Re: [PATCH] x86emul: correct put_fpu()'s segment selector handling

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.01.2025 16:37, Andrew Cooper wrote:
> On 07/01/2025 2:33 pm, Jan Beulich wrote:
>> All selector fields under ctxt->regs are (normally) poisoned in the HVM
>> case, and the four ones besides CS and SS are potentially stale for PV.
>> Avoid using them in the hypervisor incarnation of the emulator, when
>> trying to cover for a missing ->read_segment() hook.
>>
>> To make sure there's always a valid ->read_segment() handler for all HVM
>> cases, add a respective function to shadow code, even if it is not
>> expected for FPU insns to be used to update page tables.
>>
>> Fixes: 0711b59b858a ("x86emul: correct FPU code/data pointers and opcode 
>> handling")
>> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> The code comment may want adjusting in the course of FRED work.
> 
> It compiles when displacing my temporary patch in the FRED branch.  I've
> not got the ABI compatibility in userspace working yet, but
> regs->{ds,es,fs,gs} will be staying, so the #else case should be fine
> (assuming they're populated properly).
> 
> So, tentatively, Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Thanks.

> That said, I think it would be nicer to see about excluding the FPU in
> these cases.  Both cases lacking read_segment() hooks are pagetable
> emulation, and I'd say it's more likely to be code corruption than there
> actually being x87 instructions in the middle of a dual 32bit PAE update.

I considered this case, but decided against going this route. We shouldn't
be stricter than necessary towards what we permit guests to do, however odd
it might look to us.

Jan