Re: [PATCH v2] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

From: Linus Torvalds
Date: Sun Apr 26 2015 - 20:51:33 EST

Just a heads-up to the x86 people: I'm going to merge this directly,
since I'm doing -rc1 momentarily, and without this patch 32-bit
user-land on a 64-bit kernel is flaky on all AMD CPU's. Which I don't
want for -rc1.


On Sun, Apr 26, 2015 at 4:47 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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 setting SS to __KERNEL_DS __switch_to, thus
> ensuring that SYSRET never happens with SS set to NULL.
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