linux-kernel - Re: [PATCH 1/9] x86/kexec: Disable global pages before writing to control page

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Message-ID: <tksesvqt266x6a6mnoi5aqa3fhsoyo7mbp277elrrft5eieoba@7ospdizq22y6>
Date: Tue, 17 Dec 2024 14:25:48 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: David Woodhouse <dwmw2@...radead.org>
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>, Eric Biederman <ebiederm@...ssion.com>, 
	David Woodhouse <dwmw@...zon.co.uk>, Sourabh Jain <sourabhjain@...ux.ibm.com>, 
	Hari Bathini <hbathini@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Thomas Zimmermann <tzimmermann@...e.de>, Andrew Morton <akpm@...ux-foundation.org>, 
	Baoquan He <bhe@...hat.com>, Yuntao Wang <ytcoode@...il.com>, 
	David Kaplan <david.kaplan@....com>, Tao Liu <ltao@...hat.com>, Kai Huang <kai.huang@...el.com>, 
	Ard Biesheuvel <ardb@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Breno Leitao <leitao@...ian.org>, Wei Yang <richard.weiyang@...il.com>, Rong Xu <xur@...gle.com>, 
	Thomas Weißschuh <thomas.weissschuh@...utronix.de>, linux-kernel@...r.kernel.org, kexec@...ts.infradead.org, 
	Simon Horman <horms@...nel.org>, Dave Young <dyoung@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, bsz@...zon.de, nathan@...nel.org
Subject: Re: [PATCH 1/9] x86/kexec: Disable global pages before writing to
 control page

On Mon, Dec 16, 2024 at 11:24:08PM +0000, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> The kernel switches to a new set of page tables during kexec. The global
> mappings (_PAGE_GLOBAL==1) can remain in the TLB after this switch. This
> is generally not a problem because the new page tables use a different
> portion of the virtual address space than the normal kernel mappings.
> 
> The critical exception to that generalisation (and the only mapping
> which isn't an identity mapping) is the kexec control page itself —
> which was ROX in the original kernel mapping, but should be RWX in the
> new page tables. If there is a global TLB entry for that in its prior
> read-only state, it definitely needs to be flushed before attempting to
> write through that virtual mapping.
> 
> It would be possible to just avoid writing to the virtual address of the
> page and defer all writes until they can be done through the identity
> mapping. But there's no good reason to keep the old TLB entries around,
> as they can cause nothing but trouble.
> 
> Clear the PGE bit in %cr4 early, before storing data in the control page.

It worth noting that flipping CR4.PGE triggers TLB flush. I was not sure
if CR3 write is required to make it happen.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov