From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by sourceware.org (Postfix) with ESMTPS id ACCBD3858028 for ; Fri, 26 Mar 2021 04:57:08 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org ACCBD3858028 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5C44361A55 for ; Fri, 26 Mar 2021 04:57:06 +0000 (UTC) Received: by mail-ej1-f44.google.com with SMTP id r12so6498087ejr.5 for ; Thu, 25 Mar 2021 21:57:06 -0700 (PDT) X-Gm-Message-State: AOAM530j/m0iPiT6qCnNSnT9aGrhFmDOlcqdlsfFDNO8AOucIGG2dxBE t0YCVm46rcpjfWURE/LGV4JJuBZ6x2EIbeAGg0jsWg== X-Google-Smtp-Source: ABdhPJzQUC5kz4KsMNdtYz20rohKj/gTGnfe27XhqBn77MDhO+QCO6+ZjKJL54krol5eOLqpgHby8Vz1sy8kLqXHsAs= X-Received: by 2002:a17:907:2809:: with SMTP id eb9mr12915334ejc.204.1616734624746; Thu, 25 Mar 2021 21:57:04 -0700 (PDT) MIME-Version: 1.0 References: <20210316065215.23768-1-chang.seok.bae@intel.com> <20210316065215.23768-6-chang.seok.bae@intel.com> <20210325185435.GB32296@zn.tnic> In-Reply-To: <20210325185435.GB32296@zn.tnic> From: Andy Lutomirski Date: Thu, 25 Mar 2021 21:56:53 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate signal stack overflow To: Borislav Petkov Cc: Andy Lutomirski , "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 Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-11.4 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 26 Mar 2021 04:57:10 -0000 On Thu, Mar 25, 2021 at 11:54 AM Borislav Petkov wrote: > > 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. Nope. on_sig_stack() is a horrible kludge and won't work here. We could have something like __on_sig_stack() or sp_is_on_sig_stack() or something, though. > > > > + 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. I figure that the people whose programs spontaneously crash should get a hint why if they look at dmesg. Maybe the message should say "overflowed sigaltstack -- try noavx512"? We really ought to have a SIGSIGFAIL signal that's sent, double-fault style, when we fail to send a signal.