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? -- Michal Hocko SUSE Labs
