linux-kernel - Re: [PATCH 4/4] x86/tdx: Disable NOTIFY

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Date:   Fri, 9 Dec 2022 20:10:29 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Elena Reshetova <elena.reshetova@...el.com>, x86@...nel.org,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] x86/tdx: Disable NOTIFY_ENABLES

On Fri, Dec 09, 2022 at 07:50:46AM -0800, Sathyanarayanan Kuppuswamy wrote:
> 
> 
> On 12/9/22 5:25 AM, Kirill A. Shutemov wrote:
> > == Background ==
> > 
> > There is a class of side-channel attacks against SGX enclaves called
> > "SGX Step"[1]. These attacks create lots of exceptions inside of
> > enclaves. Basically, run an in-enclave instruction, cause an exception.
> > Over and over.
> > 
> > There is a concern that a VMM could attack a TDX guest in the same way
> > by causing lots of #VE's. The TDX architecture includes new
> > countermeasures for these attacks. It basically counts the number of
> > exceptions and can send another *special* exception once the number of
> > VMM-induced #VE's hits a critical threshold[2].
> > 
> > == Problem ==
> > 
> > But, these special exceptions are independent of any action that the
> > guest takes. They can occur anywhere that the guest executes. This
> > includes sensitive areas like the entry code. The (non-paranoid) #VE
> > handler is incapable of handling exceptions in these areas.
> > 
> > == Solution ==
> > 
> > Fortunately, the special exceptions can be disabled by the guest via
> > write to NOTIFY_ENABLES TDCS field. NOTIFY_ENABLES is disabled by
> > default, but might be enabled by a bootloader, firmware or an earlier
> > kernel before the current kernel runs.
> > 
> > Disable NOTIFY_ENABLES feature explicitly and unconditionally. Any
> > NOTIFY_ENABLES-based #VE's that occur before this point will end up
> > in the early #VE exception handler and die due to unexpected exit
> > reason.
> > 
> > [1] https://github.com/jovanbulck/sgx-step
> > [2] https://intel.github.io/ccc-linux-guest-hardening-docs/security-spec.html#safety-against-ve-in-kernel-code
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> 
> I don't think you need to explicitly use section names (Background,
> problem or solution) in the commit log. But it is up to you.
> 
> Rest looks good.
> 

I've checked git log and some people leave them in. I've decided to keep
them too.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov