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. >> NOTE: This vma (VM_LOCAL_CPU) is never used during a page_fault. It is >> always used in a synchronous way from a thread pinned to a single core. > > It's not a question of how your app is going to use this flag. It's a > question about how another app can abuse this flag (or how your app is > going to be exploited to abuse this flag) to break into the kernel. > If you look at the zuf user you will see that the faults all return SIG_BUS. These can never fault. The server has access to this mapping from a single thread pinned to a core. Again it is not an app visible flag in anyway Thanks for looking Boaz
