Re: [PATCH] x86/mm: fix lookup_address() to handle physical memory holes in direct mapping

[Kalra, Ashish]

On 7/1/2024 1:38 PM, Jürgen Groß wrote:
> On 01.07.24 19:57, Kalra, Ashish wrote:
>>
>> On 6/29/2024 5:20 AM, Jürgen Groß wrote:
>>> On 28.06.24 22:52, Ashish Kalra wrote:
>>>> From: Ashish Kalra <ashish.kalra@xxxxxxx>
>>>>
>>>> lookup_address_in_pgd_attr() at pte level it is simply returning
>>>> pte_offset_kernel() and there does not seem to be a check for
>>>> returning NULL if pte_none().
>>>>
>>>> Fix lookup_address_in_pgd_attr() to add check for pte_none()
>>>> after pte_offset_kernel() and return NULL if it is true.
>>>
>>> Please have a look at the comment above lookup_address(). You should not
>>> break the documented behavior without verifying that no caller is relying
>>> on the current behavior. If this is fine, please update the comment.
>>>
>>>
>> I don't get that, in this case the PTE does not exist, so as per the comments here lookup_address() should have returned NULL.
>
> There is a PTE, but it is all 0.
>
> There is no _valid_ PTE. No PTE would mean that the related PMD entry (or any
> other higher level entry) is invalid.

Then what is the caller supposed to do in this case ?

As the return from lookup_address() is non-NULL in this case, accessing it causes a fatal #PF.

Is the caller supposed to add the check for a valid PTE using pte_none(*pte) ?

Thanks, Ashish

>
> Remember that the W^X checking needs to be performed _before_ a new PTE is
> written.
>
>
> Juergen