From: Sean Christopherson <sean.j.christopherson@intel.com>
To: "Xing, Cedric" <cedric.xing@intel.com>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
Andy Lutomirski <luto@kernel.org>,
Stephen Smalley <sds@tycho.nsa.gov>,
James Morris <jmorris@namei.org>,
"Serge E . Hallyn" <serge@hallyn.com>,
LSM List <linux-security-module@vger.kernel.org>,
Paul Moore <paul@paul-moore.com>,
Eric Paris <eparis@parisplace.org>,
"selinux@vger.kernel.org" <selinux@vger.kernel.org>,
Jethro Beekman <jethro@fortanix.com>,
"Hansen, Dave" <dave.hansen@intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
"linux-sgx@vger.kernel.org" <linux-sgx@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
"nhorman@redhat.com" <nhorman@redhat.com>,
"npmccallum@redhat.com" <npmccallum@redhat.com>,
"Ayoun, Serge" <serge.ayoun@intel.com>,
"Katz-zamir, Shay" <shay.katz-zamir@intel.com>,
"Huang, Haitao" <haitao.huang@intel.com>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
"Svahn, Kai" <kai.svahn@intel.com>,
Borislav Petkov <bp@alien8.de>,
Josh Triplett <josh@joshtriplett.org>,
"Huang, Kai" <kai.huang@intel.com>,
David Rientjes <rientjes@google.com>,
"Roberts, William C" <william.c.roberts@intel.com>,
"Tricca, Philip B" <philip.b.tricca@intel.com>
Subject: Re: [RFC PATCH 0/9] security: x86/sgx: SGX vs. LSM
Date: Mon, 3 Jun 2019 18:36:50 -0700 [thread overview]
Message-ID: <20190604013650.GC24521@linux.intel.com> (raw)
In-Reply-To: <960B34DE67B9E140824F1DCDEC400C0F654ED042@ORSMSX116.amr.corp.intel.com>
On Mon, Jun 03, 2019 at 11:30:54AM -0700, Xing, Cedric wrote:
> > From: Christopherson, Sean J
> > Sent: Monday, June 03, 2019 10:16 AM
> >
> > On Sun, Jun 02, 2019 at 12:29:35AM -0700, Xing, Cedric wrote:
> > > Hi Sean,
> > >
> > > Generally I agree with your direction but think ALLOW_* flags are
> > > completely internal to LSM because they can be both produced and
> > > consumed inside an LSM module. So spilling them into SGX driver and
> > > also user mode code makes the solution ugly and in some cases
> > > impractical because not every enclave host process has a priori
> > > knowledge on whether or not an enclave page would be EMODPE'd at
> > runtime.
> >
> > In this case, the host process should tag *all* pages it *might* convert
> > to executable as ALLOW_EXEC. LSMs can (and should/will) be written in
> > such a way that denying ALLOW_EXEC is fatal to the enclave if and only
> > if the enclave actually attempts mprotect(PROT_EXEC).
>
> What if those pages contain self-modifying code but the host doesn't know
> ahead of time? Would it require ALLOW_WRITE|ALLOW_EXEC at EADD? Then would it
> prevent those pages to start with PROT_EXEC?
Without ALLOW_WRITE+ALLOW_EXEC, the enclave would build and launch, but
fail at mprotect(..., PROT_WRITE), e.g. when it attempted to gain write
access to do self-modifying code. And it would would fail irrespective of
LSM restrictions.
> Anyway, my point is that it is unnecessary even if it works.
Unnecessary in an ideal world, yes. Realistically, it's the least bad
option.
> > Take the SELinux path for example. The only scenario in which
> > PROT_WRITE is cleared from @allowed_prot is if the page *starts* with
> > PROT_EXEC.
> > If PROT_EXEC is denied on a page that starts RW, e.g. an EAUG'd page,
> > then PROT_EXEC will be cleared from @allowed_prot.
> >
> > As Stephen pointed out, auditing the denials on @allowed_prot means the
> > log will contain false positives of a sort. But this is more of a noise
> > issue than true false positives. E.g. there are three possible outcomes
> > for the enclave.
> >
> > - The enclave does not do EMODPE[PROT_EXEC] in any scenario, ever.
> > Requesting ALLOW_EXEC is either a straightforward a userspace bug or
> > a poorly written generic enclave loader.
> >
> > - The enclave conditionally performs EMODPE[PROT_EXEC]. In this case
> > the denial is a true false positive.
> >
> > - The enclave does EMODPE[PROT_EXEC] and its host userspace then fails
> > on mprotect(PROT_EXEC), i.e. the LSM denial is working as intended.
> > The audit log will be noisy, but viewed as a whole the denials
> > aren't
> > false positives.
>
> What I was talking about was EMODPE[PROT_WRITE] on an RX page.
As above, mprotect(..., PROT_WRITE) would fail without ALLOW_WRITE.
> > The potential for noisy audit logs and/or false positives is unfortunate,
> > but it's (by far) the lesser of many evils.
> >
> > > Theoretically speaking, what you really need is a per page flag (let's
> > > name it WRITTEN?) indicating whether a page has ever been written to
> > > (or more precisely, granted PROT_WRITE), which will be used to decide
> > > whether to grant PROT_EXEC when requested in future. Given the fact
> > > that all mprotect() goes through LSM and mmap() is limited to
> > > PROT_NONE, it's easy for LSM to capture that flag by itself instead of
> > asking user mode code to provide it.
> > >
> > > That said, here is the summary of what I think is a better approach.
> > > * In hook security_file_alloc(), if @file is an enclave, allocate some
> > data
> > > structure to store for every page, the WRITTEN flag as described
> > above.
> > > WRITTEN is cleared initially for all pages.
> >
> > This would effectively require *every* LSM to duplicate the SGX driver's
> > functionality, e.g. track per-page metadata, implement locking to
> > prevent races between multiple mm structs, etc...
>
> Architecturally we shouldn't dictate how LSM makes decisions. ALLOW_* are no
> difference than PROCESS__* or FILE__* flags, which are just artifacts to
> assist particular LSMs in decision making. They are never considered part of
> the LSM interface, even if other LSMs than SELinux may adopt the same/similar
> approach.
No, the flags are tracked and managed by SGX. We are not dictating LSM
behavior in any way, e.g. an LSM could completely ignore @allowed_prot and
nothing would break.
> If code duplication is what you are worrying about, you can put them in a
> library, or implement/export them in some new file (maybe
> security/enclave.c?) as utility functions.
Code duplication is the least of my concerns. Tracking file pointers
would require a global list/tree of some form, along with a locking and/or
RCU scheme to protect accesses to that container. Another lock would be
needed to prevent races between mprotect() calls from different processes.
> But spilling them into user mode is what I think is unacceptable.
Why is it unacceptable? There's effectively no cost to userspace for SGX1.
The ALLOW_* flags only come into play in the event of a noexec or LSM
restriction, i.e. worst case scenario an enclave that wants to do arbitrary
self-modifying code can declare RWX on everything.
next prev parent reply other threads:[~2019-06-04 1:36 UTC|newest]
Thread overview: 77+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-31 23:31 [RFC PATCH 0/9] security: x86/sgx: SGX vs. LSM Sean Christopherson
2019-05-31 23:31 ` [RFC PATCH 1/9] x86/sgx: Remove unused local variable in sgx_encl_release() Sean Christopherson
2019-06-04 11:41 ` Jarkko Sakkinen
2019-05-31 23:31 ` [RFC PATCH 2/9] x86/sgx: Do not naturally align MAP_FIXED address Sean Christopherson
2019-06-04 11:49 ` Jarkko Sakkinen
2019-06-04 20:16 ` Andy Lutomirski
2019-06-04 22:10 ` Xing, Cedric
2019-06-05 14:08 ` Sean Christopherson
2019-06-05 15:17 ` Jarkko Sakkinen
2019-06-05 20:14 ` Andy Lutomirski
2019-06-06 15:37 ` Jarkko Sakkinen
2019-06-13 13:48 ` Jarkko Sakkinen
2019-06-13 16:47 ` Xing, Cedric
2019-06-13 17:14 ` Sean Christopherson
2019-06-14 15:18 ` Jarkko Sakkinen
2019-06-05 15:15 ` Jarkko Sakkinen
2019-05-31 23:31 ` [RFC PATCH 3/9] x86/sgx: Allow userspace to add multiple pages in single ioctl() Sean Christopherson
2019-06-03 6:26 ` Xing, Cedric
2019-06-03 20:08 ` Sean Christopherson
2019-06-03 20:39 ` Sean Christopherson
2019-06-03 23:45 ` Xing, Cedric
2019-06-04 0:54 ` Sean Christopherson
2019-06-04 20:18 ` Andy Lutomirski
2019-06-04 22:02 ` Xing, Cedric
2019-06-03 20:14 ` Dave Hansen
2019-06-03 20:37 ` Sean Christopherson
2019-06-03 20:39 ` Dave Hansen
2019-06-03 23:48 ` Xing, Cedric
2019-06-04 0:55 ` Sean Christopherson
2019-06-04 11:55 ` Jarkko Sakkinen
2019-05-31 23:31 ` [RFC PATCH 4/9] mm: Introduce vm_ops->mprotect() Sean Christopherson
2019-06-03 6:27 ` Xing, Cedric
2019-06-04 12:24 ` Jarkko Sakkinen
2019-06-04 14:51 ` Andy Lutomirski
2019-05-31 23:31 ` [RFC PATCH 5/9] x86/sgx: Restrict mapping without an enclave page to PROT_NONE Sean Christopherson
2019-06-03 6:28 ` Xing, Cedric
2019-06-04 15:32 ` Jarkko Sakkinen
2019-05-31 23:31 ` [RFC PATCH 6/9] x86/sgx: Require userspace to provide allowed prots to ADD_PAGES Sean Christopherson
2019-06-03 6:28 ` Xing, Cedric
2019-06-04 16:23 ` Jarkko Sakkinen
2019-06-04 16:45 ` Sean Christopherson
2019-06-05 15:06 ` Jarkko Sakkinen
2019-06-04 20:23 ` Andy Lutomirski
2019-06-05 11:10 ` Ayoun, Serge
2019-06-05 23:58 ` Sean Christopherson
2019-05-31 23:31 ` [RFC PATCH 7/9] x86/sgx: Enforce noexec filesystem restriction for enclaves Sean Christopherson
2019-06-03 6:29 ` Xing, Cedric
2019-06-04 20:26 ` Andy Lutomirski
2019-06-04 16:25 ` Jarkko Sakkinen
2019-06-04 20:25 ` Andy Lutomirski
2019-06-04 20:34 ` Sean Christopherson
2019-06-04 21:54 ` Xing, Cedric
2019-06-05 15:10 ` Jarkko Sakkinen
2019-06-06 1:01 ` Sean Christopherson
2019-05-31 23:31 ` [RFC PATCH 8/9] LSM: x86/sgx: Introduce ->enclave_load() hook for Intel SGX Sean Christopherson
2019-06-03 6:28 ` Xing, Cedric
2019-06-03 14:19 ` Stephen Smalley
2019-06-03 14:42 ` Sean Christopherson
2019-06-03 18:38 ` Stephen Smalley
2019-06-03 18:45 ` Dave Hansen
2019-06-04 20:29 ` Andy Lutomirski
2019-06-04 20:36 ` Sean Christopherson
2019-06-04 21:43 ` Xing, Cedric
2019-06-06 2:04 ` Sean Christopherson
2019-05-31 23:31 ` [RFC PATCH 9/9] security/selinux: Add enclave_load() implementation Sean Christopherson
2019-06-03 15:01 ` Stephen Smalley
2019-06-03 15:50 ` Sean Christopherson
2019-06-02 7:29 ` [RFC PATCH 0/9] security: x86/sgx: SGX vs. LSM Xing, Cedric
2019-06-03 17:15 ` Sean Christopherson
2019-06-03 18:30 ` Xing, Cedric
2019-06-04 1:36 ` Sean Christopherson [this message]
2019-06-04 15:33 ` Stephen Smalley
2019-06-04 16:30 ` Sean Christopherson
2019-06-04 21:38 ` Xing, Cedric
2019-06-03 17:47 ` Stephen Smalley
2019-06-03 18:02 ` Xing, Cedric
2019-06-04 11:15 ` Jarkko Sakkinen
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