Audit: add TTY input auditing

Add TTY input auditing, used to audit system administrator's actions.  This is
required by various security standards such as DCID 6/3 and PCI to provide
non-repudiation of administrator's actions and to allow a review of past
actions if the administrator seems to overstep their duties or if the system
becomes misconfigured for unknown reasons.  These requirements do not make it
necessary to audit TTY output as well.

Compared to an user-space keylogger, this approach records TTY input using the
audit subsystem, correlated with other audit events, and it is completely
transparent to the user-space application (e.g.  the console ioctls still
work).

TTY input auditing works on a higher level than auditing all system calls
within the session, which would produce an overwhelming amount of mostly
useless audit events.

Add an "audit_tty" attribute, inherited across fork ().  Data read from TTYs
by process with the attribute is sent to the audit subsystem by the kernel.
The audit netlink interface is extended to allow modifying the audit_tty
attribute, and to allow sending explanatory audit events from user-space (for
example, a shell might send an event containing the final command, after the
interactive command-line editing and history expansion is performed, which
might be difficult to decipher from the TTY input alone).

Because the "audit_tty" attribute is inherited across fork (), it would be set
e.g.  for sshd restarted within an audited session.  To prevent this, the
audit_tty attribute is cleared when a process with no open TTY file
descriptors (e.g.  after daemon startup) opens a TTY.

See https://www.redhat.com/archives/linux-audit/2007-June/msg00000.html for a
more detailed rationale document for an older version of this patch.

[akpm@linux-foundation.org: build fix]
Signed-off-by: Miloslav Trmac <mitr@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Paul Fulghum <paulkf@microgate.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Steve Grubb <sgrubb@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 345 insertions, 0 deletions

diff --git a/drivers/char/tty_audit.c b/drivers/char/tty_audit.c
new file mode 100644
index 000000000000..d222012c1b0c
--- /dev/null
+++ b/drivers/char/tty_audit.c
@@ -0,0 +1,345 @@
+/*
+ * Creating audit events from TTY input.
+ *
+ * Copyright (C) 2007 Red Hat, Inc.  All rights reserved.  This copyrighted
+ * material is made available to anyone wishing to use, modify, copy, or
+ * redistribute it subject to the terms and conditions of the GNU General
+ * Public License v.2.
+ *
+ * Authors: Miloslav Trmac <mitr@redhat.com>
+ */
+
+#include <linux/audit.h>
+#include <linux/file.h>
+#include <linux/tty.h>
+
+struct tty_audit_buf {
+        atomic_t count;
+        struct mutex mutex;     /* Protects all data below */
+        int major, minor;       /* The TTY which the data is from */
+        unsigned icanon:1;
+        size_t valid;
+        unsigned char *data;    /* Allocated size N_TTY_BUF_SIZE */
+};
+
+static struct tty_audit_buf *tty_audit_buf_alloc(int major, int minor,
+                                                 int icanon)
+{
+        struct tty_audit_buf *buf;
+
+        buf = kmalloc(sizeof (*buf), GFP_KERNEL);
+        if (!buf)
+                goto err;
+        if (PAGE_SIZE != N_TTY_BUF_SIZE)
+                buf->data = kmalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
+        else
+                buf->data = (unsigned char *)__get_free_page(GFP_KERNEL);
+        if (!buf->data)
+                goto err_buf;
+        atomic_set(&buf->count, 1);
+        mutex_init(&buf->mutex);
+        buf->major = major;
+        buf->minor = minor;
+        buf->icanon = icanon;
+        buf->valid = 0;
+        return buf;
+
+err_buf:
+        kfree(buf);
+err:
+        return NULL;
+}
+
+static void tty_audit_buf_free(struct tty_audit_buf *buf)
+{
+        WARN_ON(buf->valid != 0);
+        if (PAGE_SIZE != N_TTY_BUF_SIZE)
+                kfree(buf->data);
+        else
+                free_page((unsigned long)buf->data);
+        kfree(buf);
+}
+
+static void tty_audit_buf_put(struct tty_audit_buf *buf)
+{
+        if (atomic_dec_and_test(&buf->count))
+                tty_audit_buf_free(buf);
+}
+
+/**
+ *      tty_audit_buf_push      -       Push buffered data out
+ *
+ *      Generate an audit message from the contents of @buf, which is owned by
+ *      @tsk with @loginuid.  @buf->mutex must be locked.
+ */
+static void tty_audit_buf_push(struct task_struct *tsk, uid_t loginuid,
+                               struct tty_audit_buf *buf)
+{
+        struct audit_buffer *ab;
+
+        if (buf->valid == 0)
+                return;
+        if (audit_enabled == 0)
+                return;
+        ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_TTY);
+        if (ab) {
+                char name[sizeof(tsk->comm)];
+
+                audit_log_format(ab, "tty pid=%u uid=%u auid=%u major=%d "
+                                 "minor=%d comm=", tsk->pid, tsk->uid,
+                                 loginuid, buf->major, buf->minor);
+                get_task_comm(name, tsk);
+                audit_log_untrustedstring(ab, name);
+                audit_log_format(ab, " data=");
+                audit_log_n_untrustedstring(ab, buf->valid, buf->data);
+                audit_log_end(ab);
+        }
+        buf->valid = 0;
+}
+
+/**
+ *      tty_audit_buf_push_current      -       Push buffered data out
+ *
+ *      Generate an audit message from the contents of @buf, which is owned by
+ *      the current task.  @buf->mutex must be locked.
+ */
+static void tty_audit_buf_push_current(struct tty_audit_buf *buf)
+{
+        tty_audit_buf_push(current, audit_get_loginuid(current->audit_context),
+                           buf);
+}
+
+/**
+ *      tty_audit_exit  -       Handle a task exit
+ *
+ *      Make sure all buffered data is written out and deallocate the buffer.
+ *      Only needs to be called if current->signal->tty_audit_buf != %NULL.
+ */
+void tty_audit_exit(void)
+{
+        struct tty_audit_buf *buf;
+
+        spin_lock_irq(&current->sighand->siglock);
+        buf = current->signal->tty_audit_buf;
+        current->signal->tty_audit_buf = NULL;
+        spin_unlock_irq(&current->sighand->siglock);
+        if (!buf)
+                return;
+
+        mutex_lock(&buf->mutex);
+        tty_audit_buf_push_current(buf);
+        mutex_unlock(&buf->mutex);
+
+        tty_audit_buf_put(buf);
+}
+
+/**
+ *      tty_audit_fork  -       Copy TTY audit state for a new task
+ *
+ *      Set up TTY audit state in @sig from current.  @sig needs no locking.
+ */
+void tty_audit_fork(struct signal_struct *sig)
+{
+        spin_lock_irq(&current->sighand->siglock);
+        sig->audit_tty = current->signal->audit_tty;
+        spin_unlock_irq(&current->sighand->siglock);
+        sig->tty_audit_buf = NULL;
+}
+
+/**
+ *      tty_audit_push_task     -       Flush task's pending audit data
+ */
+void tty_audit_push_task(struct task_struct *tsk, uid_t loginuid)
+{
+        struct tty_audit_buf *buf;
+
+        spin_lock_irq(&tsk->sighand->siglock);
+        buf = tsk->signal->tty_audit_buf;
+        if (buf)
+                atomic_inc(&buf->count);
+        spin_unlock_irq(&tsk->sighand->siglock);
+        if (!buf)
+                return;
+
+        mutex_lock(&buf->mutex);
+        tty_audit_buf_push(tsk, loginuid, buf);
+        mutex_unlock(&buf->mutex);
+
+        tty_audit_buf_put(buf);
+}
+
+/**
+ *      tty_audit_buf_get       -       Get an audit buffer.
+ *
+ *      Get an audit buffer for @tty, allocate it if necessary.  Return %NULL
+ *      if TTY auditing is disabled or out of memory.  Otherwise, return a new
+ *      reference to the buffer.
+ */
+static struct tty_audit_buf *tty_audit_buf_get(struct tty_struct *tty)
+{
+        struct tty_audit_buf *buf, *buf2;
+
+        buf = NULL;
+        buf2 = NULL;
+        spin_lock_irq(&current->sighand->siglock);
+        if (likely(!current->signal->audit_tty))
+                goto out;
+        buf = current->signal->tty_audit_buf;
+        if (buf) {
+                atomic_inc(&buf->count);
+                goto out;
+        }
+        spin_unlock_irq(&current->sighand->siglock);
+
+        buf2 = tty_audit_buf_alloc(tty->driver->major,
+                                   tty->driver->minor_start + tty->index,
+                                   tty->icanon);
+        if (buf2 == NULL) {
+                audit_log_lost("out of memory in TTY auditing");
+                return NULL;
+        }
+
+        spin_lock_irq(&current->sighand->siglock);
+        if (!current->signal->audit_tty)
+                goto out;
+        buf = current->signal->tty_audit_buf;
+        if (!buf) {
+                current->signal->tty_audit_buf = buf2;
+                buf = buf2;
+                buf2 = NULL;
+        }
+        atomic_inc(&buf->count);
+        /* Fall through */
+ out:
+        spin_unlock_irq(&current->sighand->siglock);
+        if (buf2)
+                tty_audit_buf_free(buf2);
+        return buf;
+}
+
+/**
+ *      tty_audit_add_data      -       Add data for TTY auditing.
+ *
+ *      Audit @data of @size from @tty, if necessary.
+ */
+void tty_audit_add_data(struct tty_struct *tty, unsigned char *data,
+                        size_t size)
+{
+        struct tty_audit_buf *buf;
+        int major, minor;
+
+        if (unlikely(size == 0))
+                return;
+
+        buf = tty_audit_buf_get(tty);
+        if (!buf)
+                return;
+
+        mutex_lock(&buf->mutex);
+        major = tty->driver->major;
+        minor = tty->driver->minor_start + tty->index;
+        if (buf->major != major || buf->minor != minor
+            || buf->icanon != tty->icanon) {
+                tty_audit_buf_push_current(buf);
+                buf->major = major;
+                buf->minor = minor;
+                buf->icanon = tty->icanon;
+        }
+        do {
+                size_t run;
+
+                run = N_TTY_BUF_SIZE - buf->valid;
+                if (run > size)
+                        run = size;
+                memcpy(buf->data + buf->valid, data, run);
+                buf->valid += run;
+                data += run;
+                size -= run;
+                if (buf->valid == N_TTY_BUF_SIZE)
+                        tty_audit_buf_push_current(buf);
+        } while (size != 0);
+        mutex_unlock(&buf->mutex);
+        tty_audit_buf_put(buf);
+}
+
+/**
+ *      tty_audit_push  -       Push buffered data out
+ *
+ *      Make sure no audit data is pending for @tty on the current process.
+ */
+void tty_audit_push(struct tty_struct *tty)
+{
+        struct tty_audit_buf *buf;
+
+        spin_lock_irq(&current->sighand->siglock);
+        if (likely(!current->signal->audit_tty)) {
+                spin_unlock_irq(&current->sighand->siglock);
+                return;
+        }
+        buf = current->signal->tty_audit_buf;
+        if (buf)
+                atomic_inc(&buf->count);
+        spin_unlock_irq(&current->sighand->siglock);
+
+        if (buf) {
+                int major, minor;
+
+                major = tty->driver->major;
+                minor = tty->driver->minor_start + tty->index;
+                mutex_lock(&buf->mutex);
+                if (buf->major == major && buf->minor == minor)
+                        tty_audit_buf_push_current(buf);
+                mutex_unlock(&buf->mutex);
+                tty_audit_buf_put(buf);
+        }
+}
+
+/**
+ *      tty_audit_opening       -       A TTY is being opened.
+ *
+ *      As a special hack, tasks that close all their TTYs and open new ones
+ *      are assumed to be system daemons (e.g. getty) and auditing is
+ *      automatically disabled for them.
+ */
+void tty_audit_opening(void)
+{
+        int disable;
+
+        disable = 1;
+        spin_lock_irq(&current->sighand->siglock);
+        if (current->signal->audit_tty == 0)
+                disable = 0;
+        spin_unlock_irq(&current->sighand->siglock);
+        if (!disable)
+                return;
+
+        task_lock(current);
+        if (current->files) {
+                struct fdtable *fdt;
+                unsigned i;
+
+                /*
+                 * We don't take a ref to the file, so we must hold ->file_lock
+                 * instead.
+                 */
+                spin_lock(&current->files->file_lock);
+                fdt = files_fdtable(current->files);
+                for (i = 0; i < fdt->max_fds; i++) {
+                        struct file *filp;
+
+                        filp = fcheck_files(current->files, i);
+                        if (filp && is_tty(filp)) {
+                                disable = 0;
+                                break;
+                        }
+                }
+                spin_unlock(&current->files->file_lock);
+        }
+        task_unlock(current);
+        if (!disable)
+                return;
+
+        spin_lock_irq(&current->sighand->siglock);
+        current->signal->audit_tty = 0;
+        spin_unlock_irq(&current->sighand->siglock);
+}