linux-kernel - Re: [RFC PATCH] x86/mm: Disable PTI for kernel_ident_mapping

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Message-ID: <5633d50d-0d61-4d18-b43f-11311b5ec920@intel.com>
Date: Mon, 25 Nov 2024 11:13:02 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: David Woodhouse <dwmw2@...radead.org>, kexec@...ts.infradead.org,
 Schönherr, Jan H. <jschoenh@...zon.de>,
 Rik van Riel <riel@...riel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Kai Huang <kai.huang@...el.com>, Nikolay Borisov <nik.borisov@...e.com>,
 linux-kernel@...r.kernel.org, Simon Horman <horms@...nel.org>,
 Dave Young <dyoung@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 jpoimboe@...nel.org, bsz@...zon.de
Subject: Re: [RFC PATCH] x86/mm: Disable PTI for kernel_ident_mapping_init()

On 11/25/24 10:53, David Woodhouse wrote:
>> I think we have a lot of software-available space in the page table
>> pointer entries. What would folks think if we set a special bit in those
>> p4d entries that said:
>>
>> 	"I don't need to be propagated to
>> 	the user portion of the page tables."
>>
>> It would obviously get set in this code that you're trying to fix. It
>> might _also_ be able to be set in in "_USR", like here:
>>
>> #define _KERNPG_TABLE_NOENC  (__PP|__RW|   0|___A|   0|___D|   0|   0)
>> #define _PAGE_TABLE_NOENC    (__PP|__RW|_USR|___A|   0|___D|   0|   0)
>>
>> like:
>>
>> #define _USR _PAGE_USER|_PAGE_SW_WHATEVER
> In fact, do we even need a separate bit? Any PTE without the _PAGE_USER
> bit set clearly doesn't need to be mirrored into the user page
> tables...?

I can't think of any exceptions where this would break off the top of my
head. It seems too simple to work. ;)