From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.skyhub.de (mail.skyhub.de [5.9.137.197]) by sourceware.org (Postfix) with ESMTPS id 734173857C48 for ; Fri, 14 May 2021 07:26:21 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 734173857C48 Received: from zn.tnic (p200300ec2f0b2c00f343c5c4aba7bf62.dip0.t-ipconnect.de [IPv6:2003:ec:2f0b:2c00:f343:c5c4:aba7:bf62]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.skyhub.de (SuperMail on ZX Spectrum 128k) with ESMTPSA id 44F091EC04DA; Fri, 14 May 2021 09:26:19 +0200 (CEST) Date: Fri, 14 May 2021 09:26:16 +0200 From: Borislav Petkov To: Thomas Gleixner Cc: "Bae, Chang Seok" , Andy Lutomirski , "mingo@kernel.org" , "x86@kernel.org" , "Brown, Len" , "Hansen, Dave" , "hjl.tools@gmail.com" , "Dave.Martin@arm.com" , "jannh@google.com" , "mpe@ellerman.id.au" , "carlos@redhat.com" , "Luck, Tony" , "Shankar, Ravi V" , "libc-alpha@sourceware.org" , "linux-arch@vger.kernel.org" , "linux-api@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal stack overflow Message-ID: References: <20210422044856.27250-1-chang.seok.bae@intel.com> <20210422044856.27250-6-chang.seok.bae@intel.com> <87pmxv65av.ffs@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87pmxv65av.ffs@nanos.tec.linutronix.de> X-Spam-Status: No, score=-3.0 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, 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, 14 May 2021 07:26:24 -0000 On Wed, May 12, 2021 at 10:55:04PM +0200, Thomas Gleixner wrote: > On Wed, May 12 2021 at 18:48, Chang Seok Bae wrote: > > On May 11, 2021, at 11:36, Borislav Petkov wrote: > >> > >> I clumsily tried to register a SIGSEGV handler with > >> > >> act.sa_sigaction = my_sigsegv; > >> sigaction(SIGSEGV, &act, NULL); > >> > >> but that doesn't fire - task gets killed. Maybe I'm doing it wrong. > > > > Since the altstack is already overflowed, perhaps set the flag like this -- not > > using it to get the handler: > > > > act.sa_sigaction = my_sigsegv; > > + act.sa_flags = SA_SIGINFO; > > sigaction(SIGSEGV, &act, NULL); > > > > FWIW, I think this is just a workaround for this case; in practice, altstack is > > rather a backup for normal stack corruption. > > That's the intended usage, but it's not limited to that and there exists > creative (ab)use of sigaltstack beyond catching the overflow of the > regular stack. Right, with the above sa_flags setting (SA_ONSTACK removed) it does run the SIGSEGV handler: # [NOTE] the stack size is 2048, AT_MINSIGSTKSZ: 3632 TAP version 13 1..3 ok 1 Initial sigaltstack state was SS_DISABLE # sstack: 0x7f4e2e4d1000, ss_size: 2048 # [NOTE] sigaltstack success # [NOTE] Will mmap user stack # [NOTE] Will getcontext # [NOTE] Will makecontext # [NOTE] Will raise SIGUSR1 # [NOTE] signal SEGV ^^^^^^^^^^^ # [NOTE] Will sigaltstack ok 2 sigaltstack is still SS_AUTODISARM after signal # Planned tests != run tests (3 != 2) # Totals: pass:2 fail:0 xfail:0 xpass:0 skip:0 error:0 and exits normally. dmesg has: [220514.661048] signal: get_sigframe: nested_altstack: 0, sp: 0x7ffc2846bca0, ka->sa.sa_flags: 0xc000004 [220514.661058] signal: get_sigframe: SA_ONSTACK, sas_ss_flags(sp): 0x0 [220514.661061] signal: get_sigframe: sp: 0x7f4e2e4d1800, entering_altstack [220514.661064] signal: get_sigframe: nested_altstack: 0, entering_altstack: 1, __on_sig_stack: 0 [220514.661067] signal: sas[77819] overflowed sigaltstack so at least we've warned that we've overflowed the sigaltstack. [220514.661072] signal: get_sigframe: nested_altstack: 0, sp: 0x7ffc2846bca0, ka->sa.sa_flags: 0x4000004 [220514.661075] signal: get_sigframe: nested_altstack: 0, entering_altstack: 0, __on_sig_stack: 0 So I'm not even going to think about claiming that this is taking care of the other productive ways of (ab)using the sigaltstack contraption but from where I'm standing, it is not making it worse, AFAICT. Thx. -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette