[BUG] ALSA: korg1212: Potential NULL pointer dereference in snd_korg1212_interrupt()

[Подгорный Алексей Олегович]

snd_korg1212_create() makes the following steps during initialization
of the card:
1) registers an interrupt handler (lines 2230-2232)
2) allocates and initializes korg1212->sharedBufferPtr (lines 2280-2287)
3) reboots the card via snd_korg1212_Send1212Command() (line 2358)

2145    static int snd_korg1212_create(struct snd_card *card, struct
                                 pci_dev *pci, struct snd_korg1212 **rchip)
2147    {
                ...
2230            err = request_irq(pci->irq, snd_korg1212_interrupt,
2231                                        IRQF_SHARED,
2232                                        KBUILD_MODNAME, korg1212);
                ...
2280            if (snd_dma_alloc_pages(SNDRV_DMA_TYPE_DEV, &pci->dev,
2281            sizeof(struct KorgSharedBuffer), &korg1212->dma_shared) < 0){

2282                         snd_printk(KERN_ERR "korg1212: can not
                                 allocate shared buffer memory (%zdbytes)\n",
                                 sizeof(struct KorgSharedBuffer));

2283                         snd_korg1212_free(korg1212);
2284                         return -ENOMEM;
2285            }
2286            korg1212->sharedBufferPtr =
                        (struct KorgSharedBuffer*)korg1212->dma_shared.area;
2287            korg1212->sharedBufferPhy = korg1212->dma_shared.addr;
                ...
2358            rc = snd_korg1212_Send1212Command(korg1212,
                        K1212_DB_RebootCard, 0, 0, 0, 0);
                ...
2412    }

But if interrupt happens when snd_korg1212_create() is still within
lines 2233-2286,
snd_korg1212_interrupt() may dereference korg1212->sharedBufferPtr before
it was initialized without any checks (line 1149):

1098    static irqreturn_t snd_korg1212_interrupt(int irq, void *dev_id)
1099    {
                ...
1116            switch (doorbellValue) {
                ...
1145                        case K1212_DB_CARDSTOPPED:
1146                            K1212_DEBUG_PRINTK_VERBOSE("K1212_DEBUG: IRQ
                                        CSTP count - %ld, %x, [%s].\n",
1147                                korg1212->irqcount, doorbellValue,
1148                                stateName[korg1212->cardState]);
1149                        korg1212->sharedBufferPtr->cardCommand = 0;
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1150                        break;
                ...
1185    }

Should we be sure that such interrupt cannot happen or
should we move the registration of the interrupt handler after
korg1212->sharedBufferPtr is initialized?

Found by Linux Driver Verification project (linuxtesting.org).