From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0538FC433ED for ; Wed, 21 Apr 2021 17:10:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 60E9861459 for ; Wed, 21 Apr 2021 17:10:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244643AbhDURLA (ORCPT ); Wed, 21 Apr 2021 13:11:00 -0400 Received: from foss.arm.com ([217.140.110.172]:38296 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237858AbhDURKv (ORCPT ); Wed, 21 Apr 2021 13:10:51 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0047E11FB; Wed, 21 Apr 2021 10:10:16 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.3.41]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7BC1C3F694; Wed, 21 Apr 2021 10:10:14 -0700 (PDT) Date: Wed, 21 Apr 2021 18:10:05 +0100 From: Mark Rutland To: Will Deacon Cc: Catalin Marinas , He Zhe , oleg@redhat.com, linux-arm-kernel@lists.infradead.org, paul@paul-moore.com, eparis@redhat.com, linux-audit@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat Message-ID: <20210421171005.GA46949@C02TD0UTHF1T.local> References: <20210416075533.7720-1-zhe.he@windriver.com> <20210416123322.GA23184@arm.com> <20210416133431.GA2303@C02TD0UTHF1T.local> <20210419121932.GA30004@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210419121932.GA30004@willie-the-truck> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 19, 2021 at 01:19:33PM +0100, Will Deacon wrote: > On Fri, Apr 16, 2021 at 02:34:41PM +0100, Mark Rutland wrote: > > I think this is a problem we created for ourselves back in commit: > > > > 15956689a0e60aa0 ("arm64: compat: Ensure upper 32 bits of x0 are zero on syscall return) > > > > AFAICT, the perf regs samples are the only place this matters, since for > > ptrace the compat regs are implicitly truncated to compat_ulong_t, and > > audit expects the non-truncated return value. Other architectures don't > > truncate here, so I think we're setting ourselves up for a game of > > whack-a-mole to truncate and extend wherever we need to. > > > > Given that, I suspect it'd be better to do something like the below. > > > > Will, thoughts? > > I think perf is one example, but this is also visible to userspace via the > native ptrace interface and I distinctly remember needing this for some > versions of arm64 strace to work correctly when tracing compat tasks. FWIW, you've convinced me on your approach (more on that below), but when I went digging here this didn't seem to be exposed via ptrace -- for any task tracing a compat task, the GPRs are exposed via compat_gpr_{get,set}(), which always truncate to compat_ulong_t, giving the lower 32 bits. See task_user_regset_view() for where we get the regset. Am I missing something, or are you thinking of another issue you fixed at the same time? > So I do think that clearing the upper bits on the return path is the right > approach, but it sounds like we need some more work to handle syscall(-1) > and audit (what exactly is the problem here after these patches have been > applied?) >From digging a bit more, I think I agree, and I think these patches are sufficient for audit. I have some comments I'll leave separately. The remaining issues are wherever we assign a signed value to a compat GPR without explicit truncation. That'll leak via perf sampling the user regs, but I haven't managed to convince myself whether that causes any functional change in behaviour for audit, seccomp, or syscall tracing. Since we mostly use compat_ulong_t for intermediate values in compat code, it does look like this is only an issue for x0 where we assign an error value, e.g. the -ENOSYS case in el0_svc_common. I'll go see if I can find any more. With those fixed up we can remove the x0 truncation from entry.S, which'd be nice too. Thanks, Mark.