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

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Date:   Thu, 6 May 2021 10:21:37 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     jpoimboe@...hat.com, mark.rutland@....com, jthierry@...hat.com,
        catalin.marinas@....com, will@...nel.org, jmorris@...ei.org,
        pasha.tatashin@...een.com, linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 2/4] arm64: Check the return PC against unreliable
 code sections



On 5/6/21 8:45 AM, Mark Brown wrote:
> 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.
> 

OK. I will implement these changes for version 4 and send it out so this
whole thing can be reviewed again with the actual changes in front of us.

Madhavan