From 2d5de004e009add27db76c5cdc9f1f7f7dc087e7 Mon Sep 17 00:00:00 2001 From: Axel Rasmussen Date: Mon, 8 Aug 2022 10:56:11 -0700 Subject: userfaultfd: add /dev/userfaultfd for fine grained access control Historically, it has been shown that intercepting kernel faults with userfaultfd (thereby forcing the kernel to wait for an arbitrary amount of time) can be exploited, or at least can make some kinds of exploits easier. So, in 37cd0575b8 "userfaultfd: add UFFD_USER_MODE_ONLY" we changed things so, in order for kernel faults to be handled by userfaultfd, either the process needs CAP_SYS_PTRACE, or this sysctl must be configured so that any unprivileged user can do it. In a typical implementation of a hypervisor with live migration (take QEMU/KVM as one such example), we do indeed need to be able to handle kernel faults. But, both options above are less than ideal: - Toggling the sysctl increases attack surface by allowing any unprivileged user to do it. - Granting the live migration process CAP_SYS_PTRACE gives it this ability, but *also* the ability to "observe and control the execution of another process [...], and examine and change [its] memory and registers" (from ptrace(2)). This isn't something we need or want to be able to do, so granting this permission violates the "principle of least privilege". This is all a long winded way to say: we want a more fine-grained way to grant access to userfaultfd, without granting other additional permissions at the same time. To achieve this, add a /dev/userfaultfd misc device. This device provides an alternative to the userfaultfd(2) syscall for the creation of new userfaultfds. The idea is, any userfaultfds created this way will be able to handle kernel faults, without the caller having any special capabilities. Access to this mechanism is instead restricted using e.g. standard filesystem permissions. [axelrasmussen@google.com: Handle misc_register() failure properly] Link: https://lkml.kernel.org/r/20220819205201.658693-3-axelrasmussen@google.com Link: https://lkml.kernel.org/r/20220808175614.3885028-3-axelrasmussen@google.com Signed-off-by: Axel Rasmussen Acked-by: Nadav Amit Acked-by: Peter Xu Acked-by: Mike Rapoport Cc: Al Viro Cc: Dave Hansen Cc: Dmitry V. Levin Cc: Gleb Fotengauer-Malinovskiy Cc: Hugh Dickins Cc: Jan Kara Cc: Jonathan Corbet Cc: Mel Gorman Cc: Mike Kravetz Cc: Shuah Khan Cc: Shuah Khan Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Zhang Yi Cc: Mike Rapoport Signed-off-by: Andrew Morton --- include/uapi/linux/userfaultfd.h | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'include/uapi/linux/userfaultfd.h') diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h index 7d32b1e797fb..005e5e306266 100644 --- a/include/uapi/linux/userfaultfd.h +++ b/include/uapi/linux/userfaultfd.h @@ -12,6 +12,10 @@ #include +/* ioctls for /dev/userfaultfd */ +#define USERFAULTFD_IOC 0xAA +#define USERFAULTFD_IOC_NEW _IO(USERFAULTFD_IOC, 0x00) + /* * If the UFFDIO_API is upgraded someday, the UFFDIO_UNREGISTER and * UFFDIO_WAKE ioctls should be defined as _IOW and not as _IOR. In -- cgit