linux-kernel - Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

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Date:	Fri, 24 Apr 2015 12:59:06 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	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>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue

On Fri, Apr 24, 2015 at 11:59:42AM +0200, Denys Vlasenko wrote:
> I propose a more conservative check:
> 
> 		if (ss_sel != __KERNEL_DS)
> 			loadsegment(ss, __KERNEL_DS);
> 
> I would propose this even if I would see no real case where it matters...
> but I even do see such a case.

...

> As in legacy mode, it is desirable to keep the stack-segment requestor privilege-level (SS.RPL)
> equal to the current privilege-level (CPL). When using a call gate to change privilege levels, the
> SS.RPL is updated to reflect the new CPL. The SS.RPL is restored from the return-target CS.RPL
> on the subsequent privilege-level-changing far return.
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ THIS ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> 3. The old values of the SS and RSP registers are pushed onto the stack pointed to by the new RSP.
> ...
> ...
> """
> 
> Thus, the NULL selector in SS may actually be not all zeros. Its RPL may be nonzero,
> if we are not in ring 0.

Yeah, that makes more sense. So I tested Andy's patch but changed it as
above and I get

$ taskset -c 0 ./sysret_ss_attrs_32
[RUN]   Syscalls followed by SS validation
[OK]    We survived

And this is on an AMD F16h and it used to fail before the patch. So
yeah, I think we can call this misfeature "architectural".

Tested-by: Borislav Petkov <bp@...e.de>

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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