linux-kernel - Re: [PATCH v41 06/24] x86/mm: x86/sgx: Signal SIGSEGV with PF

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Date:   Tue, 17 Nov 2020 20:12:58 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     x86@...nel.org, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Serge Ayoun <serge.ayoun@...el.com>, akpm@...ux-foundation.org
Subject: Re: [PATCH v41 06/24] x86/mm: x86/sgx: Signal SIGSEGV with PF_SGX

On Sat, Nov 14, 2020 at 05:07:08PM +0800, Hillf Danton wrote:
> On Fri, 13 Nov 2020 00:01:17 +0200 
> > 
> > From: Sean Christopherson <sean.j.christopherson@...el.com>
> > 
> > The x86 architecture has a set of page fault error codes.  These indicate
> > things like whether the fault occurred from a write, or whether it
> > originated in userspace.
> > 
> > The SGX hardware architecture has its own per-page memory management
> > metadata (EPCM) [*] and hardware which is separate from the normal x86 MMU.
> > The architecture has a new page fault error code: PF_SGX.  This new error
> > code bit is set whenever a page fault occurs as the result of the SGX MMU.
> > 
> > These faults occur for a variety of reasons.  For instance, an access
> > attempt to enclave memory from outside the enclave causes a PF_SGX fault.
> > PF_SGX would also be set for permission conflicts, such as if a write to an
> > enclave page occurs and the page is marked read-write in the x86 page
> > tables but is read-only in the EPCM.
> > 
> > These faults do not always indicate errors, though.  SGX pages are
> > encrypted with a key that is destroyed at hardware reset, including
> > suspend. Throwing a SIGSEGV allows user space software to react and recover
> > when these events occur.
> > 
> > Include PF_SGX in the PF error codes list and throw SIGSEGV when it is
> > encountered.
> > 
> > [*] Intel SDM: 36.5.1 Enclave Page Cache Map (EPCM)
> > 
> > Acked-by: Jethro Beekman <jethro@...tanix.com> # v40
> > # Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
> > ---
> >  arch/x86/include/asm/trap_pf.h |  1 +
> >  arch/x86/mm/fault.c            | 12 ++++++++++++
> >  2 files changed, 13 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/trap_pf.h b/arch/x86/include/asm/trap_pf.h
> > index 305bc1214aef..1794777b2a85 100644
> > --- a/arch/x86/include/asm/trap_pf.h
> > +++ b/arch/x86/include/asm/trap_pf.h
> > @@ -19,6 +19,7 @@ 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_TRAP_PF_H */
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index 82bf37a5c9ec..9339fee83784 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -1101,6 +1101,18 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
> >  	if (error_code & X86_PF_PK)
> >  		return 1;
> >  
> > +	/*
> > +	 * SGX hardware blocked the access.  This usually happens
> > +	 * when the enclave memory contents have been destroyed, like
> > +	 * after a suspend/resume cycle. In any case, the kernel can't
> > +	 * fix the cause of the fault.  Handle the fault as an access
> > +	 * error even in cases where no actual access violation
> > +	 * occurred.  This allows userspace to rebuild the enclave in
> > +	 * response to the signal.
> > +	 */
> 
> Given SIGSEGV, it helps much understand PF_SGX to specify the rebuild
> in userspace a bit more.

I'm sorry but I'm not exactly sure what you are suggesting.

/Jarkko