linux-kernel - Re: [PATCH v22 03/24] x86/mm: x86/sgx: Signal SIGSEGV with PF

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Message-ID: <20190924160442.GH19317@zn.tnic>
Date:   Tue, 24 Sep 2019 18:04:42 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, sean.j.christopherson@...el.com,
        nhorman@...hat.com, npmccallum@...hat.com, serge.ayoun@...el.com,
        shay.katz-zamir@...el.com, haitao.huang@...el.com,
        andriy.shevchenko@...ux.intel.com, tglx@...utronix.de,
        kai.svahn@...el.com, josh@...htriplett.org, luto@...nel.org,
        kai.huang@...el.com, rientjes@...gle.com, cedric.xing@...el.com,
        Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v22 03/24] x86/mm: x86/sgx: Signal SIGSEGV with PF_SGX

On Tue, Sep 03, 2019 at 05:26:34PM +0300, Jarkko Sakkinen wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> Include SGX bit to the PF error codes and throw SIGSEGV with PF_SGX when
> a #PF with SGX set happens.
> 
> CPU throws a #PF with the SGX bit in the event of Enclave Page Cache Map
> (EPCM) conflict. The EPCM is a CPU-internal table, which describes the
> properties for a enclave page. Enclaves are measured and signed software
> entities, which SGX hosts. [1]
> 
> Although the primary purpose of the EPCM conflict checks  is to prevent
> malicious accesses to an enclave, an illegit access can happen also for
> legit reasons.
> 
> All SGX reserved memory, including EPCM is encrypted with a transient
> key that does not survive from the power transition. Throwing a SIGSEGV
> allows user space software react when this happens (e.g. rec-create the
> enclave, which was invalidated).
> 
> [1] Intel SDM: 36.5.1 Enclave Page Cache Map (EPCM)
> 
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> ---
>  arch/x86/include/asm/traps.h |  1 +
>  arch/x86/mm/fault.c          | 13 +++++++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index b25e633033c3..81472cae4024 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -171,5 +171,6 @@ enum x86_pf_error_code {
>  	X86_PF_RSVD	=		1 << 3,
>  	X86_PF_INSTR	=		1 << 4,
>  	X86_PF_PK	=		1 << 5,
> +	X86_PF_SGX	=		1 << 15,
>  };
>  #endif /* _ASM_X86_TRAPS_H */
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 9ceacd1156db..c2dea3f9e263 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1178,6 +1178,19 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
>  	if (error_code & X86_PF_PK)
>  		return 1;
>  
> +	/*
> +	 * Access is blocked by the Enclave Page Cache Map (EPCM), i.e. the
> +	 * access is allowed by the PTE but not the EPCM.  This usually happens
> +	 * when the EPCM is yanked out from under us, e.g. by hardware after a
> +	 * suspend/resume cycle.  In any case, software, i.e. the kernel, can't
> +	 * fix the source of the fault as the EPCM can't be directly modified
> +	 * by software.  Handle the fault as an access error in order to signal
> +	 * userspace, e.g. so that userspace can rebuild their enclave(s), even

s/, e.g.//

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette