[BUG][PATCH][RFC] audit: hang up in audit_log_start executed on auditd

Toshiyuki Okajima
2013-10-11T01:36:45Z

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@jp.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..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;
+
if (audit_initialized != AUDIT_INITIALIZED)
return NULL;

--
1.5.5.6