ALSA: dice: fix kernel NULL pointer dereference due to invalid calculation for array index

At a commit f91c9d7610a ('ALSA: firewire-lib: cache maximum length of
payload to reduce function calls'), maximum size of payload for tx
isochronous packet is cached to reduce the number of function calls.

This cache was programmed to updated at a first callback of ohci1394 IR
context. However, the maximum size is required to queueing packets before
starting the isochronous context.

As a result, the cached value is reused to queue packets in next time to
starting the isochronous context. Then the cache is updated in a first
callback of the isochronous context. This can cause kernel NULL pointer
dereference in a below call graph:

(sound/firewire/amdtp-stream.c)
amdtp_stream_start()
->queue_in_packet()
  ->queue_packet()
    (drivers/firewire/core-iso.c)
    ->fw_iso_context_queue()
      ->struct fw_card_driver.queue_iso()
      (drivers/firewire/ohci.c)
      = ohci_queue_iso()
        ->queue_iso_packet_per_buffer()
          buffer->pages[page]

The issued dereference occurs in a case that:
 - target unit supports different stream formats for sampling transmission
   frequency.
 - maximum length of payload for tx stream in a first trial is bigger
   than the length in a second trial.

In this case, correct number of pages are allocated for DMA and the 'pages'
array has enough elements, while index of the element is wrongly calculated
according to the old value of length of payload in a call of
'queue_in_packet()'. Then it causes the issue.

This commit fixes the critical bug. This affects all of drivers in ALSA
firewire stack in Linux kernel v4.12 or later.

[12665.302360] BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
[12665.302415] IP: ohci_queue_iso+0x47c/0x800 [firewire_ohci]
[12665.302439] PGD 0
[12665.302440] P4D 0
[12665.302450]
[12665.302470] Oops: 0000 [#1] SMP PTI
[12665.302487] Modules linked in: ...
[12665.303096] CPU: 1 PID: 12760 Comm: jackd Tainted: P           OE   4.13.0-38-generic #43-Ubuntu
[12665.303154] Hardware name:                  /DH77DF, BIOS KCH7710H.86A.0069.2012.0224.1825 02/24/2012
[12665.303215] task: ffff9ce87da2ae80 task.stack: ffffb5b8823d0000
[12665.303258] RIP: 0010:ohci_queue_iso+0x47c/0x800 [firewire_ohci]
[12665.303301] RSP: 0018:ffffb5b8823d3ab8 EFLAGS: 00010086
[12665.303337] RAX: ffff9ce4f4876930 RBX: 0000000000000008 RCX: ffff9ce88a3955e0
[12665.303384] RDX: 0000000000000000 RSI: 0000000034877f00 RDI: 0000000000000000
[12665.303427] RBP: ffffb5b8823d3b68 R08: ffff9ce8ccb390a0 R09: ffff9ce877639ab0
[12665.303475] R10: 0000000000000108 R11: 0000000000000000 R12: 0000000000000003
[12665.303513] R13: 0000000000000000 R14: ffff9ce4f4876950 R15: 0000000000000000
[12665.303554] FS:  00007f2ec467f8c0(0000) GS:ffff9ce8df280000(0000) knlGS:0000000000000000
[12665.303600] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[12665.303633] CR2: 0000000000000030 CR3: 00000002dcf90004 CR4: 00000000000606e0
[12665.303674] Call Trace:
[12665.303698]  fw_iso_context_queue+0x18/0x20 [firewire_core]
[12665.303735]  queue_packet+0x88/0xe0 [snd_firewire_lib]
[12665.303770]  amdtp_stream_start+0x19b/0x270 [snd_firewire_lib]
[12665.303811]  start_streams+0x276/0x3c0 [snd_dice]
[12665.303840]  snd_dice_stream_start_duplex+0x1bf/0x480 [snd_dice]
[12665.303882]  ? vma_gap_callbacks_rotate+0x1e/0x30
[12665.303914]  ? __rb_insert_augmented+0xab/0x240
[12665.303936]  capture_prepare+0x3c/0x70 [snd_dice]
[12665.303961]  snd_pcm_do_prepare+0x1d/0x30 [snd_pcm]
[12665.303985]  snd_pcm_action_single+0x3b/0x90 [snd_pcm]
[12665.304009]  snd_pcm_action_nonatomic+0x68/0x70 [snd_pcm]
[12665.304035]  snd_pcm_prepare+0x68/0x90 [snd_pcm]
[12665.304058]  snd_pcm_common_ioctl1+0x4c0/0x940 [snd_pcm]
[12665.304083]  snd_pcm_capture_ioctl1+0x19b/0x250 [snd_pcm]
[12665.304108]  snd_pcm_capture_ioctl+0x27/0x40 [snd_pcm]
[12665.304131]  do_vfs_ioctl+0xa8/0x630
[12665.304148]  ? entry_SYSCALL_64_after_hwframe+0xe9/0x139
[12665.304172]  ? entry_SYSCALL_64_after_hwframe+0xe2/0x139
[12665.304195]  ? entry_SYSCALL_64_after_hwframe+0xdb/0x139
[12665.304218]  ? entry_SYSCALL_64_after_hwframe+0xd4/0x139
[12665.304242]  ? entry_SYSCALL_64_after_hwframe+0xcd/0x139
[12665.304265]  ? entry_SYSCALL_64_after_hwframe+0xc6/0x139
[12665.304288]  ? entry_SYSCALL_64_after_hwframe+0xbf/0x139
[12665.304312]  ? entry_SYSCALL_64_after_hwframe+0xb8/0x139
[12665.304335]  ? entry_SYSCALL_64_after_hwframe+0xb1/0x139
[12665.304358]  SyS_ioctl+0x79/0x90
[12665.304374]  ? entry_SYSCALL_64_after_hwframe+0x72/0x139
[12665.304397]  entry_SYSCALL_64_fastpath+0x24/0xab
[12665.304417] RIP: 0033:0x7f2ec3750ef7
[12665.304433] RSP: 002b:00007fff99e31388 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[12665.304465] RAX: ffffffffffffffda RBX: 00007fff99e312f0 RCX: 00007f2ec3750ef7
[12665.304494] RDX: 0000000000000000 RSI: 0000000000004140 RDI: 0000000000000007
[12665.304522] RBP: 0000556ebc63fd60 R08: 0000556ebc640560 R09: 0000000000000000
[12665.304553] R10: 0000000000000001 R11: 0000000000000246 R12: 0000556ebc63fcf0
[12665.304584] R13: 0000000000000000 R14: 0000000000000007 R15: 0000000000000000
[12665.304612] Code: 01 00 00 44 89 eb 45 31 ed 45 31 db 66 41 89 1e 66 41 89 5e 0c 66 45 89 5e 0e 49 8b 49 08 49 63 d4 4d 85 c0 49 63 ff 48 8b 14 d1 <48> 8b 72 30 41 8d 14 37 41 89 56 04 48 63 d3 0f 84 ce 00 00 00
[12665.304713] RIP: ohci_queue_iso+0x47c/0x800 [firewire_ohci] RSP: ffffb5b8823d3ab8
[12665.304743] CR2: 0000000000000030
[12665.317701] ---[ end trace 9d55b056dd52a19f ]---

Fixes: f91c9d7610a ('ALSA: firewire-lib: cache maximum length of payload to reduce function calls')
Cc: <stable@vger.kernel.org> # v4.12+
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/amdtp-stream.c

@@ -773,8 +773,6 @@ static void amdtp_stream_first_callback(struct fw_iso_context *context,
 	u32 cycle;
 	unsigned int packets;
 
-	s->max_payload_length = amdtp_stream_get_max_payload(s);
-
 	/*
 	 * For in-stream, first packet has come.
 	 * For out-stream, prepared to transmit first packet
@@ -879,6 +877,9 @@ int amdtp_stream_start(struct amdtp_stream *s, int channel, int speed)
 
 	amdtp_stream_update(s);
 
+	if (s->direction == AMDTP_IN_STREAM)
+		s->max_payload_length = amdtp_stream_get_max_payload(s);
+
 	if (s->flags & CIP_NO_HEADER)
 		s->tag = TAG_NO_CIP_HEADER;
 	else