audit: report audit wait metric in audit status reply

In environments where the preservation of audit events and predictable usage of system memory are prioritized, admins may use a combination of --backlog_wait_time and -b options at the risk of degraded performance resulting from backlog waiting. In some cases, this risk may be preferred to lost events or unbounded memory usage. Ideally, this risk can be mitigated by making adjustments when backlog waiting is detected.

However, detection can be difficult using the currently available metrics. For example, an admin attempting to debug degraded performance may falsely believe a full backlog indicates backlog waiting. It may turn out the backlog frequently fills up but drains quickly.

To make it easier to reliably track degraded performance to backlog waiting, this patch makes the following changes:

Add a new field backlog_wait_time_total to the audit status reply. Initialize this field to zero. Add to this field the total time spent by the current task on scheduled timeouts while the backlog limit is exceeded. Reset field to zero upon request via AUDIT_SET.

Tested on Ubuntu 18.04 using complementary changes to the audit-userspace and audit-testsuite:
- https://github.com/linux-audit/audit-userspace/pull/134
- https://github.com/linux-audit/audit-testsuite/pull/97

Signed-off-by Max Englander <[email protected]>

---
Patch changelogs between v1 and v2:
- Instead of printing a warning when backlog waiting occurs, add duration of backlog waiting to cumulative sum, and report this sum in audit status reply.

Patch changelogs between v2 and v3:
- Rename backlog_wait_sum to backlog_wait_time_actual.
- Drop unneeded and unwanted header flags AUDIT_FEATURE_BITMAP_BACKLOG_WAIT_SUM and AUDIT_VERSION_BACKLOG_WAIT_SUM.
- Increment backlog_wait_time_actual counter after every call to schedule_timeout rather than once after enqueuing (or losing) an audit record.
- Add support for resetting backlog_wait_time_actual counter to zero upon request via AUDIT_SET.

 include/uapi/linux/audit.h | 18 +++++++++++-------
 kernel/audit.c             | 35 +++++++++++++++++++++++++----------
 2 files changed, 36 insertions(+), 17 deletions(-)

include/uapi/linux/audit.h +11 -7

index a534d71e689a..92d72965ad44 100644
@@ -332,14 +332,15 @@ enum {
 };
 
 /* Status symbols */
-                               /* Mask values */
-#define AUDIT_STATUS_ENABLED           0x0001
-#define AUDIT_STATUS_FAILURE           0x0002
-#define AUDIT_STATUS_PID               0x0004
+                                               /* Mask values */
+#define AUDIT_STATUS_ENABLED                   0x0001
+#define AUDIT_STATUS_FAILURE                   0x0002
+#define AUDIT_STATUS_PID                       0x0004
 #define AUDIT_STATUS_RATE_LIMIT                0x0008
-#define AUDIT_STATUS_BACKLOG_LIMIT     0x0010
-#define AUDIT_STATUS_BACKLOG_WAIT_TIME 0x0020
-#define AUDIT_STATUS_LOST              0x0040
+#define AUDIT_STATUS_BACKLOG_LIMIT             0x0010
+#define AUDIT_STATUS_BACKLOG_WAIT_TIME         0x0020
+#define AUDIT_STATUS_LOST                      0x0040
+#define AUDIT_STATUS_BACKLOG_WAIT_TIME_ACTUAL  0x0080
 
 #define AUDIT_FEATURE_BITMAP_BACKLOG_LIMIT     0x00000001
 #define AUDIT_FEATURE_BITMAP_BACKLOG_WAIT_TIME 0x00000002

@@ -466,6 +467,9 @@ struct audit_status {
                __u32   feature_bitmap; /* bitmap of kernel audit features */
        };
        __u32           backlog_wait_time;/* message queue wait timeout */
+       __u32           backlog_wait_time_actual;/* time spent waiting while
+                                                 * message limit exceeded
+                                                 */
 };
 
 struct audit_features {

kernel/audit.c +25 -11

index 87f31bf1f0a0..33c640fdacf7 100644
@@ -136,6 +136,11 @@ u32                audit_sig_sid = 0;
 */
 static atomic_t        audit_lost = ATOMIC_INIT(0);
 
+/* Monotonically increasing sum of time the kernel has spent
+ * waiting while the backlog limit is exceeded.
+ */
+static atomic_t audit_backlog_wait_time_actual = ATOMIC_INIT(0);
+
 /* Hash for inode-based rules */
 struct list_head audit_inode_hash[AUDIT_INODE_BUCKETS];
 

@@ -1193,17 +1198,18 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
        case AUDIT_GET: {
                struct audit_status     s;
                memset(&s, 0, sizeof(s));
-               s.enabled               = audit_enabled;
-               s.failure               = audit_failure;
+               s.enabled                  = audit_enabled;
+               s.failure                  = audit_failure;
                /* NOTE: use pid_vnr() so the PID is relative to the current
                 *       namespace */
-               s.pid                   = auditd_pid_vnr();
-               s.rate_limit            = audit_rate_limit;
-               s.backlog_limit         = audit_backlog_limit;
-               s.lost                  = atomic_read(&audit_lost);
-               s.backlog               = skb_queue_len(&audit_queue);
-               s.feature_bitmap        = AUDIT_FEATURE_BITMAP_ALL;
-               s.backlog_wait_time     = audit_backlog_wait_time;
+               s.pid                      = auditd_pid_vnr();
+               s.rate_limit               = audit_rate_limit;
+               s.backlog_limit            = audit_backlog_limit;
+               s.lost                     = atomic_read(&audit_lost);
+               s.backlog                  = skb_queue_len(&audit_queue);
+               s.feature_bitmap           = AUDIT_FEATURE_BITMAP_ALL;
+               s.backlog_wait_time        = audit_backlog_wait_time;
+               s.backlog_wait_time_actual = atomic_read(&audit_backlog_wait_time_actual);
                audit_send_reply(skb, seq, AUDIT_GET, 0, 0, &s, sizeof(s));
                break;
        }

@@ -1307,6 +1313,12 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
                        audit_log_config_change("lost", 0, lost, 1);
                        return lost;
                }
+               if (s.mask == AUDIT_STATUS_BACKLOG_WAIT_TIME_ACTUAL) {
+                       u32 actual = atomic_xchg(&audit_backlog_wait_time_actual, 0);
+
+                       audit_log_config_change("backlog_wait_time_actual", 0, actual, 1);
+                       return actual;
+               }
                break;
        }
        case AUDIT_GET_FEATURE:

@@ -1778,12 +1790,15 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
                        /* sleep if we are allowed and we haven't exhausted our
                         * backlog wait limit */
                        if (gfpflags_allow_blocking(gfp_mask) && (stime > 0)) {
+                               long rtime = stime;
+
                                DECLARE_WAITQUEUE(wait, current);
 
                                add_wait_queue_exclusive(&audit_backlog_wait,
                                                         &wait);
                                set_current_state(TASK_UNINTERRUPTIBLE);
-                               stime = schedule_timeout(stime);
+                               stime = schedule_timeout(rtime);
+                               atomic_add(rtime - stime, &audit_backlog_wait_time_actual);
                                remove_wait_queue(&audit_backlog_wait, &wait);
                        } else {
                                if (audit_rate_check() && printk_ratelimit())
--

2.17.1

Quoted 44 lines · Max Englander

In environments where the preservation of audit events and predictable usage of system memory are prioritized, admins may use a combination of --backlog_wait_time and -b options at the risk of degraded performance resulting from backlog waiting. In some cases, this risk may be preferred to lost events or unbounded memory usage. Ideally, this risk can be mitigated by making adjustments when backlog waiting is detected.

However, detection can be difficult using the currently available metrics. For example, an admin attempting to debug degraded performance may falsely believe a full backlog indicates backlog waiting. It may turn out the backlog frequently fills up but drains quickly.

To make it easier to reliably track degraded performance to backlog waiting, this patch makes the following changes:

Add a new field backlog_wait_time_total to the audit status reply. Initialize this field to zero. Add to this field the total time spent by the current task on scheduled timeouts while the backlog limit is exceeded. Reset field to zero upon request via AUDIT_SET.

Tested on Ubuntu 18.04 using complementary changes to the audit-userspace and audit-testsuite:
- https://github.com/linux-audit/audit-userspace/pull/134
- https://github.com/linux-audit/audit-testsuite/pull/97

Signed-off-by Max Englander <[email protected]>

---
Patch changelogs between v1 and v2:
- Instead of printing a warning when backlog waiting occurs, add duration of backlog waiting to cumulative sum, and report this sum in audit status reply.

Patch changelogs between v2 and v3:
- Rename backlog_wait_sum to backlog_wait_time_actual.
- Drop unneeded and unwanted header flags AUDIT_FEATURE_BITMAP_BACKLOG_WAIT_SUM and AUDIT_VERSION_BACKLOG_WAIT_SUM.
- Increment backlog_wait_time_actual counter after every call to schedule_timeout rather than once after enqueuing (or losing) an audit record.
- Add support for resetting backlog_wait_time_actual counter to zero upon request via AUDIT_SET.

Quoted 3 lines

 include/uapi/linux/audit.h | 18 +++++++++++-------
 kernel/audit.c             | 35 +++++++++++++++++++++++++----------
 2 files changed, 36 insertions(+), 17 deletions(-)

This looks okay to me, thanks for the fixes Max.

Steve, does the associated userspace patch look okay to you?

Quoted 51 lines · Paul Moore

Quoted 44 lines · Max Englander

In environments where the preservation of audit events and predictable usage of system memory are prioritized, admins may use a combination of --backlog_wait_time and -b options at the risk of degraded performance resulting from backlog waiting. In some cases, this risk may be preferred to lost events or unbounded memory usage. Ideally, this risk can be mitigated by making adjustments when backlog waiting is detected.

However, detection can be difficult using the currently available metrics. For example, an admin attempting to debug degraded performance may falsely believe a full backlog indicates backlog waiting. It may turn out the backlog frequently fills up but drains quickly.

To make it easier to reliably track degraded performance to backlog waiting, this patch makes the following changes:

Add a new field backlog_wait_time_total to the audit status reply. Initialize this field to zero. Add to this field the total time spent by the current task on scheduled timeouts while the backlog limit is exceeded. Reset field to zero upon request via AUDIT_SET.

Tested on Ubuntu 18.04 using complementary changes to the audit-userspace and audit-testsuite:
- https://github.com/linux-audit/audit-userspace/pull/134
- https://github.com/linux-audit/audit-testsuite/pull/97

Signed-off-by Max Englander <[email protected]>

---
Patch changelogs between v1 and v2:
- Instead of printing a warning when backlog waiting occurs, add duration of backlog waiting to cumulative sum, and report this sum in audit status reply.

Patch changelogs between v2 and v3:
- Rename backlog_wait_sum to backlog_wait_time_actual.
- Drop unneeded and unwanted header flags AUDIT_FEATURE_BITMAP_BACKLOG_WAIT_SUM and AUDIT_VERSION_BACKLOG_WAIT_SUM.
- Increment backlog_wait_time_actual counter after every call to schedule_timeout rather than once after enqueuing (or losing) an audit record.
- Add support for resetting backlog_wait_time_actual counter to zero upon request via AUDIT_SET.

Quoted 3 lines

 include/uapi/linux/audit.h | 18 +++++++++++-------
 kernel/audit.c             | 35 +++++++++++++++++++++++++----------
 2 files changed, 36 insertions(+), 17 deletions(-)

This looks okay to me, thanks for the fixes Max.

Quoted 1 line

Steve, does the associated userspace patch look okay to you?

Steve, any comments on the userspace patch? Did I miss a reply in my inbox perhaps?

If I don't see any feedback by the end of the week I'll plan on merging this into audit/next.

Quoted 54 lines · Paul Moore

Quoted 52 lines · Paul Moore

Quoted 44 lines · Max Englander

In environments where the preservation of audit events and predictable usage of system memory are prioritized, admins may use a combination of --backlog_wait_time and -b options at the risk of degraded performance resulting from backlog waiting. In some cases, this risk may be preferred to lost events or unbounded memory usage. Ideally, this risk can be mitigated by making adjustments when backlog waiting is detected.

However, detection can be difficult using the currently available metrics. For example, an admin attempting to debug degraded performance may falsely believe a full backlog indicates backlog waiting. It may turn out the backlog frequently fills up but drains quickly.

To make it easier to reliably track degraded performance to backlog waiting, this patch makes the following changes:

Add a new field backlog_wait_time_total to the audit status reply. Initialize this field to zero. Add to this field the total time spent by the current task on scheduled timeouts while the backlog limit is exceeded. Reset field to zero upon request via AUDIT_SET.

Tested on Ubuntu 18.04 using complementary changes to the audit-userspace and audit-testsuite:
- https://github.com/linux-audit/audit-userspace/pull/134
- https://github.com/linux-audit/audit-testsuite/pull/97

Signed-off-by Max Englander <[email protected]>

---
Patch changelogs between v1 and v2:
- Instead of printing a warning when backlog waiting occurs, add duration of backlog waiting to cumulative sum, and report this sum in audit status reply.

Patch changelogs between v2 and v3:
- Rename backlog_wait_sum to backlog_wait_time_actual.
- Drop unneeded and unwanted header flags AUDIT_FEATURE_BITMAP_BACKLOG_WAIT_SUM and AUDIT_VERSION_BACKLOG_WAIT_SUM.
- Increment backlog_wait_time_actual counter after every call to schedule_timeout rather than once after enqueuing (or losing) an audit record.
- Add support for resetting backlog_wait_time_actual counter to zero upon request via AUDIT_SET.

Quoted 3 lines

 include/uapi/linux/audit.h | 18 +++++++++++-------
 kernel/audit.c             | 35 +++++++++++++++++++++++++----------
 2 files changed, 36 insertions(+), 17 deletions(-)

This looks okay to me, thanks for the fixes Max.

Steve, does the associated userspace patch look okay to you?

Quoted 5 lines

Steve, any comments on the userspace patch? Did I miss a reply in my inbox perhaps?

If I don't see any feedback by the end of the week I'll plan on merging this into audit/next.

It's been over two weeks with no comment, so I went ahead and merged this into audit/next. Thanks for your patience Max!

Signature · 3 lines

-- 
paul moore
www.paul-moore.com