Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

[Vlastimil Babka]

On 5.1.2019 20:24, Jiri Kosina wrote:
> On Sat, 5 Jan 2019, Vlastimil Babka wrote:
> 
>>> There are possibilities [1] how mincore() could be used as a converyor of 
>>> a sidechannel information about pagecache metadata.
>>>
>>> Provide vm.mincore_privileged sysctl, which makes it possible to mincore() 
>>> start returning -EPERM in case it's invoked by a process lacking 
>>> CAP_SYS_ADMIN.
>>
>> Haven't checked the details yet, but wouldn't it be safe if anonymous private
>> mincore() kept working, and restrictions were applied only to page cache?
> 
> I was considering that, but then I decided not to do so, as that'd make 
> the interface even more confusing and semantics non-obvious in the 
> 'privileged' case.
> 
>>> The default behavior stays "mincore() can be used by anybody" in order to 
>>> be conservative with respect to userspace behavior.
>>
>> What if we lied instead of returned -EPERM, to not break userspace so 
>> obviously? I guess false positive would be the safer lie?
> 
> So your proposal basically would be
> 
> if (privileged && !CAP_SYS_ADMIN)
> 	if (pagecache)
> 		return false;

I was thinking about "return true" here, assuming that userspace generally wants
to ensure itself there won't be page faults when it starts doing something
critical, and if it sees a "false" it will try to do some kind of prefaulting,
possibly in a loop. There might be somebody trying to make sure something is out
of pagecache (it wants to see "false"), but can't think of anything except
benchmarks?

> 	else
> 		return do_mincore()
> 
> right ?
> 
> I think userspace would hate us for that semantics, but on the other hand 
> I can sort of understand the 'mincore() is racy anyway, so what' argument, 
> if that's what you are suggesting.
> 
> But then, I have no idea what userspace is using mincore() for. 
> https://codesearch.debian.net/search?q=mincore might provide some insight 
> I guess (thanks Matthew).
>