linux-kernel - Re: [PATCH 14/25] x86/sgx: Tighten accessible memory range after enclave initialization

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Date:   Sun, 5 Dec 2021 01:14:58 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     dave.hansen@...ux.intel.com, tglx@...utronix.de, bp@...en8.de,
        luto@...nel.org, mingo@...hat.com, linux-sgx@...r.kernel.org,
        x86@...nel.org, seanjc@...gle.com, kai.huang@...el.com,
        cathy.zhang@...el.com, cedric.xing@...el.com,
        haitao.huang@...el.com, mark.shanahan@...el.com, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 14/25] x86/sgx: Tighten accessible memory range after
 enclave initialization

On Wed, Dec 01, 2021 at 11:23:12AM -0800, Reinette Chatre wrote:
> Before an enclave is initialized the enclave's memory range is unknown.
> The enclave's memory range is learned at the time it is created via the
> SGX_IOC_ENCLAVE_CREATE ioctl where the provided memory range is obtained
> from an earlier mmap() of the sgx_enclave device. After an enclave is
> initialized its memory can be mapped into user space (mmap()) from where
> it can be entered at its defined entry points.
> 
> With the enclave's memory range known after it is initialized there is
> no reason why it should be possible to map memory outside this range.
> 
> Lock down access to the initialized enclave's memory range by denying
> any attempt to map memory outside its memory range.
> 
> Locking down the memory range also makes adding pages to an initialized
> enclave more efficient. Pages are added to an initialized enclave by
> accessing memory that belongs to the enclave's memory range but not yet
> backed by an enclave page. If it is possible for user space to map
> memory that does not form part of the enclave then an access to this
> memory would eventually fail. Failures range from a prompt general
> protection fault if the access was an ENCLU[EACCEPT] from within the
> enclave, or a page fault via the vDSO if it was another access from
> within the enclave, or a SIGBUS (also resulting from a page fault) if
> the access was from outside the enclave.
> 
> Disallowing invalid memory to be mapped in the first place avoids
> preventable failures.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
> ---
>  arch/x86/kernel/cpu/sgx/encl.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index 342b97dd4c33..37203da382f8 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -403,6 +403,10 @@ int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
>  
>  	XA_STATE(xas, &encl->page_array, PFN_DOWN(start));
>  

Please write a comment here.

> +	if (test_bit(SGX_ENCL_INITIALIZED, &encl->flags) &&
> +	    (start < encl->base || end > encl->base + encl->size))
> +		return -EACCES;
> +
>  	/*
>  	 * Disallow READ_IMPLIES_EXEC tasks as their VMA permissions might
>  	 * conflict with the enclave page permissions.
> -- 
> 2.25.1
> 

Otherwise, makes sense.

/Jarkko