On Wed, Feb 12, 2020 at 8:51 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Tue, Feb 11, 2020 at 02:55:41PM -0800, Daniel Colascione wrote: > > Let userfaultfd opt out of handling kernel-mode faults > > Add a new sysctl for limiting userfaultfd to user mode faults > > Now this I'm very interested in. Can you go into more detail about two > things: [...] > - Why is this needed in addition to the existing vm.unprivileged_userfaultfd > sysctl? (And should this maybe just be another setting for that > sysctl, like "2"?) > > As to the mechanics of the change, I'm not sure I like the idea of adding > a UAPI flag for this. Why not just retain the permission check done at > open() and if kernelmode faults aren't allowed, ignore them? This would > require no changes to existing programs and gains the desired defense. > (And, I think, the sysctl value could be bumped to "2" as that's a > better default state -- does qemu actually need kernelmode traps?) I think this might be necessary for I/O emulation? As in, if before getting migrated, the guest writes some data into a buffer, then the guest gets migrated, and then while the postcopy migration stuff is still running, the guest tells QEMU to write that data from guest-physical memory to disk or whatever; I think in that case, QEMU will do something like a pwrite() syscall where the userspace pointer points into the memory area containing guest-physical memory, which would return -EFAULT if userfaultfd was restricted to userspace accesses. This was described in this old presentation about why userfaultfd is better than a SIGSEGV handler: https://drive.google.com/file/d/0BzyAwvVlQckeSzlCSDFmRHVybzQ/view (slide 6) (recording at https://youtu.be/pC8cWWRVSPw?t=463)
