On Thu 03-01-19 21:31:58, Roman Penyaev wrote: > On 2019-01-03 20:40, Michal Hocko wrote: > > On Thu 03-01-19 20:27:26, Roman Penyaev wrote: > > > On 2019-01-03 16:13, Michal Hocko wrote: > > > > On Thu 03-01-19 15:59:52, Roman Penyaev wrote: > > > > > area->size can include adjacent guard page but get_vm_area_size() > > > > > returns actual size of the area. > > > > > > > > > > This fixes possible kernel crash when userspace tries to map area > > > > > on 1 page bigger: size check passes but the following > > > > > vmalloc_to_page() > > > > > returns NULL on last guard (non-existing) page. > > > > > > > > Can this actually happen? I am not really familiar with all the callers > > > > of this API but VM_NO_GUARD is not really used wildly in the kernel. > > > > > > Exactly, by default (VM_NO_GUARD is not set) each area has guard page, > > > thus the area->size will be bigger. The bug is not reproduced if > > > VM_NO_GUARD is set. > > > > > > > All I can see is kasan na arm64 which doesn't really seem to use it > > > > for vmalloc. > > > > > > > > So is the problem real or this is a mere cleanup? > > > > > > This is the real problem, try this hunk for any file descriptor which > > > provides > > > mapping, or say modify epoll as example: > > > > OK, my response was more confusing than I intended. I meant to say. Is > > there any in kernel code that would allow the bug have had in mind? > > In other words can userspace trick any existing code? > > In theory any existing caller of remap_vmalloc_range() which does > not have an explicit size check should trigger an oops, e.g. this is > a good candidate: > > *** drivers/media/usb/stkwebcam/stk-webcam.c: > v4l_stk_mmap[789] ret = remap_vmalloc_range(vma, sbuf->buffer, > 0); Hmm, sbuf->buffer is allocated in stk_setup_siobuf to have buf->v4lbuf.length. mmap callback maps this buffer to the vma size and that is indeed not enforced to be <= length AFAICS. So you are right! Can we have an example in the changelog please? Thanks! -- Michal Hocko SUSE Labs
