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Date:   Tue, 2 Nov 2021 16:15:30 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Mark Rutland <mark.rutland@arm.com>, X86 ML <x86@kernel.org>,
        Kees Cook <keescook@chromium.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
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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
> 
> 
> 
> 	# arg: r11
> 	# clobbers: r10, r11
> __x86_indirect_ibt_deadbeef:
> 	movl $0xdeadbeef, %r10
> 	subq $0x10, %r11
> 	ALTERNATIVE "", "lfence", X86_FEATURE_RETPOLINE
> 	jmp *%r11
> 

These two thunks could of course be one big alternative.

> And have the actual indirect callsite look like:
> 
> 	# r11 - &foo
> 	ALTERNATIVE_2	"cs call __x86_indirect_thunk_r11",
> 			"cs call __x86_indirect_cfi_deadbeef", X86_FEATURE_CFI
> 			"cs call __x86_indirect_ibt_deadbeef", X86_FEATURE_IBT

Also simplifying this.

> Although if the compiler were to emit:
> 
> 	cs call __x86_indirect_cfi_deadbeef
> 
> we could probaly fix it up from there.
> 
> 
> Then we can at runtime decide between:
> 
>   {!cfi, cfi, ibt} x {!retpoline, retpoline, retpoline-amd}
>