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

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Message-ID: <88a87314-45ad-47f7-a93a-a9a53d7e8745@intel.com>
Date: Mon, 25 Nov 2024 10:31:13 -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 09:05, David Woodhouse wrote:
> Not sure I like this very much, but it works, and mirrors what
> arch/x86/boot/compressed/ident_map_64.c already does.

I don't like it much, either.

arch/x86/boot/compressed/ is already on the road to sharing no code with
the core kernel and it's full of horrors. It should be an example of
what *not* to do. ;)

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