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Date:   Tue, 2 Nov 2021 10:35:30 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>, X86 ML <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev, joao@overdrivepizza.com
Subject: Re: [PATCH] static_call,x86: Robustify trampoline patching
Message-ID: <202111021029.79D81E590@keescook>
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On Sat, Oct 30, 2021 at 10:16:31AM +0200, Peter Zijlstra wrote:
> foo.cfi:
> 	endbr
> 	xorl $0xdeadbeef, %r10d
> 	jz foo
> 	ud2
> 	nop	# make it an even 16 bytes
> foo:
> 	# actual function text
> 
> 
> Then have the address of foo, be the address of foo, like any normal
> sane person would expect. Have direct calls to foo, go to foo, again, as
> expected.
> 
> When doing an indirect call (to r11, as clang does), then, and only
> then, do:
> 
> 	movl $0xdeadbeef, %r10d
> 	subq $0x10, %r11
> 	call *%r11
> 
> 	# if the r11 lives, add:
> 	addq $0x10, %r11
> 
> 
> Then only when caller and callee agree 0xdeadbeef is the password, does
> the indirect call go through.
> 
> Why isn't this a suitable CFI scheme even without IBT?

The trouble is that the callee is doing the verification. There's no
protection against calling into a callee that doesn't perform a check
(e.g. BPF JIT, or otherwise constructed executable memory, etc). The
caller needs to do the verification that what they're calling into is
safe before it makes the call.

-- 
Kees Cook