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.
