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=-8.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 B9A34C433E0 for ; Wed, 10 Jun 2020 13:43:57 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 812B720734 for ; Wed, 10 Jun 2020 13:43:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="bfpgK53K" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 812B720734 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:50298 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jj11I-0005D5-Kk for qemu-devel@archiver.kernel.org; Wed, 10 Jun 2020 09:43:56 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:33926) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jj0wr-0006W0-6m for qemu-devel@nongnu.org; Wed, 10 Jun 2020 09:39:21 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:60256 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jj0wp-0002Ze-DV for qemu-devel@nongnu.org; Wed, 10 Jun 2020 09:39:20 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1591796357; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=R9yM/vimBTrGy0ToxSJZ758QhqcLxosiKbfMZXSWhq4=; b=bfpgK53KMnk0n7vbteWSiquEKCPfqnOsrBuvs7PGZZCo2ngMZn5uz10wKmUHEgWdYo6TPH Z8qMK+qFAOH2GJCVWwSDQfmoH/O7Xf+1069CDXBkatPRczdJp8Hq7YgVmqrlYqI5MPBM8w Pk3CSpXAmFBxVmST+d1z+KGg/zBp394= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-514-spwqjRdqMyOGRkqxQkcDGQ-1; Wed, 10 Jun 2020 09:39:13 -0400 X-MC-Unique: spwqjRdqMyOGRkqxQkcDGQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 9342B100CCC0; Wed, 10 Jun 2020 13:39:12 +0000 (UTC) Received: from [10.3.113.22] (ovpn-113-22.phx2.redhat.com [10.3.113.22]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E9E377F0BE; Wed, 10 Jun 2020 13:39:11 +0000 (UTC) Subject: Re: [PATCH 1/2] nbd/server: Avoid long error message assertions CVE-2020-10761 To: Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org References: <20200608182638.3256473-1-eblake@redhat.com> <20200608182638.3256473-2-eblake@redhat.com> <678021fb-d34d-4067-31b3-f864efe13dbd@virtuozzo.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <5a8ce391-a16e-1af9-78bb-8e1aab5b213c@redhat.com> Date: Wed, 10 Jun 2020 08:39:11 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <678021fb-d34d-4067-31b3-f864efe13dbd@virtuozzo.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=205.139.110.61; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/09 23:51:15 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , qemu-block@nongnu.org, qemu-stable@nongnu.org, Max Reitz , ppandit@redhat.com, xuwei@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/10/20 3:57 AM, Vladimir Sementsov-Ogievskiy wrote: > 08.06.2020 21:26, Eric Blake wrote: >> Ever since commit 36683283 (v2.8), the server code asserts that error >> strings sent to the client are well-formed per the protocol by not >> exceeding the maximum string length of 4096.  At the time the server >> first started sending error messages, the assertion could not be >> triggered, because messages were completely under our control. >> However, over the years, we have added latent scenarios where a client >> could trigger the server to attempt an error message that would >> include the client's information if it passed other checks first: >> >> - requesting NBD_OPT_INFO/GO on an export name that is not present >>    (commit 0cfae925 in v2.12 echoes the name) >> >> - requesting NBD_OPT_LIST/SET_META_CONTEXT on an export name that is >>    not present (commit e7b1948d in v2.12 echoes the name) >> >> At the time, those were still safe because we flagged names larger >> than 256 bytes with a different message; but that changed in commit >> 93676c88 (v4.2) when we raised the name limit to 4096 to match the NBD >> string limit.  (That commit also failed to change the magic number >> 4096 in nbd_negotiate_send_rep_err to the just-introduced named >> constant.)  So with that commit, long client names appended to server >> text can now trigger the assertion, and thus be used as a denial of >> service attack against a server.  As a mitigating factor, if the >> server requires TLS, the client cannot trigger the problematic paths >> unless it first supplies TLS credentials, and such trusted clients are >> less likely to try to intentionally crash the server. >> >> Reported-by: Xueqiang Wei >> CC: qemu-stable@nongnu.org >> Fixes: https://bugzilla.redhat.com/1843684 CVE-2020-10761 >> Fixes: 93676c88d7 >> Signed-off-by: Eric Blake >> --- >>   nbd/server.c               | 28 +++++++++++++++++++++++++--- >>   tests/qemu-iotests/143     |  4 ++++ >>   tests/qemu-iotests/143.out |  2 ++ >>   3 files changed, 31 insertions(+), 3 deletions(-) >> >> diff --git a/nbd/server.c b/nbd/server.c >> index 02b1ed080145..ec130303586d 100644 >> --- a/nbd/server.c >> +++ b/nbd/server.c >> @@ -217,7 +217,7 @@ nbd_negotiate_send_rep_verr(NBDClient *client, >> uint32_t type, >> >>       msg = g_strdup_vprintf(fmt, va); >>       len = strlen(msg); >> -    assert(len < 4096); >> +    assert(len < NBD_MAX_STRING_SIZE); >>       trace_nbd_negotiate_send_rep_err(msg); >>       ret = nbd_negotiate_send_rep_len(client, type, len, errp); >>       if (ret < 0) { >> @@ -231,6 +231,27 @@ nbd_negotiate_send_rep_verr(NBDClient *client, >> uint32_t type, >>       return 0; >>   } >> >> +/* >> + * Truncate a potentially-long user-supplied string into something >> + * more suitable for an error reply. >> + */ >> +static const char * >> +nbd_truncate_name(const char *name) >> +{ >> +#define SANE_LENGTH 80 >> +    static char buf[SANE_LENGTH + 3 + 1]; /* Trailing '...', NUL */ > > s/NUL/NULL/ NULL is the pointer (typically 4 or 8 bytes); NUL is the character (exactly one byte in all multi-byte-encodings like UTF-8, or sizeof(wchar_t) when using wide characters). > > Hmm. It may break if we use it in parallel in two coroutines or > threads.. Not sure, is it possible now, neither of course will it be > possible in future. After some testing (including adding some temporary sleep() into the code), it looks like 'qemu-nbd -e 2' is currently serialized (we don't start responding to a second client until we are done negotiating with the first); on that grounds, we are not risking that information leaks from one client to another. But you are correct that it is not obvious, and that if we do have a situation where two threads can try to allow an NBD connection, then this static buffer could leak information from one client to another. So I'll need to post a v2. > > I'd avoid creating functions returning  instead use g_strdup_printf(), like > > char *tmp = g_strdup_printf("%.80s...", name); > >   ( OR, if you want explicit constant: g_strdup_printf("%.*s...", > SANE_LENGTH, name) ) > > ... report error ... > > g_free(tmp) > > Using g_strdup_printf also is safer as we don't need to care about buf > size. malloc'ing the buffer is not too bad; error messages are not the hot path. I'll change it along those lines for v2. >> @@ -996,7 +1017,8 @@ static int nbd_negotiate_meta_queries(NBDClient >> *client, >>       meta->exp = nbd_export_find(export_name); >>       if (meta->exp == NULL) { >>           return nbd_opt_drop(client, NBD_REP_ERR_UNKNOWN, errp, >> -                            "export '%s' not present", export_name); >> +                            "export '%s' not present", >> +                            nbd_truncate_name(export_name)); >>       } >> > > Hmm, maybe instead of assertion, shrink message in > nbd_negotiate_send_rep_verr() too? > This will save us from forgotten (or future) uses of the function. Truncating in nbd_negotiate_send_rep_verr is arbitrary; it does not have the context of what makes sense to truncate. With an artificially short length, and a client request for "longname_from_the_client", the difference would be between: export "longnam..." not present export "longname_from_the_cl... As a user, I prefer the first form, but truncating in nbd_negotiate_send_rep_verr is the second form. > > Shrinking name is better, as it provides better message on result. But > generally shrink > all two long messages in nbd_negotiate_send_rep_verr() (maybe, together > with error_report()) > seems a good thing for me. What would error_report() accomplish? Logging on the server that we are truncating a message sent back to the client doesn't help the client. This is not a case of protocol violation, and a server sending an error message to the client is not out of the ordinary. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org