Re: [RFC PATCH v3 2/4] arm64: Check the return PC against unreliable code sections

From: Mark Brown <broonie@kernel.org>
To: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Cc: jpoimboe@redhat.com, mark.rutland@arm.com, jthierry@redhat.com,
	catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org,
	pasha.tatashin@soleen.com, linux-arm-kernel@lists.infradead.org,
	live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v3 2/4] arm64: Check the return PC against unreliable code sections
Date: Thu, 6 May 2021 14:45:42 +0100	[thread overview]
Message-ID: <20210506134542.GD4642@sirena.org.uk> (raw)
In-Reply-To: <9781011e-2d99-7f46-592c-621c66ea66c3@linux.microsoft.com>

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On Wed, May 05, 2021 at 01:48:21PM -0500, Madhavan T. Venkataraman wrote:
> On 5/5/21 11:46 AM, Mark Brown wrote:

> > I think that works even if it's hard to love the goto, might want some
> > defensiveness to ensure we can't somehow end up in an infinite loop with
> > a sufficiently badly formed stack.

> I could do something like this:

> unwind_frame()
> {
> 	int	i;
> 	...
> 
> 	for (i = 0; i < MAX_CHECKS; i++) {
> 		if (!check_frame(tsk, frame))
> 			break;
> 	}

I think that could work, yes.  Have to see the actual code (and other
people's opinions!).

> If this is acceptable, then the only question is - what should be the value of
> MAX_CHECKS (I will rename it to something more appropriate)?

I'd expect something like 10 to be way more than we'd ever need, or we
could define it down to the 2 checks we expect to be possible ATM to be
conservative.  I'm tempted to be permissive if we have sufficient other
checks but I'm not 100% sure on that.

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