linux-kernel - Re: [PATCH] lkdtm: cfi: add test for HW landing pad CFI

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Message-ID: <Ys/+qghGmH4kK4DN@FVFF77S0Q05N>
Date:   Thu, 14 Jul 2022 12:31:54 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Mark Brown <broonie@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Kees Cook <keescook@...omium.org>, peterz@...radead.org
Subject: Re: [PATCH] lkdtm: cfi: add test for HW landing pad CFI

On Wed, Jul 13, 2022 at 05:13:57PM +0100, Mark Brown wrote:
> On Wed, Jul 13, 2022 at 04:18:15PM +0100, Mark Rutland wrote:
> > Some architectures have coarse-grained HW CFI schemes where indirect
> > branches must target a "landing pad" instruction (e.g. BTI on arm64,
> > ENDBR on x86). These prevent gadgetization of arbitrary portions of
> > functions.
> 
> > Add a test which checks these work as expected.
> 
> > For example, on arm64 HW with BTI this should result in a BTI exception
> > being taken:
> 
> > +/*
> > + * This tries to call an indirect function with an address which is not a
> > + * function entry point. This should be caught by architectures with "landing
> > + * pad" instructions (e.g. BTI on arm64, or ENDBR on x86).
> > + */
> > +static void lkdtm_CFI_FORWARD_LANDING_PAD(void)
> > +{
> > +	void (*func)(int *);
> > +
> > +	func = (void *)((unsigned long)lkdtm_increment_void + 4);
> > +
> > +	pr_info("Calling gadget address ...\n");
> > +	func(&called_count);
> > +
> > +	pr_err("FAIL: survived gadget function call!\n");
> > +}
> 
> Incrementing the address by 4 here is the right number for arm64 and it
> looks like it's also right for the x86_64 ENDBR64 instruction but are we
> guaranteed that it'll do the right thing for other architectures,
> especially those with variable length instructions - couldn't we just
> get an illegal instruction exception due to ending up pointing at
> something that isn't the start of an instruction even if CFI isn't
> active?
> 
> Not sure that worrying about that at this point isn't making perfect the
> enemy of good though, it could be dealt with later.  Perhaps just put
> the offset behind a #define to make it a tiny bit more discoverable?

How about I just add a comment for now? e.g.

	/*
	 * Skip past a landing pad instruction.
	 * On arm64 all instructions are 4-byte aligned, and x86's ENDBR is 4
	 * bytes.
	 */

Thanks,
Mark.