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Date:   Wed, 22 Mar 2023 11:09:22 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Jason Baron <jbaron@akamai.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Will McVicker <willmcvicker@google.com>,
        Kees Cook <keescook@chromium.org>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 08/11] arm64/static_call: Fix static call CFI
 violations
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On Wed, Mar 22, 2023 at 12:22:07PM +0000, Mark Rutland wrote:
> > The problem is that the __perf_guest_state() static call does an
> > indirect branch to __static_call_return0(), which isn't CFI-compliant.
> 
> IIUC that'd be broken even with the old CFI mechanism, since commit:
> 
>   87b940a0675e2526 ("perf/core: Use static_call to optimize perf_guest_info_callbacks")
> 
> If so, we probably want a Fixes tag?

Yeah, it should definitely get a Fixes tag.  I wasn't quite sure if this
bug started with the above commit or with the CFI_CLANG switch to kcfi.
And then I forgot to investigate.

> > +/* Generate a CFI-compliant static call NOP function */
> > +#define __ARCH_DEFINE_STATIC_CALL_CFI(name, insns)			\
> > +	asm(".align 4						\n"	\
> > +	    ".word __kcfi_typeid_" name "			\n"	\
> > +	    ".globl " name "					\n"	\
> > +	    name ":						\n"	\
> > +	    "bti c						\n"	\
> > +	    insns "						\n"	\
> > +	    "ret						\n"	\
> > +	    ".type " name ", @function				\n"	\
> > +	    ".size " name ", . - " name "			\n")
> > +
> > +#define __ARCH_DEFINE_STATIC_CALL_RET0_CFI(name)			\
> > +	GEN_CFI_SYM(STATIC_CALL_RET0_CFI(name));			\
> > +	__ARCH_DEFINE_STATIC_CALL_CFI(STATIC_CALL_RET0_CFI_STR(name), "mov x0, xzr")
> 
> This looks correct, but given we're generating a regular functions it's
> unfortunate we can't have the compiler generate the actual code with something
> like:
> 
> #define __ARCH_DEFINE_STATIC_CALL_RET0_CFI(rettype, name, args...)	\
> rettype name(args)							\
> {									\
> 	return (rettype)0;						\
> }
> 
> ... but I guess passing the rettype and args around is painful.

Hm, I hadn't considered that.  I'll play around with it.

> Regardless, I gave this a spin atop v6.3-rc3 using LLVM 16.0.0 and CFI_CLANG,
> and it does seem to work, so:
> 
> Tested-by: Mark Rutland <mark.rutland@arm.com>

Thanks!

-- 
Josh