Linux-Sgx Archive on lore.kernel.org
 help / color / Atom feed
From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	x86@kernel.org, linux-sgx@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Jethro Beekman <jethro@fortanix.com>,
	akpm@linux-foundation.org, andriy.shevchenko@linux.intel.com,
	asapek@google.com, cedric.xing@intel.com,
	chenalexchen@google.com, conradparker@google.com,
	cyhanish@google.com, dave.hansen@intel.com,
	haitao.huang@intel.com, josh@joshtriplett.org,
	kai.huang@intel.com, kai.svahn@intel.com, kmoy@google.com,
	ludloff@google.com, luto@kernel.org, nhorman@redhat.com,
	npmccallum@redhat.com, puiterwijk@redhat.com,
	rientjes@google.com, tglx@linutronix.de, yaozhangx@google.com
Subject: Re: [PATCH v33 03/21] x86/mm: x86/sgx: Signal SIGSEGV with PF_SGX
Date: Thu, 25 Jun 2020 08:34:31 -0700
Message-ID: <20200625153431.GA3437@linux.intel.com> (raw)
In-Reply-To: <20200625085931.GB20319@zn.tnic>

On Thu, Jun 25, 2020 at 10:59:31AM +0200, Borislav Petkov wrote:
> On Thu, Jun 18, 2020 at 01:08:25AM +0300, Jarkko Sakkinen wrote:
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index 66be9bd60307..25d48aae36c1 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -1055,6 +1055,19 @@ 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 so that userspace can rebuild their enclave(s), even though
> > +	 * userspace may not have actually violated access permissions.
> > +	 */
> 
> Lemme check whether I understand this correctly: userspace must check
> whether the SIGSEGV is generated on an access to an enclave page?

Sort of.  Technically it's that's an accurate statement, but practically
speaking userspace can only access enclave pages when it is executing in
the enclave, and exceptions in enclaves have unique behavior.  Exceptions
in enclaves essentially bounce through a userspace-software-defined
location prior to being delivered to the kernel.  The trampoline is done
by the CPU so that the CPU can scrub the GPRs, XSAVE state, etc... and
hide the true RIP of the exception.  The pre-exception enclave state is
saved into protected memory and restored when userspace resumes the enclave.

Enterring or resuming an enclave can only be done through dedicted ENCLU
instructions, so really it ends up being that the SIGSEGV handler needs to
check the IP that "caused" the fault, which is actually the IP of the
trampoline.

But, that's only the first half of the story...
 
> Also, do I see it correctly that when this happens, dmesg will have
> 
>         printk("%s%s[%d]: segfault at %lx ip %px sp %px error %lx",
> 
> due to:
> 
>        if (likely(show_unhandled_signals))
>                show_signal_msg(regs, error_code, address, tsk);
> 
> which does:
> 
>         if (!unhandled_signal(tsk, SIGSEGV))
>                 return;
> 
> or is the task expected to register a SIGSEGV handler so that the
> segfault doesn't land in dmesg?

Yes, without extra help, any task running an enclave is expected to register
a SIGSEGV handler so that the task can restart the enclave if the EPC is
"lost".

However, building and running enclaves is complex, and the vast majority of
SGX enabled applications are expected to leverage a library of one kind or
another to hand the bulk of the gory details.  But, signal handling in
libraries is a mess, e.g. requires filtering/forwarding, resignaling, etc...

To that end, in v14 of this patch[1], Andy Lutomirski came up with the idea
of adding a vDSO function to provide the low level enclave EENTER/ERESUME and
trampoline, and then teaching the kernel to do exception fixup on the
relevant instructions in the vDSO.  The vDSO's exception fixup then returns
to normal userspace, with a (technically optional) struct holding the details
of the exception.  That allows for synchronous delivery of exceptions in
enclaves, obviates the need for userspace to regsiter a SIGSEGV handler, and
also means the SIGSEGV will never show up in dmesg so long as userspace is
using the vDSO.  The kernel still supports direct EENTER/ERESUME, but AFAIK
everyone is moving (or has moved) to the vDSO interface.

The vDSO stuff is in patches 15-18 of this series.

There's a gigantic thread on all the alternatives that were considered[2].

[1] https://lkml.kernel.org/r/CALCETrXByb2UVuZ6AXUeOd8y90NAikbZuvdN3wf_TjHZ+CxNhA@mail.gmail.com
[2] https://lkml.kernel.org/r/CALCETrWdpoDkbZjkucKL91GWpDPG9p=VqYrULade2pFDR7S=GQ@mail.gmail.com

> 
> If so, are we documenting this?
> 
> If not, then we should not issue any "segfault" messages to dmesg
> because that would be wrong.
> 
> Or maybe I'm not seeing it right but I don't have the hardware to test
> this out...
> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette

  reply index

Thread overview: 84+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-17 22:08 [PATCH v33 00/21] Intel SGX foundations Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 01/21] x86/cpufeatures: x86/msr: Add Intel SGX hardware bits Jarkko Sakkinen
2020-06-22 17:37   ` Borislav Petkov
2020-06-25  1:25     ` Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 02/21] x86/cpufeatures: x86/msr: Add Intel SGX Launch Control " Jarkko Sakkinen
2020-06-24 13:04   ` Borislav Petkov
2020-06-24 14:34     ` Sean Christopherson
2020-06-25  1:28       ` Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 03/21] x86/mm: x86/sgx: Signal SIGSEGV with PF_SGX Jarkko Sakkinen
2020-06-25  8:59   ` Borislav Petkov
2020-06-25 15:34     ` Sean Christopherson [this message]
2020-06-25 16:49       ` Borislav Petkov
2020-06-25 20:52     ` Jarkko Sakkinen
2020-06-25 21:11       ` Borislav Petkov
2020-06-26 13:34         ` Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 04/21] x86/sgx: Add SGX microarchitectural data structures Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 05/21] x86/sgx: Add wrappers for ENCLS leaf functions Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 06/21] x86/cpu/intel: Detect SGX support Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 07/21] x86/cpu/intel: Add nosgx kernel parameter Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 08/21] x86/sgx: Initialize metadata for Enclave Page Cache (EPC) sections Jarkko Sakkinen
2020-06-25 10:14   ` Borislav Petkov
2020-06-25 20:11     ` Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 09/21] x86/sgx: Add __sgx_alloc_epc_page() and sgx_free_epc_page() Jarkko Sakkinen
2020-06-25 17:06   ` Borislav Petkov
2020-06-25 20:55     ` Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 10/21] mm: Introduce vm_ops->may_mprotect() Jarkko Sakkinen
2020-06-25 17:14   ` Borislav Petkov
2020-06-25 17:30     ` Matthew Wilcox
2020-06-25 18:06       ` Sean Christopherson
2020-06-25 22:40         ` Jarkko Sakkinen
2020-06-25 22:26     ` Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 11/21] x86/sgx: Linux Enclave Driver Jarkko Sakkinen
2020-06-25 17:23   ` Borislav Petkov
2020-06-25 18:34     ` Sean Christopherson
2020-06-25 18:45       ` Borislav Petkov
2020-06-26 14:19       ` Jarkko Sakkinen
2020-06-25 20:21     ` Jarkko Sakkinen
2020-06-25 20:25       ` Borislav Petkov
2020-06-26 13:40         ` Jarkko Sakkinen
2020-06-25 18:53   ` Borislav Petkov
2020-06-26 14:17     ` Jarkko Sakkinen
2020-06-26  9:14   ` Borislav Petkov
2020-06-26 14:16     ` Sean Christopherson
2020-06-26 14:20       ` Borislav Petkov
2020-07-03 23:04         ` Jarkko Sakkinen
2020-07-03  3:09     ` Jarkko Sakkinen
2020-06-26 15:34   ` Borislav Petkov
2020-07-04  0:13     ` Jarkko Sakkinen
2020-06-27 17:43   ` Borislav Petkov
2020-06-29 15:27     ` Sean Christopherson
2020-06-29 15:37       ` Borislav Petkov
2020-07-04  1:43       ` Jarkko Sakkinen
2020-07-07  1:38         ` Sean Christopherson
2020-07-07  3:29           ` Jarkko Sakkinen
2020-07-04  1:42     ` Jarkko Sakkinen
2020-07-02  3:59   ` Sean Christopherson
2020-07-04  3:31     ` Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 12/21] x86/sgx: Allow a limited use of ATTRIBUTE.PROVISIONKEY for attestation Jarkko Sakkinen
2020-06-29 16:02   ` Borislav Petkov
2020-06-29 22:04     ` Sean Christopherson
2020-06-30  8:49       ` Borislav Petkov
2020-06-30 14:20         ` Sean Christopherson
2020-06-30 17:13           ` Andy Lutomirski
2020-07-02 20:47         ` Dr. Greg
2020-07-03  2:43         ` Jarkko Sakkinen
2020-07-03  2:38       ` Jarkko Sakkinen
2020-07-03  2:32     ` Jarkko Sakkinen
2020-07-03  2:55       ` Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 13/21] x86/sgx: Add a page reclaimer Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 14/21] x86/sgx: ptrace() support for the SGX driver Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 15/21] x86/vdso: Add support for exception fixup in vDSO functions Jarkko Sakkinen
2020-06-29 17:10   ` Borislav Petkov
2020-06-30  6:00     ` Sean Christopherson
2020-06-30  8:41       ` Borislav Petkov
2020-06-30 14:55         ` Sean Christopherson
2020-06-30 16:48         ` Andy Lutomirski
2020-06-30 17:23           ` Sean Christopherson
2020-07-02 12:52           ` Thomas Gleixner
2020-06-17 22:08 ` [PATCH v33 16/21] x86/fault: Add helper function to sanitize error code Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 17/21] x86/traps: Attempt to fixup exceptions in vDSO before signaling Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 18/21] x86/vdso: Implement a vDSO for Intel SGX enclave call Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 19/21] selftests/x86: Add a selftest for SGX Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 20/21] docs: x86/sgx: Document SGX micro architecture and kernel internals Jarkko Sakkinen
2020-06-17 22:08 ` [PATCH v33 21/21] x86/sgx: Update MAINTAINERS Jarkko Sakkinen

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200625153431.GA3437@linux.intel.com \
    --to=sean.j.christopherson@intel.com \
    --cc=akpm@linux-foundation.org \
    --cc=andriy.shevchenko@linux.intel.com \
    --cc=asapek@google.com \
    --cc=bp@alien8.de \
    --cc=cedric.xing@intel.com \
    --cc=chenalexchen@google.com \
    --cc=conradparker@google.com \
    --cc=cyhanish@google.com \
    --cc=dave.hansen@intel.com \
    --cc=haitao.huang@intel.com \
    --cc=jarkko.sakkinen@linux.intel.com \
    --cc=jethro@fortanix.com \
    --cc=josh@joshtriplett.org \
    --cc=kai.huang@intel.com \
    --cc=kai.svahn@intel.com \
    --cc=kmoy@google.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-sgx@vger.kernel.org \
    --cc=ludloff@google.com \
    --cc=luto@kernel.org \
    --cc=nhorman@redhat.com \
    --cc=npmccallum@redhat.com \
    --cc=puiterwijk@redhat.com \
    --cc=rientjes@google.com \
    --cc=tglx@linutronix.de \
    --cc=x86@kernel.org \
    --cc=yaozhangx@google.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Sgx Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-sgx/0 linux-sgx/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-sgx linux-sgx/ https://lore.kernel.org/linux-sgx \
		linux-sgx@vger.kernel.org
	public-inbox-index linux-sgx

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-sgx


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git