From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752744AbdJMRx7 (ORCPT ); Fri, 13 Oct 2017 13:53:59 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39448 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751543AbdJMRx5 (ORCPT ); Fri, 13 Oct 2017 13:53:57 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 7D3057E423 Authentication-Results: ext-mx03.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx03.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=fweimer@redhat.com Subject: Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call To: Andy Lutomirski , Mathieu Desnoyers Cc: "Paul E. McKenney" , Boqun Feng , Peter Zijlstra , Paul Turner , Andrew Hunter , Dave Watson , Josh Triplett , Will Deacon , linux-kernel , Thomas Gleixner , Andi Kleen , Chris Lameter , Ingo Molnar , "H. Peter Anvin" , Ben Maurer , rostedt , Linus Torvalds , Andrew Morton , Russell King , Catalin Marinas , Michael Kerrisk , Alexander Viro , linux-api References: <20171012230326.19984-1-mathieu.desnoyers@efficios.com> <20171012230326.19984-2-mathieu.desnoyers@efficios.com> <19edaac0-98d7-e7a0-aceb-b861a2befce4@redhat.com> <695804241.40580.1507902016119.JavaMail.zimbra@efficios.com> <0043559c-c4e0-523a-b634-eded6ced886c@redhat.com> <66195899.40613.1507904878681.JavaMail.zimbra@efficios.com> From: Florian Weimer Message-ID: <3358e696-43e9-15d3-9634-68e9da79e121@redhat.com> Date: Fri, 13 Oct 2017 19:53:50 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Fri, 13 Oct 2017 17:53:57 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/13/2017 07:24 PM, Andy Lutomirski wrote: > On Fri, Oct 13, 2017 at 7:27 AM, Mathieu Desnoyers > wrote: >> ----- On Oct 13, 2017, at 9:56 AM, Florian Weimer fweimer@redhat.com wrote: >> >>> On 10/13/2017 03:40 PM, Mathieu Desnoyers wrote: >>>> The proposed ABI does not require to store any function pointer. For a given >>>> rseq_finish() critical section, pointers to specific instructions (within a >>>> function) are emitted at link-time into a struct rseq_cs: >>>> >>>> struct rseq_cs { >>>> RSEQ_FIELD_u32_u64(start_ip); >>>> RSEQ_FIELD_u32_u64(post_commit_ip); >>>> RSEQ_FIELD_u32_u64(abort_ip); >>>> uint32_t flags; >>>> } __attribute__((aligned(4 * sizeof(uint64_t)))); >>>> >>>> Then, at runtime, the fast-path stores the address of that struct rseq_cs >>>> into the TLS struct rseq "rseq_cs" field. >>>> >>>> So all we store at runtime is a pointer to data, not a pointer to functions. >>>> >>>> But you seem to hint that having a pointer to data containing pointers to code >>>> may still be making it easier for exploit writers. Can you elaborate on the >>>> scenario ? >>> >>> I'm concerned that the exploit writer writes a totally made up struct >>> rseq_cs object into writable memory, along with function pointers, and >>> puts the address of that in to the rseq_cs field. >>> >>> This would be comparable to how C++ vtable pointers are targeted >>> (including those in the glibc libio implementation of stdio streams). >>> >>> Does this answer your questions? >> >> Yes, it does. How about we add a "canary" field to the TLS struct rseq, e.g.: >> >> struct rseq { >> union rseq_cpu_event u; >> RSEQ_FIELD_u32_u64(rseq_cs); -> pointer to struct rseq_cs >> uint32_t flags; >> uint32_t canary; -> 32 low bits of rseq_cs ^ canary_mask >> }; >> >> We could then add a "uint32_t canary_mask" argument to sys_rseq, e.g.: >> >> SYSCALL_DEFINE3(rseq, struct rseq __user *, rseq, uint32_t, canary_mask, int, flags); >> >> So a thread which does not care about hardening would simply register its >> struct rseq TLS with a canary mask of "0". Nothing changes on the fast-path. >> >> A thread belonging to a process that cares about hardening could use a random >> value as canary, and pass it as canary_mask argument to the syscall. The >> fast-path could then set the struct rseq "canary" value to >> (32-low-bits of rseq_cs) ^ canary_mask just surrounding the critical section, >> and set it back to 0 afterward. >> >> In the kernel, whenever the rseq_cs pointer would be loaded, its 32 low bits >> would be checked to match (canary ^ canary_mask). If it differs, then the >> kernel kills the process with SIGSEGV. >> >> Would that take care of your concern ? >> > > I would propose a slightly different solution: have the kernel verify > that it jumps to a code sequence that occurs just after some > highly-unlikely magic bytes in the text *and* that those bytes have > some signature that matches a signature in the struct rseq that's > passed in. And the signature is fixed at the time of the rseq syscall? Yes, that would be far more reliable. Thanks, Florian