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

Brian Gerst
2015-04-24T03:58:11Z

On Thu, Apr 23, 2015 at 10:15 PM, Andy Lutomirski <luto@kernel.org> wrote:
> AMD CPUs don't reinitialize the SS descriptor on SYSRET, so SYSRET
> with SS == 0 results in an invalid usermode state in which SS is
> apparently equal to __USER_DS but causes #SS if used.
>
> Work around the issue by replacing NULL SS values with __KERNEL_DS
> in __switch_to, thus ensuring that SYSRET never happens with SS set
> to NULL.
>
> This was exposed by a recent vDSO cleanup.
>
> Fixes: e7d6eefaaa44 x86/vdso32/syscall.S: Do not load __USER32_DS to %ss
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>
> Tested only on Intel, which isn't very interesting. I'll tidy up
> and send a test case, too, once Borislav confirms that it works.
>
> Please don't actually apply this until we're sure we understand the
> scope of the issue. If this doesn't affect SYSRETQ, then we might
> to fix it on before SYSRETL to avoid impacting 64-bit processes
> at all.

It works with Wine. Tested on an AMD Phenom II.

--
Brian Gerst