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=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 B2FA2C3A5A5 for ; Thu, 5 Sep 2019 13:36:51 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 154CC2070C for ; Thu, 5 Sep 2019 13:36:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="rx6lqYPC" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 154CC2070C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=cyphar.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender:Content-Type:Cc: List-Subscribe:List-Help:List-Post:List-Archive:List-Unsubscribe:List-Id: In-Reply-To:MIME-Version:References:Message-ID:Subject:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=Gb0vm2+aDOnM6E7A3NXfa7cVVJp1WI9OV+4drI0gzis=; b=rx6lqYPCcZe5Epl6qYMOAn8Rl T9NePsxZ5GGZ6/Fcp6Ob4qr9xfYnCVT7OEzUubRm7732KpUnjpr96y6JFKyDk6pUvAFKPmMI57LcP frbo4t1FBkgIK/ZfItJO6iHVbhIeEBR69UssgFtHeEMXQoYX2uLRXwv5fYOKxcM+3KshxwS2UcaQZ mx8sSOJbEHVVvRzz1iIZjcmzc0y2KdjJqb9xN+ttUP6aRNBEdy7lyLHc0pR+vxeC9IdIrYdHx7OEk aUfbEAFhP60gHR2Mf8EluxHN21tuylUP6ix+FEkacXpqcQTjgSFl+S/y3WXJ4K8r1FPJT6U9lt7Iv 22xjxWIXA==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1i5rwN-0004zI-UL; Thu, 05 Sep 2019 13:36:48 +0000 Received: from mx2a.mailbox.org ([2001:67c:2050:104:0:2:25:2] helo=mx2.mailbox.org) by bombadil.infradead.org with esmtps (Exim 4.92 #3 (Red Hat Linux)) id 1i5rw6-0004kw-PA for linux-arm-kernel@lists.infradead.org; Thu, 05 Sep 2019 13:36:33 +0000 Received: from smtp2.mailbox.org (smtp2.mailbox.org [80.241.60.241]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by mx2.mailbox.org (Postfix) with ESMTPS id 812EBA1762; Thu, 5 Sep 2019 15:36:21 +0200 (CEST) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp2.mailbox.org ([80.241.60.241]) by spamfilter04.heinlein-hosting.de (spamfilter04.heinlein-hosting.de [80.241.56.122]) (amavisd-new, port 10030) with ESMTP id QEpWHY7VDe2j; Thu, 5 Sep 2019 15:36:16 +0200 (CEST) Date: Thu, 5 Sep 2019 23:35:52 +1000 From: Aleksa Sarai To: Peter Zijlstra Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Message-ID: <20190905133552.xlckmxfzar4wh5ju@yavin.dot.cyphar.com> References: <20190904201933.10736-1-cyphar@cyphar.com> <20190904201933.10736-2-cyphar@cyphar.com> <20190905073205.GY2332@hirez.programming.kicks-ass.net> <20190905092622.tlb6nn3uisssdfbu@yavin.dot.cyphar.com> <20190905094305.GJ2349@hirez.programming.kicks-ass.net> MIME-Version: 1.0 In-Reply-To: <20190905094305.GJ2349@hirez.programming.kicks-ass.net> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190905_063631_118804_841E51BC X-CRM114-Status: GOOD ( 29.47 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, Alexander Shishkin , Rasmus Villemoes , Alexei Starovoitov , linux-kernel@vger.kernel.org, David Howells , linux-kselftest@vger.kernel.org, sparclinux@vger.kernel.org, Jiri Olsa , linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, Tycho Andersen , Aleksa Sarai , Shuah Khan , Ingo Molnar , linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org, linux-xtensa@linux-xtensa.org, Kees Cook , Arnd Bergmann , Jann Horn , linuxppc-dev@lists.ozlabs.org, linux-m68k@lists.linux-m68k.org, Al Viro , Andy Lutomirski , Shuah Khan , Namhyung Kim , David Drysdale , Christian Brauner , "J. Bruce Fields" , linux-parisc@vger.kernel.org, linux-api@vger.kernel.org, Chanho Min , Jeff Layton , Oleg Nesterov , Eric Biederman , linux-alpha@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Linus Torvalds , containers@lists.linux-foundation.org Content-Type: multipart/mixed; boundary="===============1784812432235317488==" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org --===============1784812432235317488== Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="p7vw24iv2smepbtp" Content-Disposition: inline --p7vw24iv2smepbtp Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-09-05, Peter Zijlstra wrote: > On Thu, Sep 05, 2019 at 07:26:22PM +1000, Aleksa Sarai wrote: > > On 2019-09-05, Peter Zijlstra wrote: > > > On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote: > > > > +/** > > > > + * copy_struct_to_user: copy a struct to user space > > > > + * @dst: Destination address, in user space. > > > > + * @usize: Size of @dst struct. > > > > + * @src: Source address, in kernel space. > > > > + * @ksize: Size of @src struct. > > > > + * > > > > + * Copies a struct from kernel space to user space, in a way that = guarantees > > > > + * backwards-compatibility for struct syscall arguments (as long a= s future > > > > + * struct extensions are made such that all new fields are *append= ed* to the > > > > + * old struct, and zeroed-out new fields have the same meaning as = the old > > > > + * struct). > > > > + * > > > > + * @ksize is just sizeof(*dst), and @usize should've been passed b= y user space. > > > > + * The recommended usage is something like the following: > > > > + * > > > > + * SYSCALL_DEFINE2(foobar, struct foo __user *, uarg, size_t, us= ize) > > > > + * { > > > > + * int err; > > > > + * struct foo karg =3D {}; > > > > + * > > > > + * // do something with karg > > > > + * > > > > + * err =3D copy_struct_to_user(uarg, usize, &karg, sizeof(kar= g)); > > > > + * if (err) > > > > + * return err; > > > > + * > > > > + * // ... > > > > + * } > > > > + * > > > > + * There are three cases to consider: > > > > + * * If @usize =3D=3D @ksize, then it's copied verbatim. > > > > + * * If @usize < @ksize, then kernel space is "returning" a newer= struct to an > > > > + * older user space. In order to avoid user space getting incom= plete > > > > + * information (new fields might be important), all trailing by= tes in @src > > > > + * (@ksize - @usize) must be zerored > > >=20 > > > s/zerored/zero/, right? > >=20 > > It should've been "zeroed". >=20 > That reads wrong to me; that way it reads like this function must take > that action and zero out the 'rest'; which is just wrong. >=20 > This function must verify those bytes are zero, not make them zero. Right, in my head I was thinking "must have been zeroed" which isn't what it says. I'll switch to "zero". > > > > , otherwise -EFBIG is retu= rned. > > >=20 > > > 'Funny' that, copy_struct_from_user() below seems to use E2BIG. > >=20 > > This is a copy of the semantics that sched_[sg]etattr(2) uses -- E2BIG = for > > a "too big" struct passed to the kernel, and EFBIG for a "too big" > > struct passed to user-space. I would personally have preferred EMSGSIZE > > instead of EFBIG, but felt using the existing error codes would be less > > confusing. >=20 > Sadly a recent commit: >=20 > 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and robustify s= ched_read_attr() ABI logic and code") >=20 > Made the situation even 'worse'. I hadn't seen this patch before, and I have a few questions taking a look at it: * An error code for a particular behaviour was changed (EFBIG -> E2BIG). Is this not a userspace breakage (I know Linus went ballistic about something similar a while ago[1]), or did I misunderstand what the issue was in [1]? * At the risk of bike-shedding -- of we are changing it, wouldn't -EMSGSIZE be more appropriate? To be fair, picking errno values has always been more of an art than a science, but to my ears "Argument list too long" doesn't make too much sense in the context of "returning" a struct back to userspace (and the cause of the error is that the argument passed by user space *isn't big enough*). If there was an E2SMALL that would also work. ;) * Do you want me to write a patch based on that, to switch it to copy_struct_to_user()? * That patch removes the "are there non-zero bytes in the tail that userspace won't know about" check (which I have included in mine). I understand that this caused issues specifically with sched_getattr(2) due to the default value not being zero -- how should we rectify that (given that we'd hopefully want to port everyone who uses that interface to copy_struct_{to,from}_user())? * Given that the [uk]attr->size construct is pretty important to the usability of the sched and perf interfaces, should we require (or encourage) it for all struct-extension syscall setups? > > > > + if (unlikely(!access_ok(src, usize))) > > > > + return -EFAULT; > > > > + > > > > + /* Deal with trailing bytes. */ > > > > + if (usize < ksize) > > > > + memset(dst + size, 0, rest); > > > > + else if (usize > ksize) { > > > > + const void __user *addr =3D src + size; > > > > + char buffer[BUFFER_SIZE] =3D {}; > > >=20 > > > Isn't that too big for on-stack? > >=20 > > Is a 64-byte buffer too big? I picked the number "at random" to be the > > size of a cache line, but I could shrink it down to 32 bytes if the size > > is an issue (I wanted to avoid needless allocations -- hence it being > > on-stack). >=20 > Ah, my ctags gave me a definition of BUFFER_SIZE that was 512. I suppose > 64 should be OK. Good to know, though I'll rename it to avoid confusion. [1]: https://lore.kernel.org/lkml/CA+55aFy98A+LJK4+GWMcbzaa1zsPBRo76q+ioEjb= x-uaMKH6Uw@mail.gmail.com/ --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --p7vw24iv2smepbtp Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXXEPNQAKCRCdlLljIbnQ EtdDAQC347lG5qRdA84KUpGgbwgprjAxcKgxqQIULhRNFfpXbgD/ZCHtkcVeJovi WTsQxqcwA375UPIXJ/SgrKfqOJOI7Q4= =dSqz -----END PGP SIGNATURE----- --p7vw24iv2smepbtp-- --===============1784812432235317488== Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Disposition: inline _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel --===============1784812432235317488==--