UBIFS: check buffer length when scanning for LPT nodes

'is_a_node()' function was reading from a buffer before
checking the buffer length, resulting in an OOPS as
follows:

BUG: unable to handle kernel paging request at f8f74002
IP: [<f8f9783f>] :ubifs:ubifs_unpack_bits+0xca/0x233
*pde = 19e95067 *pte = 00000000
Oops: 0000 [#1] PREEMPT SMP
Modules linked in: ubifs ubi mtdchar bio2mtd mtd brd video output
[last unloaded: mtd]

Pid: 6414, comm: integck Not tainted (2.6.27-rc6ubifs34 #23)
EIP: 0060:[<f8f9783f>] EFLAGS: 00010246 CPU: 0
EIP is at ubifs_unpack_bits+0xca/0x233 [ubifs]
EAX: 00000000 EBX: f6090630 ECX: d9badcfc EDX: 00000000
ESI: 00000004 EDI: f8f74002 EBP: d9badcec ESP: d9badcc0
 DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
Process integck (pid: 6414, ti=d9bac000 task=f727dae0 task.ti=d9bac000)
Stack: 00000006 f7306240 00000002 00000000 d9badcfc d9badd00 0000001c 00000000
       f6090630 f6090630 f8f74000 d9badd10 f8fa1cc9 00000000 f8f74002 00000000
       f8f74002 f60fe128 f6090630 f8f74000 d9badd68 f8fa1e46 00000000 0001e000
Call Trace:
 [<f8fa1cc9>] ? is_a_node+0x30/0x90 [ubifs]
 [<f8fa1e46>] ? dbg_check_ltab+0x11d/0x5bd [ubifs]
 [<f8fa388f>] ? ubifs_lpt_start_commit+0x42/0xed3 [ubifs]
 [<c038e76a>] ? mutex_unlock+0x8/0xa
 [<f8f9625d>] ? ubifs_tnc_start_commit+0x1c8/0xedb [ubifs]
 [<f8f8d90b>] ? do_commit+0x187/0x523 [ubifs]
 [<c038e76a>] ? mutex_unlock+0x8/0xa
 [<f8f7ca17>] ? bud_wbuf_callback+0x22/0x28 [ubifs]
 [<f8f8dd1d>] ? ubifs_run_commit+0x76/0xc0 [ubifs]
 [<f8f8032c>] ? ubifs_sync_fs+0xd2/0xe6 [ubifs]
 [<c01a2e97>] ? vfs_quota_sync+0x0/0x17e
 [<c01a5ba6>] ? quota_sync_sb+0x26/0xbb
 [<c01a2e97>] ? vfs_quota_sync+0x0/0x17e
 [<c01a5c5d>] ? sync_dquots+0x22/0x12c
 [<c0173d1b>] ? __fsync_super+0x19/0x68
 [<c0173d75>] ? fsync_super+0xb/0x19
 [<c0174065>] ? generic_shutdown_super+0x22/0xe7
 [<c01a31fc>] ? vfs_quota_off+0x0/0x5fd
 [<f8f7cf4d>] ? ubifs_kill_sb+0x31/0x35 [ubifs]
 [<c01741f9>] ? deactivate_super+0x5e/0x71
 [<c0187610>] ? mntput_no_expire+0x82/0xe4
 [<c0187905>] ? sys_umount+0x4c/0x2f6
 [<c0187bc8>] ? sys_oldumount+0x19/0x1b
 [<c0103b71>] ? sysenter_do_call+0x12/0x25
 =======================
Code: c1 f8 03 8d 04 07 8b 4d e8 89 01 8b 45 e4 89 10 89 d8 89 f1 d3 e8 85 c0
      74 07 29 d6 83 fe 20 75 2a 89 d8 83 c4 20 5b 5e 5f 5d
EIP: [<f8f9783f>] ubifs_unpack_bits+0xca/0x233 [ubifs] SS:ESP 0068:d9badcc0
---[ end trace 1f02572436518c13 ]---

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>

fs/ubifs/lpt_commit.c

@@ -1089,6 +1089,8 @@ static int is_a_node(struct ubifs_info *c, uint8_t *buf, int len)
 	int pos = 0, node_type, node_len;
 	uint16_t crc, calc_crc;
 
+	if (len < UBIFS_LPT_CRC_BYTES + (UBIFS_LPT_TYPE_BITS + 7) / 8)
+		return 0;
 	node_type = ubifs_unpack_bits(&addr, &pos, UBIFS_LPT_TYPE_BITS);
 	if (node_type == UBIFS_LPT_NOT_A_NODE)
 		return 0;