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Date:   Sun, 23 Jan 2022 01:28:36 +0200
From:   Jarkko Sakkinen <jarkko@kernel.org>
To:     Reinette Chatre <reinette.chatre@intel.com>
Cc:     tony.luck@intel.com, dave.hansen@linux.intel.com,
        tglx@linutronix.de, bp@alien8.de, luto@kernel.org,
        mingo@redhat.com, linux-sgx@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH V2] x86/sgx: Add poison handling to reclaimer
Message-ID: <YeyTJCmIi/R94d9S@iki.fi>
References: <be5af586f667c7bcb8ef01286ce75675de5d100f.1642630582.git.reinette.chatre@intel.com>
 <3bfe66204ee84a0bbccaf7cd20af0d8300fb9f26.camel@kernel.org>
 <8ac3fc3e-9d90-48cf-d84e-40f9cbddd9ac@intel.com>
 <YeySy2lGjEbNbK1G@iki.fi>
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On Sun, Jan 23, 2022 at 01:27:10AM +0200, Jarkko Sakkinen wrote:
> On Thu, Jan 20, 2022 at 10:20:01AM -0800, Reinette Chatre wrote:
> > Hi Jarkko,
> > 
> > On 1/20/2022 5:09 AM, Jarkko Sakkinen wrote:
> > > On Wed, 2022-01-19 at 14:23 -0800, Reinette Chatre wrote:
> > >> The SGX reclaimer code lacks page poison handling in its main
> > >> free path. This can lead to avoidable machine checks if a
> > >> poisoned page is freed and reallocated instead of being
> > >> isolated.
> > >>
> > >> A troublesome scenario is:
> > >> ?1. Machine check (#MC) occurs (asynchronous, !MF_ACTION_REQUIRED)
> > >> ?2. arch_memory_failure() is eventually called
> > >> ?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 (poison pages should be isolated on the
> > >> ??? sgx_poison_page_list instead)
> > >> ?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 by replacing the open coded
> > >> enclave page freeing code in the reclaimer with sgx_free_epc_page()
> > >> to obtain support for poison page handling that includes placing the
> > >> poisoned page on the correct list.
> > >>
> > >> Fixes: d6d261bded8a ("x86/sgx: Add new sgx_epc_page flag bit to mark
> > >> free pages")
> > >> Fixes: 992801ae9243 ("x86/sgx: Initial poison handling for dirty and
> > >> free pages")
> > > 
> > > Same comment as for the first version: remove the first fixes tag.
> > > 
> > 
> > For completeness I'll duplicate my response also:
> > 
> > The commit you refer to, commit d6d261bded8a ("x86/sgx: Add new
> > sgx_epc_page flag bit to mark free pages", introduced a new page flag bit
> > (SGX_EPC_PAGE_IS_FREE) that should be set when an EPC page is freed. The
> > commit also sets the bit in sgx_free_epc_page() when an EPC page is freed.
> > The commit should also have set that bit when the EPC page is freed in the
> > reclaimer, which contains an open coded version of sgx_free_epc_page(),
> > but it did not. This fix adds the snippet that was omitted from that
> > commit.
> > 
> > Reinette
> 
> Let me check. Yes, I can see it in lore, but somehow it has slipped out
> of my inbox. My apologies.

And thanks for re-elaborating this.

/Jarkko