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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.11.2020 12:15, Roger Pau Monné wrote:
> 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

I can't see what's different from

void test(void) {
        asm volatile (".macro ret; .error; .endm; .macro retq; .error; .endm");
}

but this one produces "Error: .error directive invoked in source file"
for me with both old and new gcc.

Jan