Feb 11, 2016 at 11:49 AM, Borislav Petkov <bp@...en8.de> wrote:
> > On Mon, Jan 25, 2016 at 01:34:14PM -0800, Andy Lutomirski wrote:
> >
> >> + * with a flat 32-bit selector.
> 
> How about:
> 
> Sigreturn restores SS as follows:
> 
> if (saved SS is valid || UC_STRICT_RESTORE_SS is set || saved CS is not 64-bit)
>  new SS = saved SS
> else
>   new SS = a flat 32-bit data segment

Much better!

> How about:
> 
> --- cut here ---
> 
> This behavior serves three purposes:
> 
>  - Legacy programs that construct a 64-bit sigcontext from scratch
> with zero or garbage in the SS slot (e.g. old CRIU) and call sigreturn
> will still work.
> 
>  - Old DOSEMU versions sometimes catch a signal from a segmented
> context, delete the old SS segment (with modify_ldt), and change the
> saved CS to a 64-bit segment.  These DOSEMU versions expect sigreturn
> to send them back to 64-bit mode without killing them, despite the
> fact that the SS selector when the signal was raised is no longer
> valid.  With UC_STRICT_RESTORE_SS clear, the kernel will fix up SS for
> these DOSEMU versions.

... and with UC_STRICT_RESTORE_SS set, they'll get __USER_DS.

>  - Old and new programs that catch a signal and return without
> modifying the saved context will end up in exactly the state they
> started in.  Old kernels would lose track of the previous SS value.
> 
> --- cut here ---

Yap, definitely better.

> FWIW, I have a DOSEMU patch that makes it use UC_STRICT_RESTORE_SS to
> get the behavior it actually wants on new kernels.  It should make it
> faster and more reliable than was possible before.

Cool.

Thanks.

-- 
Regards/Gruss,
    Boris.

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