linux-kernel - Re: [PATCH v2 1/2] x86/sgx: Resolve EAUG race where losing thread returns SIGBUS

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Date: Wed, 15 May 2024 07:15:42 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Jarkko Sakkinen <jarkko@...nel.org>,
 Dmitrii Kuvaiskii <dmitrii.kuvaiskii@...el.com>,
 dave.hansen@...ux.intel.com, 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,
 Marcelina Kościelnicka <mwk@...isiblethingslab.com>
Subject: Re: [PATCH v2 1/2] x86/sgx: Resolve EAUG race where losing thread
 returns SIGBUS

On 5/15/24 06:54, Jarkko Sakkinen wrote:
> I'd cut out 90% of the description out and just make the argument of
> the wrong error code, and done. The sequence is great for showing
> how this could happen. The prose makes my head hurt tbh.

The changelog is too long, but not fatally so.  I'd much rather have a
super verbose description than something super sparse.

Would something like this make more sense to folks?

	Imagine an mmap()'d file. Two threads touch the same address at
	the same time and fault. Both allocate a physical page and race
	to install a PTE for that page. Only one will win the race. The
	loser frees its page, but still continues handling the fault as
	a success and returns VM_FAULT_NOPAGE from the fault handler.

	The same race can happen with SGX. But there's a bug: the loser
	in the SGX steers into a failure path. The loser EREMOVE's the
	winner's EPC page, then returns SIGBUS, likely killing the app.

	Fix the SGX loser's behavior. Change the return code to
	VM_FAULT_NOPAGE to avoid SIGBUS and call sgx_free_epc_page()
	which avoids EREMOVE'ing the winner's page and only frees the
	page that the loser allocated.