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

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Message-ID: <2C6F6BDC-16B5-41B4-AA3B-F2C23F4EBB75@infradead.org>
Date: Mon, 25 Nov 2024 19:36:03 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Dave Hansen <dave.hansen@...el.com>, 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 25 November 2024 19:13:02 GMT, Dave Hansen <dave.hansen@...el.com> wrote:
>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. ;)

I'll throw something together and see if it explodes.