On Mon, Mar 11, 2019 at 05:36:58PM +0800, Peter Xu wrote: > Hi, > > (The idea comes from Andrea, and following discussions with Mike and > other people) > > This patchset introduces a new sysctl flag to allow the admin to > forbid users from using userfaultfd: > > $ cat /proc/sys/vm/unprivileged_userfaultfd > [disabled] enabled kvm CC linux-api@ This is unusual way to return current value for sysctl. Does it work fine with sysctl tool? Have you considered to place the switch into /sys/kernel/mm instead? I doubt it's the last tunable for userfaultfd. Maybe we should have an directory for it under /sys/kernel/mm? > - When set to "disabled", all unprivileged users are forbidden to > use userfaultfd syscalls. > > - When set to "enabled", all users are allowed to use userfaultfd > syscalls. > > - When set to "kvm", all unprivileged users are forbidden to use the > userfaultfd syscalls, except the user who has permission to open > /dev/kvm. > > This new flag can add one more layer of security to reduce the attack > surface of the kernel by abusing userfaultfd. Here we grant the > thread userfaultfd permission by checking against CAP_SYS_PTRACE > capability. By default, the value is "disabled" which is the most > strict policy. Distributions can have their own perferred value. > > The "kvm" entry is a bit special here only to make sure that existing > users like QEMU/KVM won't break by this newly introduced flag. What > we need to do is simply set the "unprivileged_userfaultfd" flag to > "kvm" here to automatically grant userfaultfd permission for processes > like QEMU/KVM without extra code to tweak these flags in the admin > code. > > Patch 1: The interface patch to introduce the flag > > Patch 2: The KVM related changes to detect opening of /dev/kvm > > Patch 3: Apply the flag to userfaultfd syscalls > > All comments would be greatly welcomed. Thanks, > > Peter Xu (3): > userfaultfd/sysctl: introduce unprivileged_userfaultfd > kvm/mm: introduce MMF_USERFAULTFD_ALLOW flag > userfaultfd: apply unprivileged_userfaultfd check > > fs/userfaultfd.c | 121 +++++++++++++++++++++++++++++++++ > include/linux/sched/coredump.h | 1 + > include/linux/userfaultfd_k.h | 5 ++ > init/Kconfig | 11 +++ > kernel/sysctl.c | 11 +++ > virt/kvm/kvm_main.c | 7 ++ > 6 files changed, 156 insertions(+) > > -- > 2.17.1 > -- Kirill A. Shutemov
