[CentOS] meaning and cause of kernel panic ??

Hello all. I have been running running centos 4.5 final for some time with
any problems. Been running great, until today. System was frozen and
/var/log/messages showed:

Aug 15 23:01:28 mydomain kernel: Unable to handle kernel NULL pointer
dereference at virtual address 00000000
Aug 15 23:01:28 mydomain kernel:  printing eip:
Aug 15 23:01:28 mydomain kernel: c01b2de4
Aug 15 23:01:28 mydomain kernel: *pde = 0790d067
Aug 15 23:01:28 mydomain kernel: Oops: 0000 [#1]
Aug 15 23:01:28 mydomain kernel: Modules linked in: md5 ipv6 autofs4
i2c_dev i2c_core uhci_hcd 3c59x mii floppy ata_piix libata dm_snapshot
dm_zero dm_mirr
or ext3 jbd dm_mod 3w_xxxx sd_mod scsi_mod
Aug 15 23:01:28 mydomain kernel: CPU:    0
Aug 15 23:01:28 mydomain kernel: EIP:    0060:[<c01b2de4>]    Not tainted VLI
Aug 15 23:01:28 mydomain kernel: EFLAGS: 00010246   (2.6.9-55.0.2.EL)
Aug 15 23:01:28 mydomain kernel: EIP is at sysfs_readdir+0x1b2/0x216
Aug 15 23:01:28 mydomain kernel: eax: 00000000   ebx: c54ee0e0   ecx:
ffffffff   edx: 00000000
Aug 15 23:01:28 mydomain kernel: esi: c54ee0e4   edi: 00000000   ebp:
c1be11e4   esp: c48f8f60
Aug 15 23:01:28 mydomain kernel: ds: 007b   es: 007b   ss: 0068
Aug 15 23:01:28 mydomain kernel: Process ossec-syscheckd (pid: 11762,
threadinfo=c48f8000 task=c75a27e0)
Aug 15 23:01:28 mydomain kernel: Stack: bff7125c c7ff5420 c0180d15
c48f8fa0 c717f920 c0372960 c717f920 c7f2b8b0
Aug 15 23:01:28 mydomain kernel:        c0180d15 c018097e c48f8fa0
ffffffda 0856a5dc c717f920 00000000 c0180fcb
Aug 15 23:01:28 mydomain kernel:        0856a644 0856a62c 00000f98
ffffffea 00000009 0856a5c0 002e8ff4 c48f8000
Aug 15 23:01:28 mydomain kernel: Call Trace:
Aug 15 23:01:28 mydomain kernel:  [<c0180d15>] filldir64+0x0/0x11a
Aug 15 23:01:28 mydomain kernel:  [<c0180d15>] filldir64+0x0/0x11a
Aug 15 23:01:28 mydomain kernel:  [<c018097e>] vfs_readdir+0x8a/0xb7
Aug 15 23:01:28 mydomain kernel:  [<c0180fcb>] sys_getdents64+0x80/0xba
Aug 15 23:01:28 mydomain kernel:  [<c031af83>] syscall_call+0x7/0xb
Aug 15 23:01:28 mydomain kernel:  [<c031007b>] xfrm_parse_spi+0xb2/0xd3
Aug 15 23:01:28 mydomain kernel: Code: 04 89 69 0c 8b 75 00 8b 44 24 04 83
c0 0c 39 c6 74 73 8d 5e fc 83 7b 14 00 74 66 89 d8 e8 ec e2 ff ff 89 c2 83
c9 ff
 31 c0 89 d7 <f2> ae f7 d1 49 0f b7 43 1c c1 e8 0c 50 ff 73 20 8b 44 24 18 ff
Aug 15 23:01:28 mydomain kernel:  <0>Fatal exception: panic in 5 seconds

# uname -a
Linux mydomain.com 2.6.9-55.0.2.EL #1 Tue Jun 26 14:08:18 EDT 2007 i686
i686 i386 GNU/Linux

I don't understand the meaning of all of this? I'm trying to find the
cause of why my system froze or crashed. The only recent change ( a few
weeks ago ) that I did was an upgrade of ossec from 1.2 to 1.3. Does this
log show or hint to the cause of the problem?

Thank You

Noel