Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755207AbbBFVz1 (ORCPT ); Fri, 6 Feb 2015 16:55:27 -0500 Received: from mail-lb0-f169.google.com ([209.85.217.169]:60181 "EHLO mail-lb0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752664AbbBFVz0 (ORCPT ); Fri, 6 Feb 2015 16:55:26 -0500 MIME-Version: 1.0 In-Reply-To: References: <20150206162301.18031.32251.stgit@buzz> <20150206162303.18031.37408.stgit@buzz> From: Andy Lutomirski Date: Fri, 6 Feb 2015 13:55:04 -0800 Message-ID: Subject: Re: [PATCH 2/2] kernel/fork: handle put_user errors for CLONE_PARENT_SETTID To: Konstantin Khlebnikov Cc: Linus Torvalds , Konstantin Khlebnikov , Linux API , Andrew Morton , Linux Kernel Mailing List , Roman Gushchin , Nikita Vetoshkin , Oleg Nesterov , Pavel Emelyanov Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2152 Lines: 43 On Fri, Feb 6, 2015 at 1:13 PM, Konstantin Khlebnikov wrote: > On Fri, Feb 6, 2015 at 11:49 PM, Linus Torvalds > wrote: >> On Fri, Feb 6, 2015 at 8:23 AM, Konstantin Khlebnikov >> wrote: >>> Handling of flag CLONE_PARENT_SETTID has the same problem: error returned >>> from put_user() is ignored. Glibc completely relies on that feature and uses >>> value returned from syscall only for error checking. >> >> I'm not seeing the advantage of the error checking part of the pacth >> patch. It generates extra code, possibly changing existing interfaces, >> and it doesn't actually buy us anything. >> >> What's the upside? If somebody passes in a bad pointer, it's their >> problem. For all we know, people used to pass in NULL, even if they >> had the SETTID bit set. This makes it now return EFAULT. > > Currently that works fine only because kernel retries 0-order allocations > endlessly. But pagefault_out_of_memory() is never called for non-user PF. > For kernel PF all oom-kills are triggered by buddy-allocator. > If buddy allocator gave up earlier then page-faults from kernel space > could fail without OOM. And in CoW area user-space will see stale data. > So, either we must handle all put_user/copy_to_user errors (which isn't > that bad idea) or kernel must force all PF to success-or-die policy. > > First patch is that ugly because kernel has never checked errors > in that place. So, I've tried to find solution which could fix problem > without breaking backward compatibility. If you're really worried about compatibility, it would be possible, if really really ugly, to check whether there's a vma at all at the requested address and to return -EFAULT only in the case where there is a vma but put_user still failed. A less awful approach might be to accept put_user failures if the address is NULL. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/