Re: [PATCH] x86/mpx: fix recursive munmap() corruption

[Michael Ellerman]

Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx> writes:
> Le 23/04/2019 Ã 18:04, Dave Hansen a ÃcritÂ:
>> On 4/23/19 4:16 AM, Laurent Dufour wrote:
...
>>> There are 2 assumptions here:
>>>   1. 'start' and 'end' are page aligned (this is guaranteed by __do_munmap().
>>>   2. the VDSO is 1 page (this is guaranteed by the union vdso_data_store on powerpc)
>> 
>> Are you sure about #2?  The 'vdso64_pages' variable seems rather
>> unnecessary if the VDSO is only 1 page. ;)
>
> Hum, not so sure now ;)
> I got confused, only the header is one page.
> The test is working as a best effort, and don't cover the case where 
> only few pages inside the VDSO are unmmapped (start > 
> mm->context.vdso_base). This is not what CRIU is doing and so this was 
> enough for CRIU support.
>
> Michael, do you think there is a need to manage all the possibility 
> here, since the only user is CRIU and unmapping the VDSO is not a so 
> good idea for other processes ?

Couldn't we implement the semantic that if any part of the VDSO is
unmapped then vdso_base is set to zero? That should be fairly easy, eg:

	if (start < vdso_end && end >= mm->context.vdso_base)
		mm->context.vdso_base = 0;


We might need to add vdso_end to the mm->context, but that should be OK.

That seems like it would work for CRIU and make sense in general?

cheers