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=-8.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 D2CB6C2D0A3 for ; Tue, 3 Nov 2020 17:54:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8BF6122226 for ; Tue, 3 Nov 2020 17:54:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728970AbgKCRyA (ORCPT ); Tue, 3 Nov 2020 12:54:00 -0500 Received: from mail.kernel.org ([198.145.29.99]:59262 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727706AbgKCRyA (ORCPT ); Tue, 3 Nov 2020 12:54:00 -0500 Received: from gaia (unknown [2.26.170.190]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B43BB20773; Tue, 3 Nov 2020 17:53:55 +0000 (UTC) Date: Tue, 3 Nov 2020 17:53:53 +0000 From: Catalin Marinas To: Peter Collingbourne Cc: Evgenii Stepanov , Kostya Serebryany , Vincenzo Frascino , Dave Martin , Will Deacon , Oleg Nesterov , "Eric W. Biederman" , "James E.J. Bottomley" , Linux ARM , Kevin Brodsky , Andrey Konovalov , Richard Henderson , linux-api@vger.kernel.org, Helge Deller , David Spickett Subject: Re: [PATCH v13 7/8] signal: define the field siginfo.si_faultflags Message-ID: <20201103175352.GA22573@gaia> References: <743fef80a8617378027d5d2b0538cfc36ea106a1.1604376407.git.pcc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <743fef80a8617378027d5d2b0538cfc36ea106a1.1604376407.git.pcc@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org Hi Peter, On Mon, Nov 02, 2020 at 08:09:43PM -0800, Peter Collingbourne wrote: > This field will contain flags that may be used by signal handlers to > determine whether other fields in the _sigfault portion of siginfo are > valid. An example use case is the following patch, which introduces > the si_addr_tag_bits{,_mask} fields. > > A new sigcontext flag, SA_FAULTFLAGS, is introduced in order to allow > a signal handler to require the kernel to set the field (but note > that the field will be set anyway if the kernel supports the flag, > regardless of its value). In combination with the previous patches, > this allows a userspace program to determine whether the kernel will > set the field. As per patch 5, a user is supposed to call sigaction() twice to figure out whether _faultflags is meaningful. That's the part I'm not particularly fond of. Are the unused parts of siginfo always zeroed when the kernel delivers a signal? If yes, we could simply check the new field for non-zero bits. > It is possible for an si_faultflags-unaware program to cause a signal > handler in an si_faultflags-aware program to be called with a provided > siginfo data structure by using one of the following syscalls: > > - ptrace(PTRACE_SETSIGINFO) > - pidfd_send_signal > - rt_sigqueueinfo > - rt_tgsigqueueinfo > > So we need to prevent the si_faultflags-unaware program from causing an > uninitialized read of si_faultflags in the si_faultflags-aware program when > it uses one of these syscalls. > > The last three cases can be handled by observing that each of these > syscalls fails if si_code >= 0. We also observe that kill(2) and > tgkill(2) may be used to send a signal where si_code == 0 (SI_USER), > so we define si_faultflags to only be valid if si_code > 0. > > There is no such check on si_code in ptrace(PTRACE_SETSIGINFO), so > we make ptrace(PTRACE_SETSIGINFO) clear the si_faultflags field if it > detects that the signal would use the _sigfault layout, and introduce > a new ptrace request type, PTRACE_SETSIGINFO2, that a si_faultflags-aware > program may use to opt out of this behavior. I find this pretty fragile but maybe I have to read it a few more times to fully understand the implications ;). Could we instead copy all the fields, potentially uninitialised, and instead filter them when delivering the signal based on the SA_FAULTFLAGS? That means that the kernel only writes si_faultflags if the user requested it. > v12: > - Change type of si_xflags to u32 to avoid increasing alignment [...] > diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h > index 7aacf9389010..f43778355b77 100644 > --- a/include/uapi/asm-generic/siginfo.h > +++ b/include/uapi/asm-generic/siginfo.h > @@ -91,7 +91,9 @@ union __sifields { > char _dummy_pkey[__ADDR_BND_PKEY_PAD]; > __u32 _pkey; > } _addr_pkey; > + void *_pad[6]; > }; > + __u32 _faultflags; > } _sigfault; Sorry, I haven't checked the previous discussion on alignment here but don't we already require 64-bit alignment because of other members in the _sigfault union? We already have void * throughout this and with the next patch we just have a gap (unless I miscalculated the offsets). -- Catalin