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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 35227C433E9 for ; Thu, 25 Mar 2021 18:55:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 14E9661A4B for ; Thu, 25 Mar 2021 18:55:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230229AbhCYSzJ (ORCPT ); Thu, 25 Mar 2021 14:55:09 -0400 Received: from mx2.suse.de ([195.135.220.15]:48274 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230166AbhCYSyn (ORCPT ); Thu, 25 Mar 2021 14:54:43 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id CC244AC16; Thu, 25 Mar 2021 18:54:40 +0000 (UTC) Date: Thu, 25 Mar 2021 19:54:35 +0100 From: Borislav Petkov To: Andy Lutomirski Cc: "Chang S. Bae" , Andrew Cooper , Boris Ostrovsky , Juergen Gross , Stefano Stabellini , Thomas Gleixner , Ingo Molnar , X86 ML , Len Brown , Dave Hansen , "H. J. Lu" , Dave Martin , Jann Horn , Michael Ellerman , Carlos O'Donell , Tony Luck , "Ravi V. Shankar" , libc-alpha , linux-arch , Linux API , LKML Subject: Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate signal stack overflow Message-ID: <20210325185435.GB32296@zn.tnic> References: <20210316065215.23768-1-chang.seok.bae@intel.com> <20210316065215.23768-6-chang.seok.bae@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 25, 2021 at 11:13:12AM -0700, Andy Lutomirski wrote: > diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c > index ea794a083c44..53781324a2d3 100644 > --- a/arch/x86/kernel/signal.c > +++ b/arch/x86/kernel/signal.c > @@ -237,7 +237,8 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size, > unsigned long math_size = 0; > unsigned long sp = regs->sp; > unsigned long buf_fx = 0; > - int onsigstack = on_sig_stack(sp); > + bool already_onsigstack = on_sig_stack(sp); > + bool entering_altstack = false; > int ret; > > /* redzone */ > @@ -246,15 +247,25 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size, > > /* This is the X/Open sanctioned signal stack switching. */ > if (ka->sa.sa_flags & SA_ONSTACK) { > - if (sas_ss_flags(sp) == 0) > + /* > + * This checks already_onsigstack via sas_ss_flags(). > + * Sensible programs use SS_AUTODISARM, which disables > + * that check, and programs that don't use > + * SS_AUTODISARM get compatible but potentially > + * bizarre behavior. > + */ > + if (sas_ss_flags(sp) == 0) { > sp = current->sas_ss_sp + current->sas_ss_size; > + entering_altstack = true; > + } > } else if (IS_ENABLED(CONFIG_X86_32) && > - !onsigstack && > + !already_onsigstack && > regs->ss != __USER_DS && > !(ka->sa.sa_flags & SA_RESTORER) && > ka->sa.sa_restorer) { > /* This is the legacy signal stack switching. */ > sp = (unsigned long) ka->sa.sa_restorer; > + entering_altstack = true; > } What a mess this whole signal handling is. I need a course in signal handling to understand what's going on here... > > sp = fpu__alloc_mathframe(sp, IS_ENABLED(CONFIG_X86_32), > @@ -267,8 +278,16 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size, > * If we are on the alternate signal stack and would overflow it, don't. > * Return an always-bogus address instead so we will die with SIGSEGV. > */ > - if (onsigstack && !likely(on_sig_stack(sp))) > + if (unlikely(entering_altstack && > + (sp <= current->sas_ss_sp || > + sp - current->sas_ss_sp > current->sas_ss_size))) { You could've simply done if (unlikely(entering_altstack && !on_sig_stack(sp))) here. > + if (show_unhandled_signals && printk_ratelimit()) { > + pr_info("%s[%d] overflowed sigaltstack", > + tsk->comm, task_pid_nr(tsk)); > + } Why do you even wanna issue that? It looks like callers will propagate an error value up and people don't look at dmesg all the time. Btw, s/tsk/current/g IOW, this builds: --- diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c index a06cb107c0e8..c00e932b5f18 100644 --- a/arch/x86/kernel/signal.c +++ b/arch/x86/kernel/signal.c @@ -234,10 +234,11 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size, void __user **fpstate) { /* Default to using normal stack */ + bool already_onsigstack = on_sig_stack(regs->sp); + bool entering_altstack = false; unsigned long math_size = 0; unsigned long sp = regs->sp; unsigned long buf_fx = 0; - int onsigstack = on_sig_stack(sp); int ret; /* redzone */ @@ -246,15 +247,24 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size, /* This is the X/Open sanctioned signal stack switching. */ if (ka->sa.sa_flags & SA_ONSTACK) { - if (sas_ss_flags(sp) == 0) + /* + * This checks already_onsigstack via sas_ss_flags(). Sensible + * programs use SS_AUTODISARM, which disables that check, and + * programs that don't use SS_AUTODISARM get compatible but + * potentially bizarre behavior. + */ + if (sas_ss_flags(sp) == 0) { sp = current->sas_ss_sp + current->sas_ss_size; + entering_altstack = true; + } } else if (IS_ENABLED(CONFIG_X86_32) && - !onsigstack && + !already_onsigstack && regs->ss != __USER_DS && !(ka->sa.sa_flags & SA_RESTORER) && ka->sa.sa_restorer) { /* This is the legacy signal stack switching. */ sp = (unsigned long) ka->sa.sa_restorer; + entering_altstack = true; } sp = fpu__alloc_mathframe(sp, IS_ENABLED(CONFIG_X86_32), @@ -267,8 +277,14 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size, * If we are on the alternate signal stack and would overflow it, don't. * Return an always-bogus address instead so we will die with SIGSEGV. */ - if (onsigstack && !likely(on_sig_stack(sp))) + if (unlikely(entering_altstack && !on_sig_stack(sp))) { + + if (show_unhandled_signals && printk_ratelimit()) + pr_info("%s[%d] overflowed sigaltstack", + current->comm, task_pid_nr(current)); + return (void __user *)-1L; + } /* save i387 and extended state */ ret = copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size); -- Regards/Gruss, Boris. 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