linux-kernel - Re: [PATCH] x86/sgx: fix a NULL pointer

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Date:   Tue, 18 Jul 2023 13:56:27 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Haitao Huang <haitao.huang@...ux.intel.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        dave.hansen@...ux.intel.com, linux-kernel@...r.kernel.org,
        linux-sgx@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Cc:     kai.huang@...el.com, reinette.chatre@...el.com,
        kristen@...ux.intel.com, seanjc@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH] x86/sgx: fix a NULL pointer

On 7/18/23 13:32, Haitao Huang wrote:
...
> Ignore VA pages for now. Say for a system with 10 page EPC, 2 enclaves,
> each needs 5 pages non-SECS so total demand would be 12 pages. The ksgxd
> would only need to swap out 2 pages at the most to get one enclave fully
> loaded with 6 pages, and the other one with 4 pages. There is no chance
> the ksgxd would swap any one of two SECS pages.
> 
> We would need at least one enclave A of 10 pages total to squeeze out
> the other B completely. For that to happen B pretty much has to be
> sleeping all the time so the LRU based reclaiming would hit it but not
> pages of A. So no chance to hit #PF on pages of B still.
> 
> So some minimal pressure is needed to ensure SECS swapped. The higher
> the pressure the higher the chance to hit #PF while SECS is swapped.

What would the second-to-last non-SECS page be?  A thread control page?
VA page?

As long as *that* page can generate a page fault, then you only need two
pages for this scenario to happen:

1. Reclaimer takes encl->lock
2. #PF occurs from another thread, blocks on encl->lock
3. SECS is reclaimed
4. encl->lock released
5. #PF sees reclaimed SECS