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Re: [PATCH] x86/IDT: Fix IDT generation for INT $0x80
- To: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 12 Mar 2025 00:09:12 +0000
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- Cc: Luca Fancellu <luca.fancellu@xxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 12 Mar 2025 00:09:40 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 11/03/2025 11:36 pm, Andrew Cooper wrote:
> On 11/03/2025 9:27 pm, Andrew Cooper wrote:
>> On 11/03/2025 9:22 pm, Andrew Cooper wrote:
>>> When PV is enabled, entry_int80 needs to be DPL3, not DPL0.
>>>
>>> This causes the XSA-259 PoC to fail with:
>>>
>>> --- Xen Test Framework ---
>>> Environment: PV 64bit (Long mode 4 levels)
>>> XSA-259 PoC
>>> Error: Unexpected fault 0x800d0802, #GP[IDT[256]]
>>> Test result: ERROR
>>>
>>> (Clearly I have a bug in XTF's rendering of the error code too.)
> Nope, it's a QEMU bug. Under QEMU, we get:
>
> --- Xen Test Framework ---
> Environment: PV 64bit (Long mode 4 levels)
> XSA-259 PoC
> (XEN) [ 3.982447] *** #GP/IDT ec 00000802
> (XEN) [ 3.982668] *** #GP/IDT ec 00000802, not permitted
> (XEN) [ 3.982849] *** #GP inject ec 00000802
> Error: Unexpected fault 0x800d0802, #GP[IDT[256]]
> Test result: ERROR
>
>
> On real hardware:
>
> (d1) --- Xen Test Framework ---
> (d1) Environment: PV 64bit (Long mode 4 levels)
> (d1) XSA-259 PoC
> (XEN) *** #GP/IDT ec 00000402
> (XEN) *** #GP/IDT ec 00000402, not permitted
> (XEN) *** #GP inject ec 00000402
> (d1) Success: Not vulnerable to XSA-259
> (d1) Test result: SUCCESS
>
> So it's by chance that Luca's testing even picked up this failure. The
> XSA-259 PoC can't distinguish between the #GP INT emulation path and the
> INT80 fastpath's error path which injects #GP.
And a patch submitted.
https://lore.kernel.org/qemu-devel/20250312000603.3666083-1-andrew.cooper3@xxxxxxxxxx/T/#u
~Andrew
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