Re: [PATCH 05/16] x86/shstk: Introduce Supervisor Shadow Stack support

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 04/05/2020 14:52, Jan Beulich wrote:
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> On 02.05.2020 00:58, Andrew Cooper wrote:
>> --- a/xen/arch/x86/Kconfig
>> +++ b/xen/arch/x86/Kconfig
>> @@ -34,6 +34,9 @@ config ARCH_DEFCONFIG
>>  config INDIRECT_THUNK
>>  	def_bool $(cc-option,-mindirect-branch-register)
>>  
>> +config HAS_AS_CET
>> +	def_bool $(as-instr,wrssq %rax$(comma)0;setssbsy;endbr64)
> I see you add as-instr here as a side effect. Until the other
> similar checks get converted, I think for the time being we should
> use the old model, to have all such checks in one place. I realize
> this means you can't have a Kconfig dependency then.

No.  That's like asking me to keep on using bool_t, and you are the
first person to point out and object to that in newly submitted patches.

> Also why do you check multiple insns, when just one (like we do
> elsewhere) would suffice?

Because CET-SS and CET-IBT are orthogonal ABIs, but you wanted a single
CET symbol, rather than a CET_SS symbol.

I picked a sample of various instructions to get broad coverage of CET
without including every instruction.

> The crucial insns to check are those which got changed pretty
> close before the release of 2.29 (in the cover letter you
> mention 2.32): One of incssp{d,q}, setssbsy, or saveprevssp.
> There weren't official binutils releases with the original
> insns, but distros may still have picked up intermediate
> snapshots.

I've got zero interest in catering to distros which are still using
obsolete pre-release toolchains.  Bleeding edge toolchains are one
thing, but this is like asking us to support the middle changeset of the
series introducing CET, which is absolutely a non-starter.

If the instructions were missing from real binutils releases, then
obviously we should exclude those releases, but that doesn't appear to
be the case.

>> --- a/xen/arch/x86/setup.c
>> +++ b/xen/arch/x86/setup.c
>> @@ -95,6 +95,36 @@ unsigned long __initdata highmem_start;
>>  size_param("highmem-start", highmem_start);
>>  #endif
>>  
>> +static bool __initdata opt_xen_shstk = true;
>> +
>> +static int parse_xen(const char *s)
>> +{
>> +    const char *ss;
>> +    int val, rc = 0;
>> +
>> +    do {
>> +        ss = strchr(s, ',');
>> +        if ( !ss )
>> +            ss = strchr(s, '\0');
>> +
>> +        if ( (val = parse_boolean("shstk", s, ss)) >= 0 )
>> +        {
>> +#ifdef CONFIG_XEN_SHSTK
>> +            opt_xen_shstk = val;
>> +#else
>> +            no_config_param("XEN_SHSTK", "xen", s, ss);
>> +#endif
>> +        }
>> +        else
>> +            rc = -EINVAL;
>> +
>> +        s = ss + 1;
>> +    } while ( *ss );
>> +
>> +    return rc;
>> +}
>> +custom_param("xen", parse_xen);
> What's the idea here going forward, i.e. why the new top level
> "xen" option? Almost all options are for Xen itself, after all.
> Did you perhaps mean this to be "cet"?

I forgot an RFC for this, as I couldn't think of anything better.  "cet"
as a top level option isn't going to gain more than {no-}shstk and
{no-}ibt as suboptions.

>> --- a/xen/scripts/Kconfig.include
>> +++ b/xen/scripts/Kconfig.include
>> @@ -31,6 +31,10 @@ cc-option = $(success,$(CC) -Werror $(CLANG_FLAGS) $(1) -E -x c /dev/null -o /de
>>  # Return y if the linker supports <flag>, n otherwise
>>  ld-option = $(success,$(LD) -v $(1))
>>  
>> +# $(as-instr,<instr>)
>> +# Return y if the assembler supports <instr>, n otherwise
>> +as-instr = $(success,printf "%b\n" "$(1)" | $(CC) $(CLANG_FLAGS) -c -x assembler -o /dev/null -)
> CLANG_FLAGS caught my eye here, then noticing that cc-option
> also uses it. Anthony - what's the deal with this? It doesn't
> look to get defined anywhere, and I also don't see what clang-
> specific about these constructs.

This is as it inherits from Linux.  There is obviously a reason.

However, I'm not interested in diving into that rabbit hole in an
unrelated series.

~Andrew