From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752641AbaG2Gut (ORCPT ); Tue, 29 Jul 2014 02:50:49 -0400 Received: from mail-pd0-f169.google.com ([209.85.192.169]:43301 "EHLO mail-pd0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752061AbaG2Gus (ORCPT ); Tue, 29 Jul 2014 02:50:48 -0400 Message-ID: <53D7440B.10006@linaro.org> Date: Tue, 29 Jul 2014 15:49:47 +0900 From: AKASHI Takahiro User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Will Deacon CC: Andy Lutomirski , Deepak Saxena , Will Drewry , Kees Cook , "linaro-kernel@lists.linaro.org" , "linux-arm-kernel@lists.infradead.org" , Catalin Marinas , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations References: <1406020499-5537-1-git-send-email-takahiro.akashi@linaro.org> <1406020499-5537-2-git-send-email-takahiro.akashi@linaro.org> <53D08358.4020902@amacapital.net> <53D0A037.2060308@linaro.org> <53D23341.4040403@linaro.org> <20140725110342.GD5269@arm.com> In-Reply-To: <20140725110342.GD5269@arm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/25/2014 08:03 PM, Will Deacon wrote: > On Fri, Jul 25, 2014 at 11:36:49AM +0100, AKASHI Takahiro wrote: >> On 07/25/2014 12:01 AM, Andy Lutomirski wrote: >>>>> If so, then you risk (at least) introducing >>>>> >>>>> a nice user-triggerable OOPS if audit is enabled. >>>> >>>> >>>> Can you please elaborate this? >>>> Since I didn't find any definition of audit's behavior when syscall is >>>> rewritten to -1, I thought it is reasonable to skip "exit tracing" of >>>> "skipped" syscall. >>>> (otherwise, "fake" seems to be more appropriate :) >>> >>> The audit entry hook will oops if you call it twice in a row without >>> calling the exit hook in between. >> >> Thank you, I could reproduce this problem which hits BUG(in_syscall) in >> audit_syscall_entry(). Really bad, and I fixed it in my next version and >> now a "skipped" system call is also traced by audit. > > Can you reproduce this on arch/arm/ too? If so, we should also fix the code > there. As far as I tried on arm with syscall auditing enabled, 1) Changing a syscall number to -1 under seccomp doesn't hit BUG_ON(in_syscall). 2) But, in fact, audit_syscall_entry() is NOT called in this case because __secure_computing() returns -1 and then it causes the succeeding tracing in syscall_trace_enter(), including audit_syscall_entry(), skipped. 3) On the other hand, calling syscall(-1) from userspace hits BUG_ON because the return path, ret_slow_syscall, doesn't contain syscall_trace_exit(). 4) When we re-write a syscall number to -1 without seccomp, we will also see BUG_ON hit, although I didn't try yet. Fixing case 3 is easy, but should we also fix case 2? Please note that, even if we call audit_syscall_exit() in case 2 or 3, no log against syscall -1 will be recorded because audit_filter_syscall() doesn't allow logging for any syscall number which is greater than 2048. This behavior was introduced by Andy's patch, a3c54931, in v3.16-rc. If the intention of "-1" is to fake a system call, this behavior seems to be a bit odd. Thanks, -Takahiro AKASHI > Will >