From: Sean Christopherson <sean.j.christopherson@intel.com> To: Andy Lutomirski <luto@kernel.org> Cc: Stephen Smalley <sds@tycho.nsa.gov>, Cedric Xing <cedric.xing@intel.com>, LSM List <linux-security-module@vger.kernel.org>, selinux@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>, linux-sgx@vger.kernel.org, Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>, James Morris <jmorris@namei.org>, "Serge E. Hallyn" <serge@hallyn.com>, Paul Moore <paul@paul-moore.com>, Eric Paris <eparis@parisplace.org>, Jethro Beekman <jethro@fortanix.com>, Dave Hansen <dave.hansen@intel.com>, Thomas Gleixner <tglx@linutronix.de>, Linus Torvalds <torvalds@linux-foundation.org>, Andrew Morton <akpm@linux-foundation.org>, nhorman@redhat.com, pmccallum@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>, Philip Tricca <philip.b.tricca@intel.com> Subject: Re: [RFC PATCH v1 2/3] LSM/x86/sgx: Implement SGX specific hooks in SELinux Date: Wed, 12 Jun 2019 15:02:42 -0700 Message-ID: <20190612220242.GJ20308@linux.intel.com> (raw) In-Reply-To: <CALCETrWQT3AG+-OKBOzuw-a6VPApkNYsKqZiBmS56-b-72bfYQ@mail.gmail.com> On Wed, Jun 12, 2019 at 12:30:20PM -0700, Andy Lutomirski wrote: > On Tue, Jun 11, 2019 at 3:02 PM Sean Christopherson > <sean.j.christopherson@intel.com> wrote: > > > > 1. Require userspace to explicitly specificy (maximal) enclave page > > permissions at build time. The enclave page permissions are provided > > to, and checked by, LSMs at enclave build time. > > > > Pros: Low-complexity kernel implementation, straightforward auditing > > Cons: Sullies the SGX UAPI to some extent, may increase complexity of > > SGX2 enclave loaders. > > In my notes, this works like this. This is similar, but not > identical, to what Sean has been sending out. ... > mmap() and mprotect() enforce the following rules: > > - Deny if a PROT_ flag is requested but the corresponding ALLOW_ flag > is not set for all pages in question. > > - Deny if PROT_WRITE, PROT_EXEC, and DENY_WX are all set. > > - Deny if PROT_EXEC, ALLOW_WRITE, and DENY_X_IF_ALLOW_WRITE are all set. > > mprotect() and mmap() do *not* call SGX-specific LSM hooks to ask for > permission, although they can optionally call an LSM hook if they hit one of > the -EPERM cases for auditing purposes. IMO, #1 only makes sense if it's stripped down to avoid auditing and locking complications, i.e. gets a pass/fail at security_enclave_load() and clears VM_MAY* flags during mmap(). If we want WX and W->X to be differentiated by security_enclave_init() as opposed to security_enclave_load(), then we should just scrap #1. > I think this model works quite well in an SGX1 world. The main thing > that makes me uneasy about this model is that, in SGX2, it requires > that an SGX2-compatible enclave loader must pre-declare to the kernel > whether it intends for its dynamically allocated memory to be > ALLOW_EXEC. If ALLOW_EXEC is set but not actually needed, it will > still fail if DENY_X_IF_ALLOW_WRITE ends up being set. The other > version below does not have this limitation. I'm not convinced this will be a meaningful limitation in practice, though that's probably obvious from my RFCs :-). That being said, the UAPI quirk is essentially a dealbreaker for multiple people, so let's drop #1. I discussed the options with Cedric offline, and he is ok with option #2 *if* the idea actually translates to acceptable code and doesn't present problems for userspace and/or future SGX features. So, I'll work on an RFC series to implement #2 as described below. If it works out, yay! If not, i.e. option #2 is fundamentally broken, I'll shift my focus to Cedric's code (option #3). > > 2. Pre-check LSM permissions and dynamically track mappings to enclave > > pages, e.g. add an SGX mprotect() hook to restrict W->X and WX > > based on the pre-checked permissions. > > > > Pros: Does not impact SGX UAPI, medium kernel complexity > > Cons: Auditing is complex/weird, requires taking enclave-specific > > lock during mprotect() to query/update tracking. > > Here's how this looks in my mind. It's quite similar, except that > ALLOW_READ, ALLOW_WRITE, and ALLOW_EXEC are replaced with a little > state machine. > > EADD does not take any special flags. It calls this LSM hook: > > int security_enclave_load(struct vm_area_struct *source); > > This hook can return -EPERM. Otherwise it 0 or ALLOC_EXEC_IF_UNMODIFIED > (i.e. 1). This hook enforces permissions (a) and (b). > > The driver tracks a state for each page, and the possible states are: > > - CLEAN_MAYEXEC /* no W or X VMAs have existed, but X is okay */ > - CLEAN_NOEXEC /* no W or X VMAs have existed, and X is not okay */ > - CLEAN_EXEC /* no W VMA has existed, but an X VMA has existed */ > - DIRTY /* a W VMA has existed */ > > The initial state for a page is CLEAN_MAYEXEC if the hook said > ALLOW_EXEC_IF_UNMODIFIED and CLEAN_NOEXEC otherwise. > > The future EAUG does not call a hook at all and puts pages into the state > CLEAN_NOEXEC. If SGX3 or later ever adds EAUG-but-don't-clear, it can > call security_enclave_load() and add CLEAN_MAYEXEC pages if appropriate. > > EINIT takes a sigstruct pointer. SGX calls a new hook: > > unsigned int security_enclave_init(struct sigstruct *sigstruct, > struct vm_area_struct *source, unsigned int flags); > > This hook can return -EPERM. Otherwise it returns 0 or a combination of > flags DENY_WX and DENY_X_DIRTY. The driver saves this value. > These represent permissions (c) and (d). > > If we want to have a permission for "execute code supplied from outside the > enclave that was not measured", we could have a flag like > HAS_UNMEASURED_CLEAN_EXEC_PAGE that the LSM could consider. > > mmap() and mprotect() enforce the following rules: > > - If VM_EXEC is requested and (either the page is DIRTY or VM_WRITE is > requested) and DENY_X_DIRTY, then deny. > > - If VM_WRITE and VM_EXEC are both requested and DENY_WX, then deny. > > - If VM_WRITE is requested, we need to update the state. If it was > CLEAN_EXEC, then we reject if DENY_X_DIRTY. Otherwise we change the > state to DIRTY. > > - If VM_EXEC is requested and the page is CLEAN_NOEXEC, then deny. > > mprotect() and mmap() do *not* call SGX-specific LSM hooks to ask for > permission, although they can optionally call an LSM hook if they hit one of > the -EPERM cases for auditing purposes. > > Before the SIGSTRUCT is provided to the driver, the driver acts as though > DENY_X_DIRTY and DENY_WX are both set.
next prev parent reply index Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-06-06 2:11 [RFC PATCH v2 0/5] security: x86/sgx: SGX vs. LSM Sean Christopherson 2019-06-06 2:11 ` [RFC PATCH v2 1/5] mm: Introduce vm_ops->may_mprotect() Sean Christopherson 2019-06-10 15:06 ` Jarkko Sakkinen 2019-06-10 15:55 ` Sean Christopherson 2019-06-10 17:47 ` Xing, Cedric 2019-06-10 19:49 ` Sean Christopherson 2019-06-10 22:06 ` Xing, Cedric 2019-06-06 2:11 ` [RFC PATCH v2 2/5] x86/sgx: Require userspace to define enclave pages' protection bits Sean Christopherson 2019-06-10 15:27 ` Jarkko Sakkinen 2019-06-10 16:15 ` Sean Christopherson 2019-06-10 17:45 ` Jarkko Sakkinen 2019-06-10 18:17 ` Sean Christopherson 2019-06-12 19:26 ` Jarkko Sakkinen 2019-06-10 18:29 ` Xing, Cedric 2019-06-10 19:15 ` Andy Lutomirski 2019-06-10 22:28 ` Xing, Cedric 2019-06-12 0:09 ` Andy Lutomirski 2019-06-12 14:34 ` Sean Christopherson 2019-06-12 18:20 ` Xing, Cedric 2019-06-06 2:11 ` [RFC PATCH v2 3/5] x86/sgx: Enforce noexec filesystem restriction for enclaves Sean Christopherson 2019-06-10 16:00 ` Jarkko Sakkinen 2019-06-10 16:44 ` Andy Lutomirski 2019-06-11 17:21 ` Stephen Smalley 2019-06-06 2:11 ` [RFC PATCH v2 4/5] LSM: x86/sgx: Introduce ->enclave_load() hook for Intel SGX Sean Christopherson 2019-06-07 19:58 ` Stephen Smalley 2019-06-10 16:21 ` Sean Christopherson 2019-06-10 16:05 ` Jarkko Sakkinen 2019-06-06 2:11 ` [RFC PATCH v2 5/5] security/selinux: Add enclave_load() implementation Sean Christopherson 2019-06-07 21:16 ` Stephen Smalley 2019-06-10 16:46 ` Sean Christopherson 2019-06-17 16:38 ` Jarkko Sakkinen 2019-06-10 7:03 ` [RFC PATCH v1 0/3] security/x86/sgx: SGX specific LSM hooks Cedric Xing 2019-06-10 7:03 ` [RFC PATCH v1 1/3] LSM/x86/sgx: Add " Cedric Xing 2019-06-10 7:03 ` [RFC PATCH v1 2/3] LSM/x86/sgx: Implement SGX specific hooks in SELinux Cedric Xing 2019-06-11 13:40 ` Stephen Smalley 2019-06-11 22:02 ` Sean Christopherson 2019-06-12 9:32 ` Dr. Greg 2019-06-12 14:25 ` Sean Christopherson 2019-06-13 7:25 ` Dr. Greg 2019-06-12 19:30 ` Andy Lutomirski 2019-06-12 22:02 ` Sean Christopherson [this message] 2019-06-13 0:10 ` Xing, Cedric 2019-06-13 1:02 ` Xing, Cedric 2019-06-13 17:02 ` Stephen Smalley 2019-06-13 23:03 ` Xing, Cedric 2019-06-13 23:17 ` Sean Christopherson 2019-06-14 0:31 ` Xing, Cedric 2019-06-14 0:46 ` Sean Christopherson 2019-06-14 15:38 ` Sean Christopherson 2019-06-16 22:14 ` Andy Lutomirski 2019-06-17 16:49 ` Sean Christopherson 2019-06-17 17:08 ` Andy Lutomirski 2019-06-18 15:40 ` Dr. Greg 2019-06-14 17:16 ` Xing, Cedric 2019-06-14 17:45 ` Sean Christopherson 2019-06-14 17:53 ` Sean Christopherson 2019-06-14 20:01 ` Sean Christopherson 2019-06-16 22:16 ` Andy Lutomirski 2019-06-14 23:19 ` Dr. Greg 2019-06-11 22:55 ` Xing, Cedric 2019-06-13 18:00 ` Stephen Smalley 2019-06-13 19:48 ` Sean Christopherson 2019-06-13 21:09 ` Xing, Cedric 2019-06-13 21:02 ` Xing, Cedric 2019-06-14 0:37 ` Sean Christopherson 2019-06-10 7:03 ` [RFC PATCH v1 3/3] LSM/x86/sgx: Call new LSM hooks from SGX subsystem Cedric Xing 2019-06-10 17:36 ` [RFC PATCH v1 0/3] security/x86/sgx: SGX specific LSM hooks Jarkko Sakkinen
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