Currently it's possible to ide-cd to set an incorrect blocksize by
reading garbage if the drive is dead:

ide_cd_probe()
 -> cdrom_read_toc()
     -> cdrom_read_capacity()
         -> cdrom_queue_packet_command()
             -> ide_do_drive_cmd()
                 -> ide_do_request()
                     -> start_request()

on start_request():

        /* bail early if we've exceeded max_failures */
        if (drive->max_failures && (drive->failures > drive->max_failures)) {
                goto kill_rq;
        }
(...)
kill_rq:
        ide_kill_rq(drive, rq);
        return ide_stopped;

ide_kill_rq() and the next calls won't set REQ_FAILED on rq->cmd_flags and thus
cdrom_queue_packet_command() won't return an error. then:

        stat = cdrom_queue_packet_command(drive, &req);
        if (stat == 0) {
                *capacity = 1 + be32_to_cpu(capbuf.lba);
                *sectors_per_frame =
                        be32_to_cpu(capbuf.blocklen) >> SECTOR_BITS;
        }

cdrom_read_capacity() ends believing capbuf is valid but in fact it's just
uninitialized data. back to cdrom_read_toc():

        /* Try to get the total cdrom capacity and sector size. */
        stat = cdrom_read_capacity(drive, &toc->capacity, &sectors_per_frame,
                                   sense);
        if (stat)
                toc->capacity = 0x1fffff;

        set_capacity(info->disk, toc->capacity * sectors_per_frame);
        /* Save a private copy of te TOC capacity for error handling */
        drive->probed_capacity = toc->capacity * sectors_per_frame;

        blk_queue_hardsect_size(drive->queue,
                                sectors_per_frame << SECTOR_BITS);

that will set drive->queue->hardsect_size to be the random value.
hardsect_size is used to calculate inode->i_blkbits. later on, on a read
path:

void create_empty_buffers(struct page *page,
                        unsigned long blocksize, unsigned long b_state)
{       
        struct buffer_head *bh, *head, *tail;

        head = alloc_page_buffers(page, blocksize, 1);
        bh = head;
        do {    
                bh->b_state |= b_state;
                tail = bh;
                bh = bh->b_this_page;
        } while (bh);
        tail->b_this_page = head;

alloc_page_buffers() will return NULL if blocksize > 4096. blocksize is
calculed based on inode->i_blkbits. that will trigger a null
dereference on create_empty_buffers().
Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
Cc: Borislav Petkov <bbpetkov@yahoo.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>