linux-kernel - Re: [PATCHv4, REBASED 1/4] x86/mm/ident_map: Fix virtual address wrap to zero

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Message-ID: <3rmdms6czfsa4hwfx5egcf5rheojpwhdhwq2gxkn2ffvvymfir@dv7x4cmby6sz>
Date: Thu, 31 Oct 2024 12:11:52 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Baoquan He <bhe@...hat.com>, Ard Biesheuvel <ardb@...nel.org>, 
	Tom Lendacky <thomas.lendacky@....com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, Sean Christopherson <seanjc@...gle.com>, 
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org, Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCHv4, REBASED 1/4] x86/mm/ident_map: Fix virtual address
 wrap to zero

On Wed, Oct 30, 2024 at 12:47:12PM +0100, Borislav Petkov wrote:
> On Wed, Oct 16, 2024 at 02:14:55PM +0300, Kirill A. Shutemov wrote:
> > Calculation of 'next' virtual address doesn't protect against wrapping
> > to zero. It can result in page table corruption and hang. The
> > problematic case is possible if user sets high x86_mapping_info::offset.
> > 
> > The wrapping to zero only occurs if the top PGD entry is accessed.
> > There are no such users in the upstream. Only hibernate_64.c uses
> > x86_mapping_info::offset, and it operates on the direct mapping range,
> > which is not the top PGD entry.
> > 
> > Replace manual 'next' calculation with p?d_addr_end() which handles
> > wrapping correctly.
> 
> So this is a fix for a theoretical issue as it cannot happen currently?

Right.

> Can we call that out in the commit message so that the stable AI doesn't pick
> it up?

Do we have magic words for that?

I tried to express that in the second paragraph: "no such users in the
upstream".

> And which commit is it fixing?
> 
> aece27851d44 ("x86, 64bit, mm: Add generic kernel/ident mapping helper")
> perhaps?

This one is closer:

e4630fdd4763 ("x86/power/64: Always create temporary identity mapping correctly")

It adds x86_mapping_info::offset.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov