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

[Andy Lutomirski]

On Thu, Apr 23, 2015 at 7:15 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 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@xxxxxxxxxx>
> ---
>
> 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.

Even if the issue affects SYSRETQ, it could be that we don't care.  If
the extent of the info leak is whether we context switched during a
64-bit syscall to a non-syscall context, then this is basically
uninteresting.  In that case, we could either ignore the 64-bit issue
entirely or fix it the other way: force SS to NULL on context switch
(much faster, I presume) and fix it up before SYSRETL as Denys
suggested.

We clearly don't have a spate of crashes in programs that do SYSCALL
from a 64-bit CS and then far jump/return to a 32-bit CS without first
reloading SS, since this bug has been here forever.  I agree that the
issue is ugly (if it exists in the first place), but maybe we don't
need to fix it.

Thoughts?

--Andy
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