Re: [PATCH v2 for-4.19 1/3] x86/EPT: correct special page checking in epte_get_entry_emt()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jun 12, 2024 at 03:16:37PM +0200, Jan Beulich wrote:
> mfn_valid() granularity is (currently) 256Mb. Therefore the start of a
> 1Gb page passing the test doesn't necessarily mean all parts of such a
> range would also pass.

How would such a superpage end up in the EPT?

I would assume this can only happen when adding a superpage MMIO that
has part of it return success from mfn_valid()?

> Yet using the result of mfn_to_page() on an MFN
> which doesn't pass mfn_valid() checking is liable to result in a crash
> (the invocation of mfn_to_page() alone is presumably "just" UB in such a
> case).
> 
> Fixes: ca24b2ffdbd9 ("x86/hvm: set 'ipat' in EPT for special pages")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> ---
> Of course we could leverage mfn_valid() granularity here to do an
> increment by more than 1 if mfn_valid() returned false. Yet doing so
> likely would want a suitable helper to be introduced first, rather than
> open-coding such logic here.

We would still need to call is_special_page() on each 4K chunk, at
which point taking advantage of the mfn_valid() granularity is likely
to make the code more complicated to follow IMO.

Thanks, Roger.