From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sean Christopherson Subject: Re: RFC: userspace exception fixups Date: Fri, 2 Nov 2018 15:04:37 -0700 Message-ID: <20181102220437.GI7393@linux.intel.com> References: <20181102163034.GB7393@linux.intel.com> <7050972d-a874-dc08-3214-93e81181da60@intel.com> <20181102170627.GD7393@linux.intel.com> <20181102173350.GF7393@linux.intel.com> <20181102182712.GG7393@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Jann Horn Cc: Andy Lutomirski , Dave Hansen , Linus Torvalds , dalias@libc.org, Dave Hansen , jethro@fortanix.com, jarkko.sakkinen@linux.intel.com, Florian Weimer , Linux API , the arch/x86 maintainers , linux-arch , kernel list , Peter Zijlstra , nhorman@redhat.com, npmccallum@redhat.com, serge.ayoun@intel.com, shay.katz-zamir@intel.com, linux-sgx@vger.kernel.org, andriy.shevchenko@linux.intel.com, Thomas Gleixner , Ingo Molnar , Borislav Petkov , carlos@redhat.com, adhemerval.zane List-Id: linux-arch.vger.kernel.org On Fri, Nov 02, 2018 at 08:02:23PM +0100, Jann Horn wrote: > On Fri, Nov 2, 2018 at 7:27 PM Sean Christopherson > wrote: > > On Fri, Nov 02, 2018 at 10:48:38AM -0700, Andy Lutomirski wrote: > > > This whole mechanism seems very complicated, and it's not clear > > > exactly what behavior user code wants. > > > > No argument there. That's why I like the approach of dumping the > > exception to userspace without trying to do anything intelligent in > > the kernel. Userspace can then do whatever it wants AND we don't > > have to worry about mucking with stacks. > > > > One of the hiccups with the VDSO approach is that the enclave may > > want to use the untrusted stack, i.e. the stack that has the VDSO's > > stack frame. For example, Intel's SDK uses the untrusted stack to > > pass parameters for EEXIT, which means an AEX might occur with what > > is effectively a bad stack from the VDSO's perspective. > > What exactly does "uses the untrusted stack to pass parameters for > EEXIT" mean? I guess you're saying that the enclave is writing to > RSP+[0...some_positive_offset], and the written data needs to be > visible to the code outside the enclave afterwards? As is, they actually do it the other way around, i.e. negative offsets relative to the untrusted %RSP. Going into the enclave there is no reserved space on the stack. The SDK uses EEXIT like a function call, i.e. pushing parameters on the stack and making an call outside of the enclave, hence the name out-call. This allows the SDK to handle any reasonable out-call without a priori knowledge of the application's maximum out-call "size". Rough outline of what happens in a non-faulting case. 1: Userspace executes EENTER -------------------- | userspace stack | -------------------- <-- %RSP at EENTER 2: Enclave does EEXIT to invoke out-call function -------------------- | userspace stack | -------------------- <-- %RSP at EENTER | out-call func ID | | param1 | | ... | | paramN | -------------------- <-- %RSP at EEXIT 3: Userspace re-EENTERs enclave after handling EEXIT request -------------------- | userspace stack | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | | paramN | -------------------- <-- %RSP at post-EEXIT EENTER 4: Enclave cleans up the stack -------------------- | userspace stack | -------------------- <-- %RSP back at original EENTER In the faulting case, an AEX can occur while the enclave is pushing parameters onto the stack for EEXIT. 1: Userspace executes EENTER -------------------- | userspace stack | -------------------- <-- %RSP at EENTER 2: AEX occurs during enclave prep for EEXIT -------------------- | userspace stack | -------------------- <-- %RSP at EENTER | out-call func ID | | param1 | | ... | -------------------- <-- %RSP at AEX 3: Userspace re-EENTERs enclave to invoke enclave fault handler -------------------- | userspace stack | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | -------------------- <-- %RSP at AEX | userspace stack | -------------------- <-- %RSP at EENTER to fault handler 4: Enclave handles the fault, EEXITs back to userspace -------------------- | userspace stack | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | -------------------- <-- %RSP at AEX | userspace stack | -------------------- <-- %RSP at EEXIT from fault handler 5: Userspace pops its stack and ERESUMEs back to the enclave -------------------- | userspace stack | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | -------------------- <-- %RSP at ERESUME 6: Enclave finishes its EEXIT to invoke out-call function -------------------- | userspace stuff | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | | paramN | -------------------- <-- %RSP at EEXIT > In other words, the vDSO helper would have to not touch the stack > pointer (only using the 128-byte redzone to store spilled data, at > least across the enclave entry), and return by decrementing the stack > pointer by 8 immediately before returning (storing the return pointer > in the redzone)? > > So you'd call the vDSO helper with a normal "call > vdso_helper_address", then the vDSO helper does "add rsp, 8", then the > vDSO helper does its magic, and then it returns with "sub rsp, 8" and > "ret"? That way you don't touch anything on the high-address side of > RSP while still avoiding running into CET problems. (I'm assuming that > you can use CET in a process that is hosting SGX enclaves?) From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga14.intel.com ([192.55.52.115]:26744 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726141AbeKCHNb (ORCPT ); Sat, 3 Nov 2018 03:13:31 -0400 Date: Fri, 2 Nov 2018 15:04:37 -0700 From: Sean Christopherson Subject: Re: RFC: userspace exception fixups Message-ID: <20181102220437.GI7393@linux.intel.com> References: <20181102163034.GB7393@linux.intel.com> <7050972d-a874-dc08-3214-93e81181da60@intel.com> <20181102170627.GD7393@linux.intel.com> <20181102173350.GF7393@linux.intel.com> <20181102182712.GG7393@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Jann Horn Cc: Andy Lutomirski , Dave Hansen , Linus Torvalds , dalias@libc.org, Dave Hansen , jethro@fortanix.com, jarkko.sakkinen@linux.intel.com, Florian Weimer , Linux API , the arch/x86 maintainers , linux-arch , kernel list , Peter Zijlstra , nhorman@redhat.com, npmccallum@redhat.com, serge.ayoun@intel.com, shay.katz-zamir@intel.com, linux-sgx@vger.kernel.org, andriy.shevchenko@linux.intel.com, Thomas Gleixner , Ingo Molnar , Borislav Petkov , carlos@redhat.com, adhemerval.zanella@linaro.org Message-ID: <20181102220437.n0KiDVCev2g4YDR89VKUKuDvkQKAitI4f2-7FtHfiIY@z> On Fri, Nov 02, 2018 at 08:02:23PM +0100, Jann Horn wrote: > On Fri, Nov 2, 2018 at 7:27 PM Sean Christopherson > wrote: > > On Fri, Nov 02, 2018 at 10:48:38AM -0700, Andy Lutomirski wrote: > > > This whole mechanism seems very complicated, and it's not clear > > > exactly what behavior user code wants. > > > > No argument there. That's why I like the approach of dumping the > > exception to userspace without trying to do anything intelligent in > > the kernel. Userspace can then do whatever it wants AND we don't > > have to worry about mucking with stacks. > > > > One of the hiccups with the VDSO approach is that the enclave may > > want to use the untrusted stack, i.e. the stack that has the VDSO's > > stack frame. For example, Intel's SDK uses the untrusted stack to > > pass parameters for EEXIT, which means an AEX might occur with what > > is effectively a bad stack from the VDSO's perspective. > > What exactly does "uses the untrusted stack to pass parameters for > EEXIT" mean? I guess you're saying that the enclave is writing to > RSP+[0...some_positive_offset], and the written data needs to be > visible to the code outside the enclave afterwards? As is, they actually do it the other way around, i.e. negative offsets relative to the untrusted %RSP. Going into the enclave there is no reserved space on the stack. The SDK uses EEXIT like a function call, i.e. pushing parameters on the stack and making an call outside of the enclave, hence the name out-call. This allows the SDK to handle any reasonable out-call without a priori knowledge of the application's maximum out-call "size". Rough outline of what happens in a non-faulting case. 1: Userspace executes EENTER -------------------- | userspace stack | -------------------- <-- %RSP at EENTER 2: Enclave does EEXIT to invoke out-call function -------------------- | userspace stack | -------------------- <-- %RSP at EENTER | out-call func ID | | param1 | | ... | | paramN | -------------------- <-- %RSP at EEXIT 3: Userspace re-EENTERs enclave after handling EEXIT request -------------------- | userspace stack | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | | paramN | -------------------- <-- %RSP at post-EEXIT EENTER 4: Enclave cleans up the stack -------------------- | userspace stack | -------------------- <-- %RSP back at original EENTER In the faulting case, an AEX can occur while the enclave is pushing parameters onto the stack for EEXIT. 1: Userspace executes EENTER -------------------- | userspace stack | -------------------- <-- %RSP at EENTER 2: AEX occurs during enclave prep for EEXIT -------------------- | userspace stack | -------------------- <-- %RSP at EENTER | out-call func ID | | param1 | | ... | -------------------- <-- %RSP at AEX 3: Userspace re-EENTERs enclave to invoke enclave fault handler -------------------- | userspace stack | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | -------------------- <-- %RSP at AEX | userspace stack | -------------------- <-- %RSP at EENTER to fault handler 4: Enclave handles the fault, EEXITs back to userspace -------------------- | userspace stack | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | -------------------- <-- %RSP at AEX | userspace stack | -------------------- <-- %RSP at EEXIT from fault handler 5: Userspace pops its stack and ERESUMEs back to the enclave -------------------- | userspace stack | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | -------------------- <-- %RSP at ERESUME 6: Enclave finishes its EEXIT to invoke out-call function -------------------- | userspace stuff | -------------------- <-- %RSP at original EENTER | out-call func ID | | param1 | | ... | | paramN | -------------------- <-- %RSP at EEXIT > In other words, the vDSO helper would have to not touch the stack > pointer (only using the 128-byte redzone to store spilled data, at > least across the enclave entry), and return by decrementing the stack > pointer by 8 immediately before returning (storing the return pointer > in the redzone)? > > So you'd call the vDSO helper with a normal "call > vdso_helper_address", then the vDSO helper does "add rsp, 8", then the > vDSO helper does its magic, and then it returns with "sub rsp, 8" and > "ret"? That way you don't touch anything on the high-address side of > RSP while still avoiding running into CET problems. (I'm assuming that > you can use CET in a process that is hosting SGX enclaves?)