linux-kernel - Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary

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Message-ID: <CA+55aFxre3Jk-CG+mBs6Bp9a1J=riCQaA3dwUTFRph1OsCMSuA@mail.gmail.com>
Date:	Thu, 23 Apr 2015 09:13:34 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary

On Thu, Apr 23, 2015 at 9:06 AM, Brian Gerst <brgerst@...il.com> wrote:
>
> So you are saying we should save and conditionally restore the
> kernel's %ss during context switch?  That shouldn't be too bad.  Half
> of the time you would be loading the null selector which is fast (no
> GDT access, no validation).

I'd almost prefer something along those lines, yes. Who knows *what*
leaks? If the present bit state leaks, then likely so does the limit
value etc etc..

                         Linus
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