linux-kernel - Re: [PATCH] x86/ibt: Implement FineIBT

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Date:   Sat, 22 Oct 2022 17:03:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT

On Fri, Oct 21, 2022 at 04:08:59PM -0700, Josh Poimboeuf wrote:
> On Tue, Oct 18, 2022 at 03:35:50PM +0200, Peter Zijlstra wrote:
> > +#ifdef CONFIG_FINEIBT
> > +/*
> > + * kCFI						FineIBT
> > + *
> > + * __cfi_\func:					__cfi_\func:
> > + *	movl   $0x12345678,%eax			     endbr64			// 4
> > + *	nop					     subl   $0x12345678,%r10d   // 7
> > + *	nop					     jz     1f			// 2
> > + *	nop					     ud2			// 2
> > + *	nop					1:   nop			// 1
> > + *	nop
> > + *	nop
> > + *	nop
> > + *	nop
> > + *	nop
> > + *	nop
> > + *	nop
> 
> All the "CFI" naming everywhere is very unfortunate.  We already have
> "call frame information" in both the toolchain and objtool.
> 
> The feature is called "kCFI" anyway, can Clang call the symbols
> '__kcfi_*'?

I think the compiler patch is already merged in clang, not sure that's
still an option, Sami?

> > +++ b/tools/objtool/builtin-check.c
> > @@ -79,6 +79,7 @@ const struct option check_options[] = {
> >  	OPT_BOOLEAN('s', "stackval", &opts.stackval, "validate frame pointer rules"),
> >  	OPT_BOOLEAN('t', "static-call", &opts.static_call, "annotate static calls"),
> >  	OPT_BOOLEAN('u', "uaccess", &opts.uaccess, "validate uaccess rules for SMAP"),
> > +	OPT_BOOLEAN(0  , "cfi", &opts.cfi, "generate cfi_sites"),
> 
> "annotate kernel control flow integrity (kCFI) function preambles" ?

Sure.

> > +++ b/tools/objtool/check.c
> > @@ -861,6 +861,62 @@ static int create_ibt_endbr_seal_section
> >  	return 0;
> >  }
> >  
> > +static int create_cfi_sections(struct objtool_file *file)
> > +{
> > +	struct section *sec, *s;
> > +	struct symbol *sym;
> > +	unsigned int *loc;
> > +	int idx;
> > +
> > +	sec = find_section_by_name(file->elf, ".cfi_sites");
> > +	if (sec) {
> > +		INIT_LIST_HEAD(&file->call_list);
> > +		WARN("file already has .cfi_sites section, skipping");
> > +		return 0;
> > +	}
> > +
> > +	idx = 0;
> > +	for_each_sec(file, s) {
> > +		if (!s->text)
> > +			continue;
> > +
> > +		list_for_each_entry(sym, &s->symbol_list, list) {
> > +			if (strncmp(sym->name, "__cfi_", 6))
> > +				continue;
> 
> Also make sure it's STT_FUNC.

OK.