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

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Message-ID: <8d970528-0e57-457f-ae00-862b4d320a2a@suse.com>
Date: Mon, 1 Jul 2024 20:38:40 +0200
From: Jürgen Groß <jgross@...e.com>
To: "Kalra, Ashish" <ashish.kalra@....com>, dave.hansen@...ux.intel.com,
 luto@...nel.org, peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com,
 bp@...en8.de
Cc: x86@...nel.org, hpa@...or.com, kirill.shutemov@...ux.intel.com,
 rick.p.edgecombe@...el.com, mhklinux@...look.com, peterx@...hat.com,
 linux-kernel@...r.kernel.org, thomas.lendacky@....com,
 linux-coco@...ts.linux.dev, jroedel@...e.de
Subject: Re: [PATCH] x86/mm: fix lookup_address() to handle physical memory
 holes in direct mapping

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@....com>
>>>
>>> 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.

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


Juergen