Re: [BUG][PATCH][RFC] audit: hang up in audit_log_start executedon auditd

[Toshiyuki Okajima (smtp-b.css)]

Gao-san,

(2013/10/11 18:33), Gao feng wrote:
> On 10/11/2013 09:36 AM, Toshiyuki Okajima wrote:
>> Hi. 
>>
>> The following reproducer causes auditd daemon hang up.
>> (But the hang up is released after the audit_backlog_wait_time passes.)
>>  # auditctl -a exit,always -S all
>>  # reboot
>>
>>
>> I reproduced the hangup on KVM, and then got a crash dump.
>> After I analyzed the dump, I found auditd daemon hung up in audit_log_start. 
>> (I have confirmed it on linux-3.12-rc4.)
>>
>> Like this:
>> crash> bt 1426
>> PID: 1426   TASK: ffff88007b63e040  CPU: 1   COMMAND: "auditd"
>>  #0 [ffff88007cb93918] __schedule at ffffffff8155d980
>>  #1 [ffff88007cb939b0] schedule at ffffffff8155de99
>>  #2 [ffff88007cb939c0] schedule_timeout at ffffffff8155b840
>>  #3 [ffff88007cb93a60] audit_log_start at ffffffff810d3ce5
>>  #4 [ffff88007cb93b20] audit_log_config_change at ffffffff810d3ece
>>  #5 [ffff88007cb93b60] audit_receive_msg at ffffffff810d4fd6
>>  #6 [ffff88007cb93c00] audit_receive at ffffffff810d5173
>>  #7 [ffff88007cb93c30] netlink_unicast at ffffffff814c5269
>>  #8 [ffff88007cb93c90] netlink_sendmsg at ffffffff814c6386
>>  #9 [ffff88007cb93d20] sock_sendmsg at ffffffff814813c0
>> #10 [ffff88007cb93e30] SYSC_sendto at ffffffff81481524
>> #11 [ffff88007cb93f70] sys_sendto at ffffffff8148157e
>> #12 [ffff88007cb93f80] system_call_fastpath at ffffffff81568052
>>     RIP: 00007f5c47f7fba3  RSP: 00007fffcf21a118  RFLAGS: 00010202
>>     RAX: 000000000000002c  RBX: ffffffff81568052  RCX: 0000000000000000
>>     RDX: 0000000000000030  RSI: 00007fffcf21e7d0  RDI: 0000000000000003
>>     RBP: 00007fffcf21e7d0   R8: 00007fffcf21a130   R9: 000000000000000c
>>     R10: 0000000000000000  R11: 0000000000000293  R12: ffffffff8148157e
>>     R13: ffff88007cb93f78  R14: 0000000000000020  R15: 0000000000000030
>>     ORIG_RAX: 000000000000002c  CS: 0033  SS: 002b
>>
>>
>> The reason is that auditd daemon itself cannot consume its backlog 
>> while audit_log_start is calling schedule_timeout on auditd daemon.  
>> So, that is a deadlock!
>>
>> Therefore, I think audit_log_start shouldn't handle auditd's backlog
>> when auditd daemon executes audit_log_start.
>>
>> For example, I made the following fix patch.
>> --------------------------------------------------------------
>> auditd daemon can execute the audit_log_start, and then it can cause 
>> a hang up because only auditd daemon can consume the backlog.
>> So, audit_log_start executed by auditd daemon should not handle the backlog 
>> in case auditd daemon hangs up (while wait_for_auditd is calling).
>>
>> Signed-off-by: Toshiyuki Okajima <toshi.okajima@xxxxxxxxxxxxxx>
>> ---
>>  kernel/audit.c |    3 +++
>>  1 files changed, 3 insertions(+), 0 deletions(-)
>>
>> diff --git a/kernel/audit.c b/kernel/audit.c
>> index 7b0e23a..86c389e 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))
>> +		return NULL;
>> +
> 
> audit_log_start can be called in interrupt context, such as iptables AUDIT module,
> we can't use current here.
> please try the patch below.
> 
> diff --git a/kernel/audit.c b/kernel/audit.c
> index 7b0e23a..1f35f3d 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -270,9 +270,13 @@ static int audit_log_config_change(char *function_name, int new, int old,
>                                    int allow_changes)
>  {
>         struct audit_buffer *ab;
> +       gfp_t gfp_mask = GFP_KERNEL;
>         int rc = 0;
> 
> -       ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_CONFIG_CHANGE);
> +       if (audit_pid && audit_pid == current->pid)
> +               gfp_mask = GFP_ATOMIC;
> +
> +       ab = audit_log_start(NULL, gfp_mask, AUDIT_CONFIG_CHANGE);
>         if (unlikely(!ab))
>                 return rc;
>         audit_log_format(ab, "%s=%d old=%d", function_name, new, old);
> 
> 

Thanks for your suggestion. I will try it.

Regards,
Toshiyuki Okajima


> Thanks
> 
> 



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