audit: Record fanotify access control decisions

The fanotify interface allows user space daemons to make access
control decisions. Under common criteria requirements, we need to
optionally record decisions based on policy. This patch adds a bit mask,
FAN_AUDIT, that a user space daemon can 'or' into the response decision
which will tell the kernel that it made a decision and record it.

It would be used something like this in user space code:

  response.response = FAN_DENY | FAN_AUDIT;
  write(fd, &response, sizeof(struct fanotify_response));

When the syscall ends, the audit system will record the decision as a
AUDIT_FANOTIFY auxiliary record to denote that the reason this event
occurred is the result of an access control decision from fanotify
rather than DAC or MAC policy.

A sample event looks like this:

type=PATH msg=audit(1504310584.332:290): item=0 name="./evil-ls"
inode=1319561 dev=fc:03 mode=0100755 ouid=1000 ogid=1000 rdev=00:00
obj=unconfined_u:object_r:user_home_t:s0 nametype=NORMAL
type=CWD msg=audit(1504310584.332:290): cwd="/home/sgrubb"
type=SYSCALL msg=audit(1504310584.332:290): arch=c000003e syscall=2
success=no exit=-1 a0=32cb3fca90 a1=0 a2=43 a3=8 items=1 ppid=901
pid=959 auid=1000 uid=1000 gid=1000 euid=1000 suid=1000
fsuid=1000 egid=1000 sgid=1000 fsgid=1000 tty=pts1 ses=3 comm="bash"
exe="/usr/bin/bash" subj=unconfined_u:unconfined_r:unconfined_t:
s0-s0:c0.c1023 key=(null)
type=FANOTIFY msg=audit(1504310584.332:290): resp=2

Prior to using the audit flag, the developer needs to call
fanotify_init or'ing in FAN_ENABLE_AUDIT to ensure that the kernel
supports auditing. The calling process must also have the CAP_AUDIT_WRITE
capability.

Signed-off-by: sgrubb <sgrubb@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

3 files changed, 25 insertions, 2 deletions

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 2fa99aeaa095..1968d21a3f37 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -9,6 +9,7 @@
 #include <linux/sched/user.h>
 #include <linux/types.h>
 #include <linux/wait.h>
+#include <linux/audit.h>
 
 #include "fanotify.h"
 
@@ -78,7 +79,7 @@ static int fanotify_get_response(struct fsnotify_group *group,
 	fsnotify_finish_user_wait(iter_info);
 out:
 	/* userspace responded, convert to something usable */
-	switch (event->response) {
+	switch (event->response & ~FAN_AUDIT) {
 	case FAN_ALLOW:
 		ret = 0;
 		break;
@@ -86,6 +87,11 @@ out:
 	default:
 		ret = -EPERM;
 	}
+
+	/* Check if the response should be audited */
+	if (event->response & FAN_AUDIT)
+		audit_fanotify(event->response & ~FAN_AUDIT);
+
 	event->response = 0;
 
 	pr_debug("%s: group=%p event=%p about to return ret=%d\n", __func__,
diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 907a481ac781..0455ea729384 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -179,7 +179,7 @@ static int process_access_response(struct fsnotify_group *group,
 	 * userspace can send a valid response or we will clean it up after the
 	 * timeout
 	 */
-	switch (response) {
+	switch (response & ~FAN_AUDIT) {
 	case FAN_ALLOW:
 	case FAN_DENY:
 		break;
@@ -190,6 +190,9 @@ static int process_access_response(struct fsnotify_group *group,
 	if (fd < 0)
 		return -EINVAL;
 
+	if ((response & FAN_AUDIT) && !group->fanotify_data.audit)
+		return -EINVAL;
+
 	event = dequeue_event(group, fd);
 	if (!event)
 		return -ENOENT;
@@ -721,7 +724,11 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
+#ifdef CONFIG_AUDITSYSCALL
+	if (flags & ~(FAN_ALL_INIT_FLAGS | FAN_ENABLE_AUDIT))
+#else
 	if (flags & ~FAN_ALL_INIT_FLAGS)
+#endif
 		return -EINVAL;
 
 	if (event_f_flags & ~FANOTIFY_INIT_ALL_EVENT_F_BITS)
@@ -805,6 +812,13 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 		group->fanotify_data.max_marks = FANOTIFY_DEFAULT_MAX_MARKS;
 	}
 
+	if (flags & FAN_ENABLE_AUDIT) {
+		fd = -EPERM;
+		if (!capable(CAP_AUDIT_WRITE))
+			goto out_destroy_group;
+		group->fanotify_data.audit = true;
+	}
+
 	fd = anon_inode_getfd("[fanotify]", &fanotify_fops, group, f_flags);
 	if (fd < 0)
 		goto out_destroy_group;
diff --git a/fs/notify/fdinfo.c b/fs/notify/fdinfo.c
index dd63aa9a6f9a..645ab561e790 100644
--- a/fs/notify/fdinfo.c
+++ b/fs/notify/fdinfo.c
@@ -156,6 +156,9 @@ void fanotify_show_fdinfo(struct seq_file *m, struct file *f)
 	if (group->fanotify_data.max_marks == UINT_MAX)
 		flags |= FAN_UNLIMITED_MARKS;
 
+	if (group->fanotify_data.audit)
+		flags |= FAN_ENABLE_AUDIT;
+
 	seq_printf(m, "fanotify flags:%x event-flags:%x\n",
 		   flags, group->fanotify_data.f_flags);