Re: [BUG][PATCH] audit: audit_log_start running on auditd should not stop

[Gao feng <gaofeng@cn.fujitsu.com>]

Hi Toshiyuki-san,
On 10/15/2013 12:43 PM, Toshiyuki Okajima wrote:
> The backlog cannot be consumed when audit_log_start is running on auditd
> even if audit_log_start calls wait_for_auditd to consume it.
> The situation is a deadlock because only auditd can consume the backlog.
> If the other process needs to send the backlog, it can be also stopped 
> by the deadlock.
> 
> So, audit_log_start running on auditd should not stop.
> 
> You can see the deadlock with the following reproducer:
>  # auditctl -a exit,always -S all
>  # reboot
> 
> Signed-off-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
> Cc: gaofeng@cn.fujitsu.com
> ---
>  kernel/audit.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/audit.c b/kernel/audit.c
> index 7b0e23a..ce1fb38 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -1098,6 +1098,9 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
>  	int reserve;
>  	unsigned long timeout_start = jiffies;
>  
> +	if (audit_pid && audit_pid == current->pid)
> +		gfp_mask &= ~__GFP_WAIT;
> +
>  	if (audit_initialized != AUDIT_INITIALIZED)
>  		return NULL;
>  
> 

Hmm, I see, There may be other code paths that auditd can call audit_log_start except
audit_log_config_change. so it's better to handle this problem in audit_log_start.

but current task is only meaningful when gfp_mask & __GFP_WAIT is true.
so maybe the below patch is what you want.

diff --git a/kernel/audit.c b/kernel/audit.c
index 7b0e23a..10b4545 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -1095,7 +1095,9 @@ struct audit_buffer *audit_log_start(struct audit_context
        struct audit_buffer     *ab     = NULL;
        struct timespec         t;
        unsigned int            uninitialized_var(serial);
-       int reserve;
+       int reserve = 5; /* Allow atomic callers to go up to five
+                           entries over the normal backlog limit */
+
        unsigned long timeout_start = jiffies;

        if (audit_initialized != AUDIT_INITIALIZED)
@@ -1104,11 +1106,12 @@ struct audit_buffer *audit_log_start(struct audit_contex
        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
-                               entries over the normal backlog limit */
+       if (gfp_mask & __GFP_WAIT) {
+               if (audit_pid && audit_pid == current->pid)
+                       gfp_mask &= ~__GFP_WAIT;
+               else
+                       reserve = 0;
+       }

        while (audit_backlog_limit
               && skb_queue_len(&audit_skb_queue) > audit_backlog_limit + reserv


Thanks