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 10:15:49 -0700 [thread overview]
Message-ID: <20190603171549.GE13384@linux.intel.com> (raw)
In-Reply-To: <960B34DE67B9E140824F1DCDEC400C0F654EC5FD@ORSMSX116.amr.corp.intel.com>
On Sun, Jun 02, 2019 at 12:29:35AM -0700, Xing, Cedric wrote:
> Hi Sean,
>
> > From: Christopherson, Sean J
> > Sent: Friday, May 31, 2019 4:32 PM
> >
> > This series is the result of a rather absurd amount of discussion over how to get SGX to play
> > nice with LSM policies, without having to resort to evil shenanigans or put undue burden on
> > userspace. The discussion definitely wandered into completely insane territory at times, but
> > I think/hope we ended up with something reasonable.
> >
> > The basic gist of the approach is to require userspace to declare what protections are
> > maximally allowed for any given page, e.g. add a flags field for loading enclave pages that
> > takes ALLOW_{READ,WRITE,EXEC}. LSMs can then adjust the allowed protections, e.g. clear
> > ALLOW_EXEC to prevent ever mapping the page with PROT_EXEC. SGX enforces the allowed perms
> > via a new mprotect() vm_ops hook, e.g. like regular mprotect() uses MAY_{READ,WRITE,EXEC}.
> >
> > ALLOW_EXEC is used to deny hings like loading an enclave from a noexec file system or from a
> > file without EXECUTE permissions, e.g. without the ALLOW_EXEC concept, on SGX2 hardware
> > (regardless of kernel support) userspace could EADD from a noexec file using read-only
> > permissions, and later use mprotect() and ENCLU[EMODPE] to gain execute permissions.
> >
> > ALLOW_WRITE is used in conjuction with ALLOW_EXEC to enforce SELinux's EXECMOD (or EXECMEM).
> >
> > This is very much an RFC series. It's only compile tested, likely has obvious bugs, the
> > SELinux patch could be completely harebrained, etc...
> > My goal at this point is to get feedback at a macro level, e.g. is the core concept
> > viable/acceptable, are there objection to hooking mprotect(), etc...
> >
> > Andy and Cedric, hopefully this aligns with your general expectations based on our last
> > discussion.
>
> I couldn't understand the real intentions of ALLOW_* flags until I saw them
> in code. I have to say C is more expressive than English in that regard :)
>
> 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).
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.
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...
> Open: Given a file of type struct file *, how to tell if it is an enclave (i.e. /dev/sgx/enclave)?
> * In hook security_mmap_file(), if @file is an enclave, make sure @prot can
> only be PROT_NONE. This is to force all protection changes to go through
> security_file_mprotect().
> * In the newly introduced hook security_enclave_load(), set WRITTEN for pages
> that are requested PROT_WRITE.
How would an LSM associate a page with a specific enclave? vma->vm_file
will point always point at /dev/sgx/enclave. vma->vm_mm is useless
because we're allowing multiple processes to map a single enclave, not to
mention that by mm would require holding a reference to the mm.
> * In hook security_file_mprotect(), if @vma->vm_file is an enclave, look up
> and use WRITTEN flags for all pages within @vma, along with other global
> flags (e.g. PROCESS__EXECMEM/FILE__EXECMOD in the case of SELinux) to decide
> on allowing/rejecting @prot.
vma->vm_file will always be /dev/sgx/enclave at this point, which means
LSMs don't have the necessary anchor back to the source file, e.g. to
enforce FILE__EXECUTE. The noexec file system case is also unaddressed.
> * In hook security_file_free(), if @file is an enclave, free storage
> allocated for WRITTEN flags.
next prev parent reply other threads:[~2019-06-03 17:15 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 [this message]
2019-06-03 18:30 ` Xing, Cedric
2019-06-04 1:36 ` Sean Christopherson
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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