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>

8 files changed, 46 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);
 
diff --git a/include/linux/audit.h b/include/linux/audit.h
index cb708eb8accc..d66220dac364 100644
--- a/include/linux/audit.h
+++ b/include/linux/audit.h
@@ -356,6 +356,7 @@ extern int __audit_log_bprm_fcaps(struct linux_binprm *bprm,
 extern void __audit_log_capset(const struct cred *new, const struct cred *old);
 extern void __audit_mmap_fd(int fd, int flags);
 extern void __audit_log_kern_module(char *name);
+extern void __audit_fanotify(unsigned int response);
 
 static inline void audit_ipc_obj(struct kern_ipc_perm *ipcp)
 {
@@ -452,6 +453,12 @@ static inline void audit_log_kern_module(char *name)
                 __audit_log_kern_module(name);
 }
 
+static inline void audit_fanotify(unsigned int response)
+{
+        if (!audit_dummy_context())
+                __audit_fanotify(response);
+}
+
 extern int audit_n_rules;
 extern int audit_signals;
 #else /* CONFIG_AUDITSYSCALL */
@@ -568,6 +575,9 @@ static inline void audit_log_kern_module(char *name)
 {
 }
 
+static inline void audit_fanotify(unsigned int response)
+{ }
+
 static inline void audit_ptrace(struct task_struct *t)
 { }
 #define audit_n_rules 0
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index c6c69318752b..4a474f972910 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -190,6 +190,7 @@ struct fsnotify_group {
                         int f_flags;
                         unsigned int max_marks;
                         struct user_struct *user;
+                        bool audit;
                 } fanotify_data;
 #endif /* CONFIG_FANOTIFY */
         };
diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
index 0714a66f0e0c..221f8b7f01b2 100644
--- a/include/uapi/linux/audit.h
+++ b/include/uapi/linux/audit.h
@@ -112,6 +112,7 @@
 #define AUDIT_FEATURE_CHANGE    1328    /* audit log listing feature changes */
 #define AUDIT_REPLACE           1329    /* Replace auditd if this packet unanswerd */
 #define AUDIT_KERN_MODULE       1330    /* Kernel Module events */
+#define AUDIT_FANOTIFY          1331    /* Fanotify access decision */
 
 #define AUDIT_AVC               1400    /* SE Linux avc denial or grant */
 #define AUDIT_SELINUX_ERR       1401    /* Internal SE Linux Errors */
diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
index 030508d195d3..5dda19a9a947 100644
--- a/include/uapi/linux/fanotify.h
+++ b/include/uapi/linux/fanotify.h
@@ -35,6 +35,7 @@
 
 #define FAN_UNLIMITED_QUEUE     0x00000010
 #define FAN_UNLIMITED_MARKS     0x00000020
+#define FAN_ENABLE_AUDIT        0x00000040
 
 #define FAN_ALL_INIT_FLAGS      (FAN_CLOEXEC | FAN_NONBLOCK | \
                                  FAN_ALL_CLASS_BITS | FAN_UNLIMITED_QUEUE |\
@@ -99,6 +100,8 @@ struct fanotify_response {
 /* Legit userspace responses to a _PERM event */
 #define FAN_ALLOW       0x01
 #define FAN_DENY        0x02
+#define FAN_AUDIT       0x10    /* Bit mask to create audit record for result */
+
 /* No fd set in event */
 #define FAN_NOFD        -1
 
diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index ecc23e25c9eb..9c723e978245 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -2390,6 +2390,12 @@ void __audit_log_kern_module(char *name)
         context->type = AUDIT_KERN_MODULE;
 }
 
+void __audit_fanotify(unsigned int response)
+{
+        audit_log(current->audit_context, GFP_KERNEL,
+                AUDIT_FANOTIFY, "resp=%u", response);
+}
+
 static void audit_log_task(struct audit_buffer *ab)
 {
         kuid_t auid, uid;