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

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Date:   Fri, 9 Dec 2022 07:50:46 -0800
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     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 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.


>  arch/x86/coco/tdx/tdx.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 0e47846ff8ff..c93c2fd2e113 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -19,6 +19,10 @@
>  #define TDX_GET_VEINFO			3
>  #define TDX_GET_REPORT			4
>  #define TDX_ACCEPT_PAGE			6
> +#define TDX_WR				8
> +
> +/* TDCS fields. To be used by TDG.VM.WR and TDG.VM.RD module calls */
> +#define TDCS_NOTIFY_ENABLES		0x9100000000000010
>  
>  /* TDX hypercall Leaf IDs */
>  #define TDVMCALL_MAP_GPA		0x10001
> @@ -858,6 +862,9 @@ void __init tdx_early_init(void)
>  	tdx_parse_tdinfo(&cc_mask);
>  	cc_set_mask(cc_mask);
>  
> +	/* Kernel does not use NOTIFY_ENABLES and does not need random #VEs */
> +	tdx_module_call(TDX_WR, 0, TDCS_NOTIFY_ENABLES, 0, -1ULL, NULL);
> +
>  	/*
>  	 * All bits above GPA width are reserved and kernel treats shared bit
>  	 * as flag, not as part of physical address.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer