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To:     Reinette Chatre <reinette.chatre@intel.com>, tony.luck@intel.com,
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References: <ef74bd9548df61f77e802e7505affcfb5159c48c.1642545829.git.reinette.chatre@intel.com>
From:   Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH] x86/sgx: Add poison handling to reclaimer
Message-ID: <9cefa244-9830-c158-6112-b2c61a464632@intel.com>
Date:   Wed, 19 Jan 2022 11:51:05 -0800
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On 1/18/22 3:05 PM, Reinette Chatre wrote:
> The machine check recovery handling in SGX added the changes
> listed below to the freeing of pages in sgx_free_epc_page().
> The SGX reclaimer contains an open coded version of
> sgx_free_epc_page() and thus did not obtain the changes in
> support of poison handling.

I was trying to decide if this is an urgent fix or not.  A more crisp
problem statement might have helped in the changelog.

But, from what I can tell, the most probable troublesome scenario here
would be something like:

 1. Machine check (#MC) occurs (asynchronous, !MF_ACTION_REQUIRED)
 2. arch_memory_failure() called is eventually
 3. (SGX) page->poison set to 1
 4. Page is reclaimed
 5. Page added to normal free lists by sgx_reclaim_pages()
    ^ This is the bug
 6. Page is reallocated by some innocent enclave, a second (synchronous)
    in-kernel #MC is induced, probably during EADD instruction.
    ^ This is the fallout from the bug

#6 is unfortunate and can be avoided if this patch is applied.

Basically, this patch ensures that a bad enclave page is isolated
quickly and causes a minimal amount of collateral damage.  Is this a
valid summary?

	The SGX reclaimer code lacks page poison handling in its free
	path.  This can lead to completely avoidable machine checks if a
	poisoned page is freed and reallocated instead of being
	isolated.