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=-8.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,T_DKIMWL_WL_MED,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=unavailable 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 83FADC28CC0 for ; Wed, 29 May 2019 16:12:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5D91622AED for ; Wed, 29 May 2019 16:12:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="f2+LkvmJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727032AbfE2QMq (ORCPT ); Wed, 29 May 2019 12:12:46 -0400 Received: from mail-ot1-f66.google.com ([209.85.210.66]:38591 "EHLO mail-ot1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727096AbfE2QMq (ORCPT ); Wed, 29 May 2019 12:12:46 -0400 Received: by mail-ot1-f66.google.com with SMTP id s19so2594526otq.5 for ; Wed, 29 May 2019 09:12:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=1dA5y3r0aKOO17C44t4tmXUXlp1J2scPYhjFw2NUwAY=; b=f2+LkvmJQcoBptCBm246fDUwbMG6hdJdrT0vD4h0TvX/VMamWpYyvvexR9WXdTf6GN a6ndExHvujI200GIB17qHDR5BRqnNQAso0RemsqKAi9lJyGCtqCI2L/Xiq6y+2qoP8uS X2itQ+NkIkTjLv0mEem5m+uz4MCQeXIAN1gYQwIu9J2V9m6Q51c48uvrxu4HUmp43XeJ OtO0k1rezsyZvoKR2+c8omaqFkJwe1VDzKCVoRlCv14hXE85n5c+4wxlaawpMFBHUfLv sZ2i3sT6G5ebk20iTs9m19hqG+D4/Npi87/mxK0+2ohPzpdxVR4pPWvJg/HBrqlBaufe 0eFQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=1dA5y3r0aKOO17C44t4tmXUXlp1J2scPYhjFw2NUwAY=; b=EWUo5aTEt8vGbtKi9YhL9wrPy7UAozil1KILwHF/bUNVGYqs6uTiQ60JsOmIwFwfF8 kGcfxgwQsehRRmQBVY3T3uO6l2p2IlcI7L6UaAveXOt1QGlKF2PqLrOU9ZmYo0o/whJJ zcJ+o5jVfBokYVRE92hLKcnFFGRaI4ach9rKHb7LA086gIxcZD6TXWC/WnHGGUpnI+RF +OdadRH4HKW5Pf2smnXO9wGSkK+UfMiPwnZ6ASlcrsNg0Pxw89i4h/aAnxV/o6lT4PkG /yDl8/HifBpPsOLAOj0r3h62pRTiDahQ2wJzZJKps3ug/yeOBgme6ddtmkCpZo9rrBXr FEaA== X-Gm-Message-State: APjAAAVELb7pksgxSDLFDluQD7WuDbaxmyZ3MSSQV/N0TqYKn/65Kg3t HbPXTLeqG2UvxPfx48mwXcPW18v2swIMuxWoFJc8Ew== X-Google-Smtp-Source: APXvYqwaOZla10Bof+mWLbbjMk94Ufa8TbjMKTZDaEYcqsOulha4oFcPoVi9fCgSIx16NNNOnT1R2rYebkMt9uJmErs= X-Received: by 2002:a9d:7347:: with SMTP id l7mr1571410otk.183.1559146365528; Wed, 29 May 2019 09:12:45 -0700 (PDT) MIME-Version: 1.0 References: <155905930702.7587.7100265859075976147.stgit@warthog.procyon.org.uk> <155905933492.7587.6968545866041839538.stgit@warthog.procyon.org.uk> <14347.1559127657@warthog.procyon.org.uk> <312a138c-e5b2-4bfb-b50b-40c82c55773f@schaufler-ca.com> In-Reply-To: <312a138c-e5b2-4bfb-b50b-40c82c55773f@schaufler-ca.com> From: Jann Horn Date: Wed, 29 May 2019 18:12:19 +0200 Message-ID: Subject: Re: [PATCH 3/7] vfs: Add a mount-notification facility To: Casey Schaufler Cc: David Howells , Al Viro , raven@themaw.net, linux-fsdevel , Linux API , linux-block@vger.kernel.org, keyrings@vger.kernel.org, linux-security-module , kernel list , Andy Lutomirski Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, May 29, 2019 at 5:53 PM Casey Schaufler wrote: > On 5/29/2019 4:00 AM, David Howells wrote: > > Jann Horn wrote: > > > >>> +void post_mount_notification(struct mount *changed, > >>> + struct mount_notification *notify) > >>> +{ > >>> + const struct cred *cred = current_cred(); > >> This current_cred() looks bogus to me. Can't mount topology changes > >> come from all sorts of places? For example, umount_mnt() from > >> umount_tree() from dissolve_on_fput() from __fput(), which could > >> happen pretty much anywhere depending on where the last reference gets > >> dropped? > > IIRC, that's what Casey argued is the right thing to do from a security PoV. > > Casey? > > You need to identify the credential of the subject that triggered > the event. If it isn't current_cred(), the cred needs to be passed > in to post_mount_notification(), or derived by some other means. > > > Maybe I should pass in NULL creds in the case that an event is being generated > > because an object is being destroyed due to the last usage[*] being removed. > > You should pass the cred of the process that removed the > last usage. If the last usage was removed by something like > the power being turned off on a disk drive a system cred > should be used. Someone or something caused the event. It can > be important who it was. The kernel's normal security model means that you should be able to e.g. accept FDs that random processes send you and perform read()/write() calls on them without acting as a subject in any security checks; let alone close(). If you send a file descriptor over a unix domain socket and the unix domain socket is garbage collected, for example, I think the close() will just come from some random, completely unrelated task that happens to trigger the garbage collector? Also, I think if someone does I/O via io_uring, I think the caller's credentials for read/write operations will probably just be normal kernel creds? Here the checks probably aren't all that important, but in other places, when people try to use an LSM as the primary line of defense, checks that don't align with the kernel's normal security model might lead to a bunch of problems.