From d0d4730ac2e404a5b0da9a87ef38c73e51cb1664 Mon Sep 17 00:00:00 2001 From: Lokesh Gidra Date: Mon, 14 Dec 2020 19:13:54 -0800 Subject: userfaultfd: add user-mode only option to unprivileged_userfaultfd sysctl knob With this change, when the knob is set to 0, it allows unprivileged users to call userfaultfd, like when it is set to 1, but with the restriction that page faults from only user-mode can be handled. In this mode, an unprivileged user (without SYS_CAP_PTRACE capability) must pass UFFD_USER_MODE_ONLY to userfaultd or the API will fail with EPERM. This enables administrators to reduce the likelihood that an attacker with access to userfaultfd can delay faulting kernel code to widen timing windows for other exploits. The default value of this knob is changed to 0. This is required for correct functioning of pipe mutex. However, this will fail postcopy live migration, which will be unnoticeable to the VM guests. To avoid this, set 'vm.userfault = 1' in /sys/sysctl.conf. The main reason this change is desirable as in the short term is that the Android userland will behave as with the sysctl set to zero. So without this commit, any Linux binary using userfaultfd to manage its memory would behave differently if run within the Android userland. For more details, refer to Andrea's reply [1]. [1] https://lore.kernel.org/lkml/20200904033438.GI9411@redhat.com/ Link: https://lkml.kernel.org/r/20201120030411.2690816-3-lokeshgidra@google.com Signed-off-by: Lokesh Gidra Reviewed-by: Andrea Arcangeli Cc: Kees Cook Cc: Jonathan Corbet Cc: Peter Xu Cc: Sebastian Andrzej Siewior Cc: Alexander Viro Cc: Stephen Smalley Cc: Eric Biggers Cc: Daniel Colascione Cc: "Joel Fernandes (Google)" Cc: Kalesh Singh Cc: Suren Baghdasaryan Cc: Jeff Vander Stoep Cc: Cc: Mike Rapoport Cc: Shaohua Li Cc: Jerome Glisse Cc: Mauro Carvalho Chehab Cc: Johannes Weiner Cc: Mel Gorman Cc: Nitin Gupta Cc: Vlastimil Babka Cc: Iurii Zaikin Cc: Luis Chamberlain Cc: Daniel Colascione Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/admin-guide/sysctl/vm.rst | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) (limited to 'Documentation/admin-guide') diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index f455fa00c00f..d06a98b2a4e7 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -873,12 +873,17 @@ file-backed pages is less than the high watermark in a zone. unprivileged_userfaultfd ======================== -This flag controls whether unprivileged users can use the userfaultfd -system calls. Set this to 1 to allow unprivileged users to use the -userfaultfd system calls, or set this to 0 to restrict userfaultfd to only -privileged users (with SYS_CAP_PTRACE capability). +This flag controls the mode in which unprivileged users can use the +userfaultfd system calls. Set this to 0 to restrict unprivileged users +to handle page faults in user mode only. In this case, users without +SYS_CAP_PTRACE must pass UFFD_USER_MODE_ONLY in order for userfaultfd to +succeed. Prohibiting use of userfaultfd for handling faults from kernel +mode may make certain vulnerabilities more difficult to exploit. -The default value is 1. +Set this to 1 to allow unprivileged users to use the userfaultfd system +calls without any restrictions. + +The default value is 0. user_reserve_kbytes -- cgit v1.2.3