Re: [PATCH] mm: Add new vma flag VM_LOCAL_CPU

[Matthew Wilcox]

On Tue, May 15, 2018 at 01:43:23PM +0300, Boaz Harrosh wrote:
> On 15/05/18 03:41, Matthew Wilcox wrote:
> > On Mon, May 14, 2018 at 10:37:38PM +0300, Boaz Harrosh wrote:
> >> On 14/05/18 22:15, Matthew Wilcox wrote:
> >>> On Mon, May 14, 2018 at 08:28:01PM +0300, Boaz Harrosh wrote:
> >>>> On a call to mmap an mmap provider (like an FS) can put
> >>>> this flag on vma->vm_flags.
> >>>>
> >>>> The VM_LOCAL_CPU flag tells the Kernel that the vma will be used
> >>>> from a single-core only, and therefore invalidation (flush_tlb) of
> >>>> PTE(s) need not be a wide CPU scheduling.
> >>>
> >>> I still don't get this.  You're opening the kernel up to being exploited
> >>> by any application which can persuade it to set this flag on a VMA.
> >>>
> >>
> >> No No this is not an application accessible flag this can only be set
> >> by the mmap implementor at ->mmap() time (Say same as VM_VM_MIXEDMAP).
> >>
> >> Please see the zuf patches for usage (Again apologise for pushing before
> >> a user)
> >>
> >> The mmap provider has all the facilities to know that this can not be
> >> abused, not even by a trusted Server.
> > 
> > I don't think page tables work the way you think they work.
> > 
> > +               err = vm_insert_pfn_prot(zt->vma, zt_addr, pfn, prot);
> > 
> > That doesn't just insert it into the local CPU's page table.  Any CPU
> > which directly accesses or even prefetches that address will also get
> > the translation into its cache.
> 
> Yes I know, but that is exactly the point of this flag. I know that this
> address is only ever accessed from a single core. Because it is an mmap (vma)
> of an O_TMPFILE-exclusive file created in a core-pinned thread and I allow
> only that thread any kind of access to this vma. Both the filehandle and the
> mmaped pointer are kept on the thread stack and have no access from outside.
> 
> So the all point of this flag is the kernel driver telling mm that this
> address is enforced to only be accessed from one core-pinned thread.

You're still thinking about this from the wrong perspective.  If you
were writing a program to attack this facility, how would you do it?
It's not exactly hard to leak one pointer's worth of information.