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Message-ID: <c57d8cc4e4bfbef028a1f226ec2c4691a7c100fe.camel@intel.com>
Date: Thu, 27 Jul 2023 02:50:02 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
"bp@...en8.de" <bp@...en8.de>,
"jarkko@...nel.org" <jarkko@...nel.org>,
"Chatre, Reinette" <reinette.chatre@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"x86@...nel.org" <x86@...nel.org>,
"haitao.huang@...ux.intel.com" <haitao.huang@...ux.intel.com>,
"hpa@...or.com" <hpa@...or.com>,
"mingo@...hat.com" <mingo@...hat.com>
CC: "kristen@...ux.intel.com" <kristen@...ux.intel.com>,
"Mehta, Sohil" <sohil.mehta@...el.com>,
"stable@...r.kernel.org" <stable@...r.kernel.org>,
"Hansen, Dave" <dave.hansen@...el.com>,
"Christopherson,, Sean" <seanjc@...gle.com>
Subject: Re: [PATCH v5] x86/sgx: Resolves SECS reclaim vs. page fault for EAUG
race
On Wed, 2023-07-26 at 18:02 -0700, Haitao Huang wrote:
> Under heavy load, the SGX EPC reclaimer (ksgxd) may reclaim the SECS EPC
If I read correctly, Dave suggested to not use "high" (heavy in this sentence)
or "low" pressure:
https://lore.kernel.org/lkml/op.179a4xs0wjvjmi@hhuan26-mobl.amr.corp.intel.com/T/#m9120eac6a4a94daa7c9fcc47709f241cd181e5dc
And I agree. For instance, consider this happens to one extremely "small"
enclave, while there's a new "big" enclave starts to run. I don't think we
should say this is "under heavy load". Just stick to the fact that the
reclaimer may reclaim the SECS page.
> page for an enclave and set encl->secs.epc_page to NULL. But the SECS
> EPC page is used for EAUG in the SGX page fault handler without checking
> for NULL and reloading.
>
> Fix this by checking if SECS is loaded before EAUG and loading it if it
> was reclaimed.
>
> The SECS page holds global enclave metadata. It can only be reclaimed
> when there are no other enclave pages remaining. At that point,
> virtually nothing can be done with the enclave until the SECS page is
> paged back in.
>
> An enclave can not run nor generate page faults without a resident SECS
> page.
>
I am not sure whether "nor generate page faults without a resident SECS page" is
accurate? When SECS is swapped out, I suppose the first EENTER should trigger a
#PF on the TSC page and in the #PF handler the SECS will be swapped in first.
I guess you can just remove this sentence?
> But it is still possible for a #PF for a non-SECS page to race
> with paging out the SECS page: when the last resident non-SECS page A
> triggers a #PF in a non-resident page B, and then page A and the SECS
> both are paged out before the #PF on B is handled.
>
> Hitting this bug requires that race triggered with a #PF for EAUG.
The above race can happen for the normal ELDU path too, thus I suppose it will
be better to mention why the normal ELDU path doesn't have this issue: it
already does what this fix does.
> Following is a trace when it happens.
>
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> RIP: 0010:sgx_encl_eaug_page+0xc7/0x210
> Call Trace:
> ? __kmem_cache_alloc_node+0x16a/0x440
> ? xa_load+0x6e/0xa0
> sgx_vma_fault+0x119/0x230
> __do_fault+0x36/0x140
> do_fault+0x12f/0x400
> __handle_mm_fault+0x728/0x1110
> handle_mm_fault+0x105/0x310
> do_user_addr_fault+0x1ee/0x750
> ? __this_cpu_preempt_check+0x13/0x20
> exc_page_fault+0x76/0x180
> asm_exc_page_fault+0x27/0x30
>
> Fixes: 5a90d2c3f5ef ("x86/sgx: Support adding of pages to an initialized enclave")
> Cc: stable@...r.kernel.org # v6.0+
> Signed-off-by: Haitao Huang <haitao.huang@...ux.intel.com>
> Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>
> Acked-by: Reinette Chatre <reinette.chatre@...el.com>
With above addressed, feel free to add:
Reviewed-by: Kai Huang <kai.huang@...el.com>
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