From patchwork Fri Oct 29 15:09:04 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Eric W. Biederman" X-Patchwork-Id: 12592951 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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 55BCEC433EF for ; Fri, 29 Oct 2021 15:09:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 382EC611C6 for ; Fri, 29 Oct 2021 15:09:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229610AbhJ2PMK (ORCPT ); Fri, 29 Oct 2021 11:12:10 -0400 Received: from out03.mta.xmission.com ([166.70.13.233]:38362 "EHLO out03.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229527AbhJ2PMJ (ORCPT ); Fri, 29 Oct 2021 11:12:09 -0400 Received: from in02.mta.xmission.com ([166.70.13.52]:44310) by out03.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1mgTVk-006xUw-G8; Fri, 29 Oct 2021 09:09:40 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95]:54384 helo=email.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1mgTVh-004SCm-Ga; Fri, 29 Oct 2021 09:09:39 -0600 From: ebiederm@xmission.com (Eric W. Biederman) To: Kees Cook Cc: Andrea Righi , Shuah Khan , Alexei Starovoitov , Andy Lutomirski , Will Drewry , linux-kselftest@vger.kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org References: <202110280955.B18CB67@keescook> <878rydm56l.fsf@disp2133> <202110281136.5CE65399A7@keescook> Date: Fri, 29 Oct 2021 10:09:04 -0500 In-Reply-To: <202110281136.5CE65399A7@keescook> (Kees Cook's message of "Thu, 28 Oct 2021 11:47:48 -0700") Message-ID: <87k0hvkgvj.fsf_-_@disp2133> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 X-XM-SPF: eid=1mgTVh-004SCm-Ga;;;mid=<87k0hvkgvj.fsf_-_@disp2133>;;;hst=in02.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/FEeRP4fHav6LdZxVWjjrMekdL2ZOhVZk= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: [PATCH] signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org As Andy pointed out that there are races between force_sig_info_to_task and sigaction[1] when force_sig_info_task. As Kees discovered[2] ptrace is also able to change these signals. In the case of seeccomp killing a process with a signal it is a security violation to allow the signal to be caught or manipulated. Solve this problem by introducing a new flag SA_IMMUTABLE that prevents sigaction and ptrace from modifying these forced signals. This flag is carefully made kernel internal so that no new ABI is introduced. Longer term I think this can be solved by guaranteeing short circuit delivery of signals in this case. Unfortunately reliable and guaranteed short circuit delivery of these signals is still a ways off from being implemented, tested, and merged. So I have implemented a much simpler alternative for now. [1] https://lkml.kernel.org/r/b5d52d25-7bde-4030-a7b1-7c6f8ab90660@www.fastmail.com [2] https://lkml.kernel.org/r/202110281136.5CE65399A7@keescook Cc: stable@vger.kernel.org Fixes: 307d522f5eb8 ("signal/seccomp: Refactor seccomp signal and coredump generation") Signed-off-by: "Eric W. Biederman" Tested-by: Andrea Righi --- I have tested this patch and this changed works for me to fix the issue. I believe this closes all of the races that force_sig_info_to_task has when sigdfl is specified. So this should be enough for anything that needs a guaranteed that userspace can not race with the kernel is handled. Can folks look this over and see if I missed something? Thank you, Eric include/linux/signal_types.h | 3 +++ include/uapi/asm-generic/signal-defs.h | 1 + kernel/signal.c | 8 +++++++- 3 files changed, 11 insertions(+), 1 deletion(-) diff --git a/include/linux/signal_types.h b/include/linux/signal_types.h index 34cb28b8f16c..927f7c0e5bff 100644 --- a/include/linux/signal_types.h +++ b/include/linux/signal_types.h @@ -70,6 +70,9 @@ struct ksignal { int sig; }; +/* Used to kill the race between sigaction and forced signals */ +#define SA_IMMUTABLE 0x008000000 + #ifndef __ARCH_UAPI_SA_FLAGS #ifdef SA_RESTORER #define __ARCH_UAPI_SA_FLAGS SA_RESTORER diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h index fe929e7b77ca..7572f2f46ee8 100644 --- a/include/uapi/asm-generic/signal-defs.h +++ b/include/uapi/asm-generic/signal-defs.h @@ -45,6 +45,7 @@ #define SA_UNSUPPORTED 0x00000400 #define SA_EXPOSE_TAGBITS 0x00000800 /* 0x00010000 used on mips */ +/* 0x00800000 used for internal SA_IMMUTABLE */ /* 0x01000000 used on x86 */ /* 0x02000000 used on x86 */ /* diff --git a/kernel/signal.c b/kernel/signal.c index 6a5e1802b9a2..056a107e3cbc 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1336,6 +1336,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool blocked = sigismember(&t->blocked, sig); if (blocked || ignored || sigdfl) { action->sa.sa_handler = SIG_DFL; + action->sa.sa_flags |= SA_IMMUTABLE; if (blocked) { sigdelset(&t->blocked, sig); recalc_sigpending_and_wake(t); @@ -2760,7 +2761,8 @@ bool get_signal(struct ksignal *ksig) if (!signr) break; /* will return 0 */ - if (unlikely(current->ptrace) && signr != SIGKILL) { + if (unlikely(current->ptrace) && (signr != SIGKILL) && + !(sighand->action[signr -1].sa.sa_flags & SA_IMMUTABLE)) { signr = ptrace_signal(signr, &ksig->info); if (!signr) continue; @@ -4110,6 +4112,10 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact) k = &p->sighand->action[sig-1]; spin_lock_irq(&p->sighand->siglock); + if (k->sa.sa_flags & SA_IMMUTABLE) { + spin_unlock_irq(&p->sighand->siglock); + return -EINVAL; + } if (oact) *oact = *k;