linux-kernel - Re: [PATCH v5 4/6] x86/tdx: Add a restriction on access to MMIO address

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Message-ID: <cpbbbdeqkgvtoeqviygn6vcagxl55wjb3nyhqd5fzk2re6luan@rmfr2sbw7tfi>
Date: Thu, 29 Aug 2024 15:30:21 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Alexey Gladkov <legion@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Yuan Yao <yuan.yao@...el.com>, 
	Geert Uytterhoeven <geert@...ux-m68k.org>, Yuntao Wang <ytcoode@...il.com>, Kai Huang <kai.huang@...el.com>, 
	Baoquan He <bhe@...hat.com>, Oleg Nesterov <oleg@...hat.com>, cho@...rosoft.com, 
	decui@...rosoft.com, John.Starks@...rosoft.com, stable@...r.kernel.org
Subject: Re: [PATCH v5 4/6] x86/tdx: Add a restriction on access to MMIO
 address

On Wed, Aug 28, 2024 at 12:44:34PM +0200, Alexey Gladkov wrote:
> From: "Alexey Gladkov (Intel)" <legion@...nel.org>
> 
> For security reasons, access from kernel space to MMIO addresses in
> userspace should be restricted. All MMIO operations from kernel space
> are considered trusted and are not validated.
> 
> For instance, if in response to a syscall, kernel does put_user() and
> the target address is MMIO mapping in userspace, current #VE handler
> threat this access as kernel MMIO which is wrong and have security
> implications.

What about this:

------------------------------------8<-----------------------------------

Subject: x86/tdx: Fix "in-kernel MMIO" check

TDX only supports kernel-initiated MMIO operations. The handle_mmio()
function checks if the #VE exception occurred in the kernel and rejects
the operation if it did not.

However, userspace can deceive the kernel into performing MMIO on its
behalf. For example, if userspace can point a syscall to an MMIO address,
syscall does get_user() or put_user() on it, triggering MMIO #VE. The
kernel will treat the #VE as in-kernel MMIO.

Ensure that the target MMIO address is within the kernel before decoding
instruction.

------------------------------------8<-----------------------------------

And please make this patch the first in the patchset. It has to be
backported to stable trees and should have zero dependencies on the rest
of the patchset.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov