Re: [PATCH v4 06/10] x86/alternative: use temporary mm for text poking

[Nadav Amit <namit@vmware.com>]

From: Peter Zijlstra
Sent: November 11, 2018 at 11:52:20 PM GMT
> To: Nadav Amit <namit@vmware.com>
> Cc: Ingo Molnar <mingo@redhat.com>, LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>, H. Peter Anvin <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, Andy Lutomirski <luto@kernel.org>, Kees Cook <keescook@chromium.org>, Dave Hansen <dave.hansen@intel.com>, Masami Hiramatsu <mhiramat@kernel.org>
> Subject: Re: [PATCH v4 06/10] x86/alternative: use temporary mm for text poking
> 
> 
> On Sun, Nov 11, 2018 at 08:53:07PM +0000, Nadav Amit wrote:
> 
>>>> +	/*
>>>> +	 * The lock is not really needed, but this allows to avoid open-coding.
>>>> +	 */
>>>> +	ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
>>>> +
>>>> +	/*
>>>> +	 * If we failed to allocate a PTE, fail. This should *never* happen,
>>>> +	 * since we preallocate the PTE.
>>>> +	 */
>>>> +	if (WARN_ON_ONCE(!ptep))
>>>> +		goto out;
>>> 
>>> Since we hard rely on init getting that right; can't we simply get rid
>>> of this?
>> 
>> This is a repeated complaint of yours, which I do not feel comfortable with.
>> One day someone will run some static analysis tool and start finding that
>> all these checks are missing.
>> 
>> The question is why do you care about them.
> 
> Mostly because they should not be happening, ever. And if they happen,
> there really isn't anything sensible we can do about it.
> 
>> If it is because they affect the
>> generated code and make it less efficient, I can fully understand and perhaps
>> we should have something like PARANOID_WARN_ON_ONCE() which compiles into nothing
>> unless a certain debug option is set.
>> 
>> If it is about the way the source code looks - I guess it doesn’t sore my
>> eyes as hard as some other stuff, and I cannot do much about it (other than
>> removing it as you asked).
> 
> And yes on the above two points. It adds both runtime overhead (albeit
> trivially small) and code complexity.

I understand. So the question is - what would you prefer: something like
PARANOID_WARN_ON_ONCE() or should I just remove the assertion?

>>>> +out:
>>>> +	if (memcmp(addr, opcode, len))
>>>> +		r = -EFAULT;
>>> 
>>> How could this ever fail? And how can we reliably recover from that?
>> 
>> This code has been there before (with slightly uglier code). Before this
>> patch, a BUG_ON() was used here. However, I noticed that kgdb actually
>> checks that text_poke() succeeded after calling it and gracefully fail.
>> However, this was useless, since text_poke() would panic before kgdb gets
>> the chance to do anything (see patch 7).
> 
> Yes, I know it was there before, and I did see kgdb do it too. But aside
> from that out-label case, which we also should never hit, how can we
> realistically ever fail that memcmp()?
> 
> If we fail here, something is _seriously_ buggered.

I agree. But as it may be useful at least to warn in such a case, as
debugging of SMC/CMC is hard. For example, if there is some sort of a race
between module (un)loading and static-keys - such a check might be
beneficial to indicate so. Having said that, changing it into VM_BUG_ON() or
something similar may make more sense.

Personally, I don’t care much - I’m just worried that I made some intrusive
changes *and* you want me to remove the assertion that checks that I didn’t
screw up.