From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1DBA568 for ; Tue, 2 Nov 2021 18:14:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=apcnU7ismDMGqhT244nxT/5OUNxzvS18BDxORL1RIIY=; b=UG4CE4sK5Ph3+yQomBR8Ofbygx eJa7ofiHw6NzIXdD4e6gpwxgKNuX+K8zMLnbBSwNvYv7Drxs69Pi6IJSGBvTzwoYPDMiYxVLX7q+V ZbEitT0HDCnIBbrhgtuIaGeBlVdHCqQZJFPuxwAYfaRBeF9tuHz5eMMds4TJI3C1o0WpSOIizc6+j /qrDzfCuj4GBmgzQNxirtD+fCwTIlPTCePR8lz5CCCQSCET4LLLQyraaVhd5LNobwCA2G9ZOKnRiU mtJrBT/t4ldrqDZ2JNpRvInKHzqe4pNQygTuOI6FJ3p9n6GXGNdATh98C0jxFwu0/U6n0o0ARB7QG TsRm3gzA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mhyIl-00DnQw-75; Tue, 02 Nov 2021 18:14:27 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (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) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 4EA0C3000D5; Tue, 2 Nov 2021 19:14:25 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 33DF22D53269E; Tue, 2 Nov 2021 19:14:25 +0100 (CET) Date: Tue, 2 Nov 2021 19:14:25 +0100 From: Peter Zijlstra To: Ard Biesheuvel Cc: Sami Tolvanen , Mark Rutland , X86 ML , Kees Cook , Josh Poimboeuf , Nathan Chancellor , Nick Desaulniers , Sedat Dilek , Steven Rostedt , linux-hardening@vger.kernel.org, Linux Kernel Mailing List , llvm@lists.linux.dev, joao@overdrivepizza.com Subject: Re: [PATCH] static_call,x86: Robustify trampoline patching Message-ID: References: <20211101090155.GW174703@worktop.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Nov 02, 2021 at 06:44:56PM +0100, Ard Biesheuvel wrote: > On Tue, 2 Nov 2021 at 16:15, Peter Zijlstra wrote: > > > > On Tue, Nov 02, 2021 at 01:57:44PM +0100, Peter Zijlstra wrote: > > > > > So how insane is something like this, have each function: > > > > > > foo.cfi: > > > endbr64 > > > xorl $0xdeadbeef, %r10d > > > jz foo > > > ud2 > > > nop # make it 16 bytes > > > foo: > > > # actual function text goes here > > > > > > > > > And for each hash have two thunks: > > > > > > > > > # arg: r11 > > > # clobbers: r10, r11 > > > __x86_indirect_cfi_deadbeef: > > > movl -9(%r11), %r10 # immediate in foo.cfi > > > xorl $0xdeadbeef, %r10 # our immediate > > > jz 1f > > > ud2 > > > 1: ALTERNATIVE_2 "jmp *%r11", > > > "jmp __x86_indirect_thunk_r11", X86_FEATURE_RETPOLINE > > > "lfence; jmp *%r11", X86_FEATURE_RETPOLINE_AMD > > > > > So are these supposed to go into the jump tables? If so, there still > needs to be a check against the boundary of the table at the call > site, to ensure that we are not calling something that we shouldn't. > > If they are not going into the jump tables, I don't see the point of > having them, as only happy flow/uncomprised code would bother to use > them. I don't understand. If you can scribble your own code, you can circumvent pretty much any range check anyway. But if you can't scribble your own code, you get to use the branch here and that checks the callsite and callee signature. The range check isn't fundamental to CFI, having a check is the important thing AFAIU.