Re: [PATCH v3 5/7] x86: guard against straight-line speculation past RET

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Oct 23, 2020 at 10:38:04AM +0200, Jan Beulich wrote:
> Under certain conditions CPUs can speculate into the instruction stream
> past a RET instruction. Guard against this just like 3b7dab93f240
> ("x86/spec-ctrl: Protect against CALL/JMP straight-line speculation")
> did - by inserting an "INT $3" insn. It's merely the mechanics of how to
> achieve this that differ: A set of macros gets introduced to post-
> process RET insns issued by the compiler (or living in assembly files).
> 
> Unfortunately for clang this requires further features their built-in
> assembler doesn't support: We need to be able to override insn mnemonics
> produced by the compiler (which may be impossible, if internally
> assembly mnemonics never get generated), and we want to use \(text)
> escaping / quoting in the auxiliary macro.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> TBD: Would be nice to avoid the additions in .init.text, but a query to
>      the binutils folks regarding the ability to identify the section
>      stuff is in (by Peter Zijlstra over a year ago:
>      https://sourceware.org/pipermail/binutils/2019-July/107528.html)
>      has been left without helpful replies.
> ---
> v3: Use .byte 0xc[23] instead of the nested macros.
> v2: Fix build with newer clang. Use int3 mnemonic. Also override retq.
> 
> --- a/xen/Makefile
> +++ b/xen/Makefile
> @@ -145,7 +145,15 @@ t2 = $(call as-insn,$(CC) -I$(BASEDIR)/i
>  # https://bugs.llvm.org/show_bug.cgi?id=36110
>  t3 = $(call as-insn,$(CC),".macro FOO;.endm"$(close); asm volatile 
> $(open)".macro FOO;.endm",-no-integrated-as)
>  
> -CLANG_FLAGS += $(call or,$(t1),$(t2),$(t3))
> +# Check whether \(text) escaping in macro bodies is supported.
> +t4 = $(call as-insn,$(CC),".macro m ret:req; \\(ret) $$\\ret; .endm; m 
> 8",,-no-integrated-as)
> +
> +# Check whether macros can override insn mnemonics in inline assembly.
> +t5 = $(call as-insn,$(CC),".macro ret; .error; .endm; .macro retq; .error; 
> .endm",-no-integrated-as)

I was going over this to post a bug report to LLVM, but it seems like
gcc also doesn't overwrite ret when using the above snippet:

https://godbolt.org/z/oqsPTv

Roger.