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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 15 May 2019 16:13:15 -0700
Message-ID: <CALCETrX2ovRx3Rre+1_xC-q6CiybyLjQ-gmB4FZF_qCZ-Qd+4A@mail.gmail.com>
Subject: Re: SGX vs LSM (Re: [PATCH v20 00/28] Intel SGX1 support)
To:     James Morris <jmorris@namei.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        LSM List <linux-security-module@vger.kernel.org>,
        Paul Moore <paul@paul-moore.com>,
        Stephen Smalley <sds@tycho.nsa.gov>,
        Eric Paris <eparis@parisplace.org>, selinux@vger.kernel.org,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Jethro Beekman <jethro@fortanix.com>,
        "Xing, Cedric" <cedric.xing@intel.com>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        "linux-sgx@vger.kernel.org" <linux-sgx@vger.kernel.org>,
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On Wed, May 15, 2019 at 3:46 PM James Morris <jmorris@namei.org> wrote:
>
> On Wed, 15 May 2019, Andy Lutomirski wrote:
>
> > > Why not just use an xattr, like security.sgx ?
> >
> > Wouldn't this make it so that only someone with CAP_MAC_ADMIN could
> > install an enclave?  I think that this decision should be left up the
> > administrator, and it should be easy to set up a loose policy where
> > anyone can load whatever enclave they want.  That's what would happen
> > in my proposal if there was no LSM loaded or of the LSM policy didn't
> > restrict what .sigstruct files were acceptable.
> >
>
> You could try user.sigstruct, which does not require any privs.
>

I don't think I understand your proposal.  What file would this
attribute be on?  What would consume it?

I'm imagining that there's some enclave in a file
crypto_thingy.enclave.  There's also a file crypto_thingy.sigstruct.
crypto_thingy.enclave has type lib_t or similar so that it's
executable.  crypto_thingy.sigstruct has type sgx_sigstruct_t.  The
enclave loader does, in effect:

void *source_data = mmap(crypto_thingy.enclave, PROT_READ | PROT_EXEC, ...);
int sigstruct_fd = open("crypto_thingy.sigstruct", O_RDONLY);
int enclave_fd = open("/dev/sgx/enclave", O_RDWR);

ioctl(enclave_fd, SGX_IOC_ADD_SOME_DATA, source_data + source_offset,
enclave_offset, len, ...);
ioctl(enclave_fd, SGX_IOC_ADD_SOME_DATA, source_data + source_offset2,
enclave_offset2, len, ...);
etc.

/* Here's where LSMs get to check that the sigstruct is acceptable.
The CPU will check that the sigstruct matches the enclave. */
ioctl(enclave_fd, SGX_INIT_THE_ENCLAVE, sigstruct_fd);

/* Actually map the thing */
mmap(enclave_fd RO section, PROT_READ, ...);
mmap(enclave_fd RW section, PROT_READ | PROT_WRITE, ...);
mmap(enclave_fd RX section, PROT_READ | PROT_EXEC, ...);

/* This should fail unless EXECMOD is available, I think */
mmap(enclave_fd RWX section, PROT_READ | PROT_WRITE | PROT_EXEC);

And the idea here is that, if the .enclave file isn't mapped
PROT_EXEC, then mmapping the RX section will also require EXECMEM or
EXECMOD.