Re: [Xen-devel] [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to zero

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 25/04/17 20:18, Borislav Petkov wrote:
> On Tue, Apr 25, 2017 at 08:34:34PM +0200, Juergen Gross wrote:
>> And what happens when there is a scheduling event right here?
>> __switch_to() will see X86_BUG_SYSRET_SS_ATTRS set and take a wrong
>> path.
> So the whole thing we're doing right now is wrong: set bit and then
> clear bit.
>
> We should not set the bit at all and there won't be any window to get it
> wrong.
>
> So can we do something like this instead:
>
> 	if (!cpu_has(c, X86_FEATURE_XENPV))
> 		set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
>
> or is XENPV the wrong thing to test?
>

X86_BUG_SYSRET_SS_ATTRS only actually causes a problem if you enter the
kernel via an IDT vector (forcing %ss to NULL), then exiting the kernel
via the optimistic sysret path, which on AMD loads the %ss selector, but
apparently doesn't update the segment cache (and %ss.dpl in particular).

The problem (for all ring-deprivileged virtuailsation; not just Xen PV),
is that

savesegment(ss, ss_sel);
if (ss_sel != __KERNEL_DS)
    loadsegment(ss, __KERNEL_DS);

tries to load %ss with an RPL of 0, and things blow up.

~Andrew