Re: [PATCH 1/2] audit: fix soft lockups due to loop in audit_log_start() wh,en audit_backlog_limit exceeded

[Dave Kleikamp <dave.kleikamp@oracle.com>]

Reviewed this before seeing Luiz' patch, so this is not an endorsement
of one approach over the other, but adding my ack.

Shaggy

On 09/02/2013 10:49 PM, Chuck Anderson wrote:
> audit: fix softlockups due to loop in audit_log_start() when
> audit_backlog_limit exceeded
> 
> author: Dan Duval <dan.duval@oracle.com>
> 
> This patch fixes a bug in kernel/audit that can cause many soft lockups
> and prevent the boot of a large memory 3.8 system:
> 
>   BUG: soft lockup - CPU#66 stuck for 22s! [udevd:9559]
>   RIP: 0010:[<ffffffff810d1d06>]  [<ffffffff810d1d06>]
> audit_log_start+0xe6/0x350
>   Call Trace:
>    [<ffffffff8108ea30>] ? try_to_wake_up+0x2d0/0x2d0
>    [<ffffffff810d8d6f>] audit_log_exit+0x3f/0x590
>    [<ffffffff810d975d>] __audit_syscall_exit+0x28d/0x2c0
>    [<ffffffff815e0440>] sysret_audit+0x17/0x21
> 
> audit_log_start() will call wait_for_auditd() to delay returning an
> audit_buffer if there are too many SKBs on audit_skb_queue.
> wait_for_auditd() puts itself on the audit_backlog_wait queue and
> sleeps for sleep_time jiffies or until it is (normally) woken when
> kauditd takes an SKB off of audit_skb_queue.  wait_for_auditd() returns
> to audit_log_start() which checks to see if audit_skb_queue still has
> too many SKBs.  If there are still too many, audit_log_start() will
> continue to call wait_for_auditd() in a loop until
> audit_backlog_wait_time has passed.  audit_log_start() will then
> complain ("backlog limit exceeded"); set audit_backlog_wait_time
> to NULL so other waiters will fall out of the loop when woken up; wake
> up any waiters in wait_for_auditd(); return NULL which tells the caller
> that an audit_buffer could not be allocated.
> 
> A bug in audit_log_start() prevents it from breaking out of the
> wait_for_auditd() loop when audit_backlog_wait_time has passed.
> Instead, it will loop in the audit_skb_queue-is-too-long while-loop
> eventually causing a soft lockup.  There can (and most likely will)
> be multiple threads looping.  The fix is to continue in the while-loop
> only if sleep_time was greater than 0 (audit_backlog_wait_time has not
> passed).
> 
> Another bug in audit_log_start() prevents audit_backlog_wait_time from
> working as expected.  audit_backlog_wait_time is normally the time
> period that audit_log_start() will wait for the number of SKBs on
> audit_skb_queue to fall below the too-many threshold.  If
> audit_backlog_wait_time passes, audit_log_start() will set it to
> audit_backlog_wait_overflow, which is zero, and wake up any waiters in
> wait_for_auditd().  audit_backlog_wait_time is now zero so the waiters
> will fall out of the loop when they return to audit_log_start().  That
> is expected behavior.  But audit_backlog_wait_time is not reset to its
> initial value when audit_skb_queue's length is no longer too long.
> Subsequent calls to audit_log_start() when audit_skb_queue is too long
> will not wait in wait_for_auditd(), instead returning NULL.  The fix
> is to set audit_backlog_wait_time to its initial value when
> audit_skb_queue passes the size test, potentially resetting it.
> 
> A third issue is to have both audit_log_start() and wait_for_auditd()
> use the same limit value for the length of audit_skb_queue.  It isn't
> necessary today but (1) assumptions may change in the future and (2)
> is one less oddity for a reader to have to verify.
> 
> Signed-off-by: Dan Duval <dan.duval@oracle.com>
> Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com>
Acked-by: Dave Kleikamp <dave.kleikamp@oracle.com>
> ---
>  kernel/audit.c |   28 ++++++++++++++++------------
>  1 files changed, 16 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/audit.c b/kernel/audit.c
> index 91e53d0..9a78dde 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -103,9 +103,11 @@ static int    audit_rate_limit;
> 
>  /* Number of outstanding audit_buffers allowed. */
>  static int    audit_backlog_limit = 64;
> -static int    audit_backlog_wait_time = 60 * HZ;
>  static int    audit_backlog_wait_overflow = 0;
> 
> +#define AUDIT_BACKLOG_WAIT_TIME (60 * HZ)
> +static int    audit_backlog_wait_time = AUDIT_BACKLOG_WAIT_TIME;
> +
>  /* The identity of the user shutting down the audit system. */
>  kuid_t        audit_sig_uid = INVALID_UID;
>  pid_t        audit_sig_pid = -1;
> @@ -1053,14 +1055,14 @@ static inline void audit_get_stamp(struct
> audit_context
> *ctx,
>  /*
>   * Wait for auditd to drain the queue a little
>   */
> -static void wait_for_auditd(unsigned long sleep_time)
> +static void wait_for_auditd(unsigned long sleep_time, int limit)
>  {
>      DECLARE_WAITQUEUE(wait, current);
>      set_current_state(TASK_UNINTERRUPTIBLE);
>      add_wait_queue(&audit_backlog_wait, &wait);
> 
>      if (audit_backlog_limit &&
> -        skb_queue_len(&audit_skb_queue) > audit_backlog_limit)
> +        skb_queue_len(&audit_skb_queue) > limit)
>          schedule_timeout(sleep_time);
> 
>      __set_current_state(TASK_RUNNING);
> @@ -1095,8 +1097,8 @@ struct audit_buffer *audit_log_start(struct
> audit_context
> *ctx, gfp_t gfp_mask,
>      struct audit_buffer    *ab    = NULL;
>      struct timespec        t;
>      unsigned int        uninitialized_var(serial);
> -    int reserve;
>      unsigned long timeout_start = jiffies;
> +    int limit;
> 
>      if (audit_initialized != AUDIT_INITIALIZED)
>          return NULL;
> @@ -1104,22 +1106,22 @@ struct audit_buffer *audit_log_start(struct
> audit_contex
> t *ctx, gfp_t gfp_mask,
>      if (unlikely(audit_filter_type(type)))
>          return NULL;
> 
> -    if (gfp_mask & __GFP_WAIT)
> -        reserve = 0;
> -    else
> -        reserve = 5; /* Allow atomic callers to go up to five
> +    limit = audit_backlog_limit;
> +    if (!(gfp_mask & __GFP_WAIT))
> +        limit += 5;  /* Allow atomic callers to go up to five
>                  entries over the normal backlog limit */
> 
>      while (audit_backlog_limit
> -           && skb_queue_len(&audit_skb_queue) > audit_backlog_limit +
> reserv
> e) {
> +           && skb_queue_len(&audit_skb_queue) > limit) {
>          if (gfp_mask & __GFP_WAIT && audit_backlog_wait_time) {
>              unsigned long sleep_time;
> 
>              sleep_time = timeout_start + audit_backlog_wait_time -
>                      jiffies;
> -            if ((long)sleep_time > 0)
> -                wait_for_auditd(sleep_time);
> -            continue;
> +            if ((long)sleep_time > 0) {
> +                wait_for_auditd(sleep_time, limit);
> +                continue;
> +            }
>          }
>          if (audit_rate_check() && printk_ratelimit())
>              printk(KERN_WARNING
> @@ -1133,6 +1135,8 @@ struct audit_buffer *audit_log_start(struct
> audit_context
> *ctx, gfp_t gfp_mask,
>          return NULL;
>      }
> 
> +    audit_backlog_wait_time = AUDIT_BACKLOG_WAIT_TIME;
> +
>      ab = audit_buffer_alloc(ctx, gfp_mask, type);
>      if (!ab) {
>          audit_log_lost("out of memory in audit_log_start");