io_uring: add a sysctl to disable io_uring system-wide

Introduce a new sysctl (io_uring_disabled) which can be either 0, 1, or
2. When 0 (the default), all processes are allowed to create io_uring
instances, which is the current behavior.  When 1, io_uring creation is
disabled (io_uring_setup() will fail with -EPERM) for unprivileged
processes not in the kernel.io_uring_group group.  When 2, calls to
io_uring_setup() fail with -EPERM regardless of privilege.

Signed-off-by: Matteo Rizzo <matteorizzo@google.com>
[JEM: modified to add io_uring_group]
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Link: https://lore.kernel.org/r/x49y1i42j1z.fsf@segfault.boston.devel.redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 3800fab..0795d79 100644 (file)
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -450,6 +450,35 @@ this allows system administrators to override the
 ``IA64_THREAD_UAC_NOPRINT`` ``prctl`` and avoid logs being flooded.
 
 
+io_uring_disabled
+=================
+
+Prevents all processes from creating new io_uring instances. Enabling this
+shrinks the kernel's attack surface.
+
+= ======================================================================
+0 All processes can create io_uring instances as normal. This is the
+  default setting.
+1 io_uring creation is disabled (io_uring_setup() will fail with
+  -EPERM) for unprivileged processes not in the io_uring_group group.
+  Existing io_uring instances can still be used.  See the
+  documentation for io_uring_group for more information.
+2 io_uring creation is disabled for all processes. io_uring_setup()
+  always fails with -EPERM. Existing io_uring instances can still be
+  used.
+= ======================================================================
+
+
+io_uring_group
+==============
+
+When io_uring_disabled is set to 1, a process must either be
+privileged (CAP_SYS_ADMIN) or be in the io_uring_group group in order
+to create an io_uring instance.  If io_uring_group is set to -1 (the
+default), only processes with the CAP_SYS_ADMIN capability may create
+io_uring instances.
+
+
 kexec_load_disabled
 ===================
 

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index c6d9e46..0f0ba31 100644 (file)
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -150,6 +150,31 @@ static void io_queue_sqe(struct io_kiocb *req);
 
 struct kmem_cache *req_cachep;
 
+static int __read_mostly sysctl_io_uring_disabled;
+static int __read_mostly sysctl_io_uring_group = -1;
+
+#ifdef CONFIG_SYSCTL
+static struct ctl_table kernel_io_uring_disabled_table[] = {
+       {
+               .procname       = "io_uring_disabled",
+               .data           = &sysctl_io_uring_disabled,
+               .maxlen         = sizeof(sysctl_io_uring_disabled),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec_minmax,
+               .extra1         = SYSCTL_ZERO,
+               .extra2         = SYSCTL_TWO,
+       },
+       {
+               .procname       = "io_uring_group",
+               .data           = &sysctl_io_uring_group,
+               .maxlen         = sizeof(gid_t),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec,
+       },
+       {},
+};
+#endif
+
 struct sock *io_uring_get_socket(struct file *file)
 {
 #if defined(CONFIG_UNIX)
@@ -4070,9 +4095,30 @@ static long io_uring_setup(u32 entries, struct io_uring_params __user *params)
        return io_uring_create(entries, &p, params);
 }
 
+static inline bool io_uring_allowed(void)
+{
+       int disabled = READ_ONCE(sysctl_io_uring_disabled);
+       kgid_t io_uring_group;
+
+       if (disabled == 2)
+               return false;
+
+       if (disabled == 0 || capable(CAP_SYS_ADMIN))
+               return true;
+
+       io_uring_group = make_kgid(&init_user_ns, sysctl_io_uring_group);
+       if (!gid_valid(io_uring_group))
+               return false;
+
+       return in_group_p(io_uring_group);
+}
+
 SYSCALL_DEFINE2(io_uring_setup, u32, entries,
                struct io_uring_params __user *, params)
 {
+       if (!io_uring_allowed())
+               return -EPERM;
+
        return io_uring_setup(entries, params);
 }
 
@@ -4666,6 +4712,10 @@ static int __init io_uring_init(void)
                                offsetof(struct io_kiocb, cmd.data),
                                sizeof_field(struct io_kiocb, cmd.data), NULL);
 
+#ifdef CONFIG_SYSCTL
+       register_sysctl_init("kernel", kernel_io_uring_disabled_table);
+#endif
+
        return 0;
 };
 __initcall(io_uring_init);