Re: [Xen-devel] [PATCH v4 1/8] public / x86: Introduce __HYPERCALL_dm_op...

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/01/2017 15:02, Paul Durrant wrote:
>
>>> +    if ( !rc &&
>>> +         !copy_buf_to_guest(bufs, nr_bufs, 0, &op, sizeof(op)) )
>> Do all ops need a copyback?  If they do, this is fine.  If not, it would
>> be better to have a copyback boolean which subops set as necessary.
> I can restrict copy-back using a boolean set for sub-ops that have 'out' 
> params, or when there needs to be a continuation but I didn't really think it 
> was worth the extra complexity.

Extraneous writebacks to PV guests are fairly cheep, but is is certainly
not the case for HVM guests.  A writeback to HVM requires a least one
guest pagetable walk (which itself most likely includes an EPT/NPT walk).

From a correctness point of view, it is reasonable for an implementation
which expects a hypercall datastructure to be read only, to put said
structure in read-only memory.  The PKRU feature in particular makes it
very easy to set something up, then switch it from RW to RO for use. 
Such an implementation should have the hypercall fail with a spurious
-EFAULT after it has otherwise completed successfully.

~Andrew

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