linux-kernel - Re: [PATCH v3 1/7] x86/sgx: Provide indication of life-cycle of EPC pages

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Date:   Wed, 28 Jul 2021 15:12:03 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tony Luck <tony.luck@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/7] x86/sgx: Provide indication of life-cycle of EPC
 pages

On 7/28/21 1:46 PM, Tony Luck wrote:
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -581,7 +581,7 @@ struct sgx_epc_page *sgx_alloc_epc_page(void *owner, bool reclaim)
>  	for ( ; ; ) {
>  		page = __sgx_alloc_epc_page();
>  		if (!IS_ERR(page)) {
> -			page->owner = owner;
> +			page->owner = owner ? owner : page;
>  			break;
>  		}

I'm a little worried about this.

Let's say we get confused about the type of the page and dereference
page->owner.  If it's NULL, we get a nice oops.  If it's a real, valid
pointer, we get real valid memory back that we can scribble on.

Wouldn't it be safer to do something like:

	page->owner = owner ? owner : (void *)-1;

-1 is non-NULL, but also invalid, which makes it harder for us to poke
ourselves in the eye.