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 Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id EF826CCA462 for ; Fri, 16 Oct 2026 09:44:34 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1xHeUC-000175-23; Fri, 16 Oct 2026 05:44:25 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xHeTo-0000wY-MJ for qemu-devel@nongnu.org; Fri, 16 Oct 2026 05:44:02 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xHeTm-0007K1-LQ for qemu-devel@nongnu.org; Fri, 16 Oct 2026 05:43:59 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1792143837; 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=/alU1FOVtQDtrfm2izDGqKUn26NeShLQ0co9p+UnUT4=; b=Pf+jnu3VOr8Qr7mkqTU5pjT2Twky/STI1JGO4fYPqmX17mY3lNXulF+smComEYe5XFVwMC PhSCXzV+JHpZGdPOELrUVe1I6dTse7F26PqkPQhnmBoxwmz3DSRW9fThvZ65WjZLumrCF1 jT09HHJw+VuRlCi2iPyVg7uR1SRYdKI= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-278-8hl8pu_iP5iRom_4kjBXzA-1; Fri, 16 Oct 2026 09:43:55 +0000 X-MC-Unique: 8hl8pu_iP5iRom_4kjBXzA-1 X-Mimecast-MFC-AGG-ID: 8hl8pu_iP5iRom_4kjBXzA_1792143835 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E42C31955F1E for ; Fri, 16 Oct 2026 09:43:54 +0000 (UTC) Received: from berrange.csb (headnet03.pony-001.prod.iad2.dc.redhat.com [10.2.32.114]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id C0E823000735; Fri, 16 Oct 2026 09:43:53 +0000 (UTC) From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= To: qemu-devel@nongnu.org Cc: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= Subject: [PULL 2/8] seccomp: drop redundant match for clone() flags == 0 Date: Fri, 16 Oct 2026 10:43:42 +0100 Message-ID: <20261016094348.2616374-3-berrange@redhat.com> In-Reply-To: <20261016094348.2616374-1-berrange@redhat.com> References: <20261016094348.2616374-1-berrange@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 10 X-Spam_score: 1.0 X-Spam_bar: + X-Spam_report: (1.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.24, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org We have seccomp matches for clone() which mandate the presence of several flags, in order to permit pthreads_create: REQUIRE_CLONE_FLAG(CLONE_VM); REQUIRE_CLONE_FLAG(CLONE_FS); ... Given those mandatory matches, it is impossible for flags == 0 to pass the filter. This is fortunate, since the explicit match on flags == 0 is in fact broken, as it has a silent size extension from 32-bits to 64-bits. This results in seccomp generating pseudo code of: if ($arch == 3221225534) # filter for syscall "clone" (56) [priority: 65531] if ($syscall == 56) if ($a0.hi32 & 0xffffffff == 0) if ($a0.lo32 & 0xffffff00 == 0) action ERRNO(1); # default action action ALLOW; # invalid architecture action action KILL; Since the clone() flag is passed as a 32-bit value in the syscall, the high 32-bits are liable to be undefined/uninitialized garbage The desired match for flags == 0 was if ($arch == 3221225534) # filter for syscall "clone" (56) [priority: 65532] if ($syscall == 56) if ($a0.hi32 & 0x00000000 == 0) if ($a0.lo32 & 0xffffff00 == 0) action ERRNO(1); # default action action ALLOW; # invalid architecture action action KILL; That is redundant, however, as can be seen if we look at the (fixed) program with the CLONE_THREAD rule also present: if ($arch == 3221225534) # filter for syscall "clone" (56) [priority: 65532] if ($syscall == 56) if ($a0.hi32 & 0x00000000 == 0) if ($a0.lo32 & 0xffffff00 == 0) action ERRNO(1); if ($a0.lo32 & 0x00010000 == 0) action ERRNO(1); # default action action ALLOW; # invalid architecture action action KILL; That "($a0.lo32 & 0x00010000 == 0)" match will catch the case of flags == 0 and do the right thing. Reported-by: Ricardo Branco (https://gitlab.com/ricardobranco777) Related: https://gitlab.com/qemu-project/qemu/-/work_items/4671 Reviewed-by: Marc-André Lureau Signed-off-by: Daniel P. Berrangé --- system/qemu-seccomp.c | 9 --------- 1 file changed, 9 deletions(-) diff --git a/system/qemu-seccomp.c b/system/qemu-seccomp.c index cd2c1b0b747..a71cf66028d 100644 --- a/system/qemu-seccomp.c +++ b/system/qemu-seccomp.c @@ -76,13 +76,6 @@ const struct scmp_arg_cmp sched_setscheduler_arg[] = { { SCMP_SYS(clone), QEMU_SECCOMP_SET_SPAWN, \ ARRAY_SIZE(clone_arg ## flag), clone_arg ## flag, SCMP_ACT_ERRNO(EPERM) } -/* If no CLONE_* flags are set, except CSIGNAL, deny */ -const struct scmp_arg_cmp clone_arg_none[] = { - { .arg = CLONE_FLAGS_ARG, - .op = SCMP_CMP_MASKED_EQ, - .datum_a = ~(CSIGNAL), .datum_b = 0 } -}; - /* * pthread_create should always set all of these. */ @@ -219,8 +212,6 @@ static const struct QemuSeccompSyscall denylist[] = { 0, NULL, SCMP_ACT_ERRNO(EPERM) }, { SCMP_SYS(execve), QEMU_SECCOMP_SET_SPAWN, 0, NULL, SCMP_ACT_ERRNO(EPERM) }, - { SCMP_SYS(clone), QEMU_SECCOMP_SET_SPAWN, - ARRAY_SIZE(clone_arg_none), clone_arg_none, SCMP_ACT_ERRNO(EPERM) }, RULE_CLONE_FLAG(CLONE_VM), RULE_CLONE_FLAG(CLONE_FS), RULE_CLONE_FLAG(CLONE_FILES), -- 2.56.0