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=-11.4 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL 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 31659C00A89 for ; Fri, 30 Oct 2020 19:42:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C17CD20702 for ; Fri, 30 Oct 2020 19:42:36 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="GEScQJl/" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727429AbgJ3TPG (ORCPT ); Fri, 30 Oct 2020 15:15:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51416 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727423AbgJ3TPG (ORCPT ); Fri, 30 Oct 2020 15:15:06 -0400 Received: from mail-lf1-x134.google.com (mail-lf1-x134.google.com [IPv6:2a00:1450:4864:20::134]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CE619C0613D5 for ; Fri, 30 Oct 2020 12:15:05 -0700 (PDT) Received: by mail-lf1-x134.google.com with SMTP id 126so9236400lfi.8 for ; Fri, 30 Oct 2020 12:15:05 -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=wG/u6QLDIeO3+GPNzepwWscTWygaEgqcNOvOguEOtzE=; b=GEScQJl/XRE+Al+2evygNVW34IiBSkoia8s5HzAl8Hv6DEbsHP21Lirga/6tIsjkoR 6pndjXo3bhKZOW1Dk5Uub2F/QyY+XpBeX+nLyHZxDUaPhBwraxjKmyUU389u1/xqXlL/ voC7OY6hmd5Fr4gE/vpOFEq+3M/K4uBWKjshMkpddljKtLvb7oEgjUepvGxdHLB+qlRi lsXaJFer4KS7uPl33OEqYL9fA3HJYLgu76FbsgqoBkyILSU1vMZhuNLi6w9qOqfkXaAU ytPJv1sI1qSIE1x7QKh8O++0G+mtEs+XlsK0dEAC2cz1JpC6LyJR8sRuqTs2bq0q6mTh QDIw== 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=wG/u6QLDIeO3+GPNzepwWscTWygaEgqcNOvOguEOtzE=; b=a2JQCYwPiM4jEiiPweBuFWyi5cfTMw+yR9yzAq+QSuUVAs76ZOOaARozLOrcIHvyBq 9g9Ut8ghBQS6hr5IMaLBAH9suvx+/oVCKrJEy45EgLa8aqgFB17+PCH+5I+KvqCBkQYX fRWYRScTEiOtp8Slsh1bh1PeGGuuoN5C919z6Cfjffk5D9J2EXBqbdUMC2EJM58+VA04 RqI/NN+0X7Ht/6Dc46MqDAHsl9AfX+uh9RjEgQab9XZ24FA0PPajjvxuVTh3XwfgNMDw X2DWnC4v0o8BySW5o//MXWLBF564Zplnx9t7sOy1dE2V3eLuLiV6ueOzJTzQpj9p/HUy UVWQ== X-Gm-Message-State: AOAM531OKJOgaEalpvz67DRrtWREL+zgwQCy4pGxmN6Ga32zKY9jHg8T 3ddSG/gf/ni4f/w2f+SFO+OeOS2qjG23hoV9mc5LOg== X-Google-Smtp-Source: ABdhPJwWwVpiw9gJ/puN8rWssNkdz8vVGHPzS0mmXDNNvTGXQ5m+dUeuCNcHNeJlQwiyM5w1WvddtFCfbNI2DKeMRtk= X-Received: by 2002:a05:6512:1054:: with SMTP id c20mr1626841lfb.576.1604085303713; Fri, 30 Oct 2020 12:15:03 -0700 (PDT) MIME-Version: 1.0 References: <63598b4f-6ce3-5a11-4552-cdfe308f68e4@gmail.com> <0de41eb1-e1fd-85da-61b7-fac4e3006726@gmail.com> In-Reply-To: <0de41eb1-e1fd-85da-61b7-fac4e3006726@gmail.com> From: Jann Horn Date: Fri, 30 Oct 2020 20:14:37 +0100 Message-ID: Subject: Re: For review: seccomp_user_notif(2) manual page [v2] To: "Michael Kerrisk (man-pages)" Cc: Kees Cook , Tycho Andersen , Sargun Dhillon , Christian Brauner , Daniel Borkmann , Giuseppe Scrivano , Song Liu , Robert Sesek , Containers , linux-man , lkml , Aleksa Sarai , Alexei Starovoitov , Will Drewry , bpf , Andy Lutomirski Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Thu, Oct 29, 2020 at 3:19 PM Michael Kerrisk (man-pages) wrote: > On 10/29/20 2:42 AM, Jann Horn wrote: > > On Mon, Oct 26, 2020 at 10:55 AM Michael Kerrisk (man-pages) > > wrote: > >> static bool > >> getTargetPathname(struct seccomp_notif *req, int notifyFd, > >> char *path, size_t len) > >> { > >> char procMemPath[PATH_MAX]; > >> > >> snprintf(procMemPath, sizeof(procMemPath), "/proc/%d/mem", req->pid); > >> > >> int procMemFd = open(procMemPath, O_RDONLY); > >> if (procMemFd == -1) > >> errExit("\tS: open"); > >> > >> /* Check that the process whose info we are accessing is still alive. > >> If the SECCOMP_IOCTL_NOTIF_ID_VALID operation (performed > >> in checkNotificationIdIsValid()) succeeds, we know that the > >> /proc/PID/mem file descriptor that we opened corresponds to the > >> process for which we received a notification. If that process > >> subsequently terminates, then read() on that file descriptor > >> will return 0 (EOF). */ > >> > >> checkNotificationIdIsValid(notifyFd, req->id); > >> > >> /* Read bytes at the location containing the pathname argument > >> (i.e., the first argument) of the mkdir(2) call */ > >> > >> ssize_t nread = pread(procMemFd, path, len, req->data.args[0]); > >> if (nread == -1) > >> errExit("pread"); > > > > As discussed at > > , > > we need to re-check checkNotificationIdIsValid() after reading remote > > memory but before using the read value in any way. Otherwise, the > > syscall could in the meantime get interrupted by a signal handler, the > > signal handler could return, and then the function that performed the > > syscall could free() allocations or return (thereby freeing buffers on > > the stack). > > > > In essence, this pread() is (unavoidably) a potential use-after-free > > read; and to make that not have any security impact, we need to check > > whether UAF read occurred before using the read value. This should > > probably be called out elsewhere in the manpage, too... > > Thanks very much for pointing me at this! > > So, I want to conform that the fix to the code is as simple as > adding a check following the pread() call, something like: > > [[ > ssize_t nread = pread(procMemFd, path, len, req->data.args[argNum]); > if (nread == -1) > errExit("Supervisor: pread"); > > if (nread == 0) { > fprintf(stderr, "\tS: pread() of /proc/PID/mem " > "returned 0 (EOF)\n"); > exit(EXIT_FAILURE); > } > > if (close(procMemFd) == -1) > errExit("Supervisor: close-/proc/PID/mem"); > > + /* Once again check that the notification ID is still valid. The > + case we are particularly concerned about here is that just > + before we fetched the pathname, the target's blocked system > + call was interrupted by a signal handler, and after the handler > + returned, the target carried on execution (past the interrupted > + system call). In that case, we have no guarantees about what we > + are reading, since the target's memory may have been arbitrarily > + changed by subsequent operations. */ > + > + if (!notificationIdIsValid(notifyFd, req->id, "post-open")) > + return false; > + > /* We have no guarantees about what was in the memory of the target > process. We therefore treat the buffer returned by pread() as > untrusted input. The buffer should be terminated by a null byte; > if not, then we will trigger an error for the target process. */ > > if (strnlen(path, nread) < nread) > return true; > ]] Yeah, that should do the job. With the caveat that a cancelled syscall could've also led to the memory being munmap()ed, so the nread==0 case could also happen legitimately - so you might want to move this check up above the nread==0 (mm went away) and nread==-1 (mm still exists, but read from address failed, errno EIO) checks if the error message shouldn't appear spuriously.