Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <6eed73b1-adee-caf7-e02f-4d2b08f1aefe@intel.com>
Date:   Tue, 13 Dec 2022 15:17:02 -0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.4.2
Subject: Re: [PATCH 4/4] x86/tdx: Disable NOTIFY_ENABLES
Content-Language: en-US
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Elena Reshetova <elena.reshetova@intel.com>, x86@kernel.org,
        linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org
References: <20221209132524.20200-1-kirill.shutemov@linux.intel.com>
 <20221209132524.20200-5-kirill.shutemov@linux.intel.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <20221209132524.20200-5-kirill.shutemov@linux.intel.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 12/9/22 05:25, 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@linux.intel.com>

Reviewed-by: Dave Hansen <dave.hansen@intel.com>