linux-kernel - Re: [PATCH v3 2/2] x86/sgx: Resolve EREMOVE page vs EAUG page data race

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Date: Tue, 28 May 2024 09:23:13 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@...el.com>,
 dave.hansen@...ux.intel.com, jarkko@...nel.org, kai.huang@...el.com,
 haitao.huang@...ux.intel.com, reinette.chatre@...el.com,
 linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: mona.vij@...el.com, kailun.qin@...el.com, stable@...r.kernel.org
Subject: Re: [PATCH v3 2/2] x86/sgx: Resolve EREMOVE page vs EAUG page data
 race

On 5/17/24 04:06, Dmitrii Kuvaiskii wrote:
..

First, why is SGX so special here?  How is the SGX problem different
than what the core mm code does?

> --- a/arch/x86/kernel/cpu/sgx/encl.h
> +++ b/arch/x86/kernel/cpu/sgx/encl.h
> @@ -25,6 +25,9 @@
>  /* 'desc' bit marking that the page is being reclaimed. */
>  #define SGX_ENCL_PAGE_BEING_RECLAIMED	BIT(3)
>  
> +/* 'desc' bit marking that the page is being removed. */
> +#define SGX_ENCL_PAGE_BEING_REMOVED	BIT(2)

Second, convince me that this _needs_ a new bit.  Why can't we just have
a bit that effectively means "return EBUSY if you see this bit when
handling a fault".

>  struct sgx_encl_page {
>  	unsigned long desc;
>  	unsigned long vm_max_prot_bits:8;
> diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
> index 5d390df21440..de59219ae794 100644
> --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> @@ -1142,6 +1142,7 @@ static long sgx_encl_remove_pages(struct sgx_encl *encl,
>  		 * Do not keep encl->lock because of dependency on
>  		 * mmap_lock acquired in sgx_zap_enclave_ptes().
>  		 */
> +		entry->desc |= SGX_ENCL_PAGE_BEING_REMOVED;

This also needs a comment, no matter what.