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

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Date: Mon, 03 Jun 2024 13:42:49 -0500
From: "Haitao Huang" <haitao.huang@...ux.intel.com>
To: "Dmitrii Kuvaiskii" <dmitrii.kuvaiskii@...el.com>,
 dave.hansen@...ux.intel.com, jarkko@...nel.org, kai.huang@...el.com,
 reinette.chatre@...el.com, linux-sgx@...r.kernel.org,
 linux-kernel@...r.kernel.org, "Dave Hansen" <dave.hansen@...el.com>
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 Tue, 28 May 2024 11:23:13 -0500, Dave Hansen <dave.hansen@...el.com>  
wrote:

> 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".
>

IIUC, reclaimer_writing_to_pcmd() also uses SGX_ENCL_PAGE_BEING_RECLAIMED  
to check if a page is about being reclaimed in order to prevent its VA  
slot fro being freed. So I think we do need separate bit for EREMOVE which  
does not write to VA slot?

BR
Haitao