crypto: ccm - Fix handling of null assoc data

Its a valid use case to have null associated data in a ccm vector, but
this case isn't being handled properly right now.

The following ccm decryption/verification test vector, using the
rfc4309 implementation regularly triggers a panic, as will any
other vector with null assoc data:

* key: ab2f8a74b71cd2b1ff802e487d82f8b9
* iv: c6fb7d800d13abd8a6b2d8
* Associated Data: [NULL]
* Tag Length: 8
* input: d5e8939fc7892e2b

The resulting panic looks like so:

Unable to handle kernel paging request at ffff810064ddaec0 RIP: 
 [<ffffffff8864c4d7>] :ccm:get_data_to_compute+0x1a6/0x1d6
PGD 8063 PUD 0 
Oops: 0002 [1] SMP 
last sysfs file: /module/libata/version
CPU 0
Modules linked in: crypto_tester_kmod(U) seqiv krng ansi_cprng chainiv rng ctr aes_generic aes_x86_64 ccm cryptomgr testmgr_cipher testmgr aead crypto_blkcipher crypto_a
lgapi des ipv6 xfrm_nalgo crypto_api autofs4 hidp l2cap bluetooth nfs lockd fscache nfs_acl sunrpc ip_conntrack_netbios_ns ipt_REJECT xt_state ip_conntrack nfnetlink xt_
tcpudp iptable_filter ip_tables x_tables dm_mirror dm_log dm_multipath scsi_dh dm_mod video hwmon backlight sbs i2c_ec button battery asus_acpi acpi_memhotplug ac lp sg 
snd_intel8x0 snd_ac97_codec ac97_bus snd_seq_dummy snd_seq_oss joydev snd_seq_midi_event snd_seq snd_seq_device snd_pcm_oss snd_mixer_oss ide_cd snd_pcm floppy parport_p
c shpchp e752x_edac snd_timer e1000 i2c_i801 edac_mc snd soundcore snd_page_alloc i2c_core cdrom parport serio_raw pcspkr ata_piix libata sd_mod scsi_mod ext3 jbd uhci_h
cd ohci_hcd ehci_hcd
Pid: 12844, comm: crypto-tester Tainted: G      2.6.18-128.el5.fips1 #1
RIP: 0010:[<ffffffff8864c4d7>]  [<ffffffff8864c4d7>] :ccm:get_data_to_compute+0x1a6/0x1d6
RSP: 0018:ffff8100134434e8  EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8100104898b0 RCX: ffffffffab6aea10
RDX: 0000000000000010 RSI: ffff8100104898c0 RDI: ffff810064ddaec0
RBP: 0000000000000000 R08: ffff8100104898b0 R09: 0000000000000000
R10: ffff8100103bac84 R11: ffff8100104898b0 R12: ffff810010489858
R13: ffff8100104898b0 R14: ffff8100103bac00 R15: 0000000000000000
FS:  00002ab881adfd30(0000) GS:ffffffff803ac000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: ffff810064ddaec0 CR3: 0000000012a88000 CR4: 00000000000006e0
Process crypto-tester (pid: 12844, threadinfo ffff810013442000, task ffff81003d165860)
Stack:  ffff8100103bac00 ffff8100104898e8 ffff8100134436f8 ffffffff00000000
 0000000000000000 ffff8100104898b0 0000000000000000 ffff810010489858
 0000000000000000 ffff8100103bac00 ffff8100134436f8 ffffffff8864c634
Call Trace:
 [<ffffffff8864c634>] :ccm:crypto_ccm_auth+0x12d/0x140
 [<ffffffff8864cf73>] :ccm:crypto_ccm_decrypt+0x161/0x23a
 [<ffffffff88633643>] :crypto_tester_kmod:cavs_test_rfc4309_ccm+0x4a5/0x559
[...]

The above is from a RHEL5-based kernel, but upstream is susceptible too.

The fix is trivial: in crypto/ccm.c:crypto_ccm_auth(), pctx->ilen contains
whatever was in memory when pctx was allocated if assoclen is 0. The tested
fix is to simply add an else clause setting pctx->ilen to 0 for the
assoclen == 0 case, so that get_data_to_compute() doesn't try doing
things its not supposed to.

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>