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=-8.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_PASS,USER_AGENT_MUTT 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 ABDB1C0044C for ; Wed, 31 Oct 2018 15:05:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4FB0C2081B for ; Wed, 31 Oct 2018 15:05:54 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="DKBCs6rz" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4FB0C2081B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=joelfernandes.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729061AbeKAAEP (ORCPT ); Wed, 31 Oct 2018 20:04:15 -0400 Received: from mail-pg1-f194.google.com ([209.85.215.194]:34472 "EHLO mail-pg1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728649AbeKAAEP (ORCPT ); Wed, 31 Oct 2018 20:04:15 -0400 Received: by mail-pg1-f194.google.com with SMTP id k1-v6so6096131pgq.1 for ; Wed, 31 Oct 2018 08:05:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=j3c11nxoD/TAnBnTTa0O8G4EJ3FYk6elSZWYqMnVshI=; b=DKBCs6rzXGAQzayzLHAcOmDMZb/7rIkVqKEmmIf0m3q2wqY42xnOjMxl59r+Zh0LEl rSzel+SUrzUVGzMLIUCLEn4qWRyVRIxFBVjgxtKdtr0LyKPbh8O7AiVNAEy1NpZK7B9S EIjbtQP2Pms5Q8lOKzAJ65PpLP5HCzIGdErSE= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=j3c11nxoD/TAnBnTTa0O8G4EJ3FYk6elSZWYqMnVshI=; b=gncj5XPA15lLa+igwE5rrbjjUcU4lnLOqlY03+PpLMkDaIO5SNpKaRFaxCZNyKt2rm dljZI+mYNuYkWJKdbz6LPCQ0bzsZH685/J6xeAJjIpQwPGFldx0no6wHVGjETDfEqf/y hdQTErG0k8j6D1BoOX2TeLnqaRH7iE7H4rTC8HZyzGg7YmDKuKthR08IYFndLCw+NVsF UlK2wPgRmrJ25PXEehNMb+GcYqislmTRHUpNXP5wcIktSQQ3PsstKJlS9znBOLZwVJC0 rM9UkDmHsTe9YippHgDuospe0vOf3ZkKpZwN5iRLlSorIaVPdbUdbrheGnV5eKKaOO4i YE2Q== X-Gm-Message-State: AGRZ1gJ7m3/i7ZqQ+s8HUKx7+EbMYMLPyXVkQzMUpLFE0lNcOmetEnXY MKzZ9fMWgRmbQ7y9e1i2/UoNsQ== X-Google-Smtp-Source: AJdET5fMLfYg6GqUrigVClTBrurikChaAcQ2P8ghh27QQzPvzZbiSFU8WQ6wY9JOxYzI3mHtVQtNWw== X-Received: by 2002:a62:2683:: with SMTP id m125-v6mr3784207pfm.74.1540998350952; Wed, 31 Oct 2018 08:05:50 -0700 (PDT) Received: from localhost ([2620:0:1000:1601:3aef:314f:b9ea:889f]) by smtp.gmail.com with ESMTPSA id 11-v6sm26720182pfr.55.2018.10.31.08.05.49 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 31 Oct 2018 08:05:49 -0700 (PDT) Date: Wed, 31 Oct 2018 08:05:48 -0700 From: Joel Fernandes To: Daniel Colascione Cc: linux-kernel@vger.kernel.org, timmurray@google.com, joelaf@google.com, surenb@google.com, cyphar@cyphar.com, christian.brauner@canonical.com, ebiederm@xmission.com, keescook@chromium.org, oleg@redhat.com Subject: Re: [PATCH v2] Implement /proc/pid/kill Message-ID: <20181031150548.GA103404@joelaf.mtv.corp.google.com> References: <20181029221037.87724-1-dancol@google.com> <20181031143744.77677-1-dancol@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181031143744.77677-1-dancol@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 31, 2018 at 02:37:44PM +0000, Daniel Colascione wrote: > Add a simple proc-based kill interface. To use /proc/pid/kill, just > write the signal number in base-10 ASCII to the kill file of the > process to be killed: for example, 'echo 9 > /proc/$$/kill'. > > Semantically, /proc/pid/kill works like kill(2), except that the > process ID comes from the proc filesystem context instead of from an > explicit system call parameter. This way, it's possible to avoid races > between inspecting some aspect of a process and that process's PID > being reused for some other process. > > Note that only the real user ID that opened a /proc/pid/kill file can > write to it; other users get EPERM. This check prevents confused > deputy attacks via, e.g., standard output of setuid programs. > > With /proc/pid/kill, it's possible to write a proper race-free and > safe pkill(1). An approximation follows. A real program might use > openat(2), having opened a process's /proc/pid directory explicitly, > with the directory file descriptor serving as a sort of "process > handle". > > #!/bin/bash > set -euo pipefail > pat=$1 > for proc_status in /proc/*/status; do ( > cd $(dirname $proc_status) > readarray proc_argv -d'' < cmdline > if ((${#proc_argv[@]} > 0)) && > [[ ${proc_argv[0]} = *$pat* ]]; > then > echo 15 > kill > fi > ) || true; done > > Signed-off-by: Daniel Colascione > --- > > Added a real-user-ID check to prevent confused deputy attacks. > > fs/proc/base.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 51 insertions(+) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index 7e9f07bf260d..74e494f24b28 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -205,6 +205,56 @@ static int proc_root_link(struct dentry *dentry, struct path *path) > return result; > } > > +static ssize_t proc_pid_kill_write(struct file *file, > + const char __user *buf, > + size_t count, loff_t *ppos) > +{ > + ssize_t res; > + int sig; > + char buffer[4]; > + > + /* This check prevents a confused deputy attack in which an > + * unprivileged process opens /proc/victim/kill and convinces > + * a privileged process to write to that kill FD, effectively > + * performing a kill with the privileges of the unwitting > + * privileged process. Here, we just fail the kill operation > + * if someone calls write(2) with a real user ID that differs > + * from the one used to open the kill FD. > + */ > + res = -EPERM; > + if (file->f_cred->user != current_user()) > + goto out; nit: You could get rid of the out label and just do direct returns. Will save a few lines and is more readable. > + > + res = -EINVAL; > + if (*ppos != 0) > + goto out; > + > + res = -EINVAL; > + if (count > sizeof(buffer) - 1) > + goto out; > + > + res = -EFAULT; > + if (copy_from_user(buffer, buf, count)) > + goto out; > + > + buffer[count] = '\0'; I think you can just zero-initialize buffer with "= {};" and get rid of this line. > + res = kstrtoint(strstrip(buffer), 10, &sig); > + if (res) > + goto out; > + > + res = kill_pid(proc_pid(file_inode(file)), sig, 0); > + if (res) > + goto out; if (res) return res; Other than the security issues which I still think you're discussing, since we need this, I suggest to maintainers we take this in as an intermediate solution since we don't have anything close to it and this is a real issue, and the fix proposed is simple. So FWIW feel free to add my reviewed-by (with the above nits and security issues taken care off) on any future respins: Reviewed-by: Joel Fernandes (Google) thanks, - Joel