[SCSI] st: fix possible memory use after free after MTSETBLK ioctl

A memory use after free bug can manifest if the MTSETBLK or SET_DENS_AND_BLK
ioctl features are used to set the tape's blocksize from 0 to non-zero.
After the driver sets the new block size, in this one case it calls
normalize_buffer() to free the device's internal data buffers. However, the
ioctl code assumes there is always a buffer and does not check or allocate
a buffer if there isn't one. So any following ioctl calls can corrupt
a part of memory by writing data to memory that the st driver had freed.

This patch removes the normalize_buffer() call and the specialness of
changing from a 0 to non-zero blocksize to fix the possible use of
memory after it has been freed by the st driver.

signed-off-by: David Jeffery <[e-mail]>
Acked-by: Kai Makisara <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>

drivers/scsi/st.c

···
 			ioctl_result = st_int_ioctl(STp, MTBSF, 1);
 
 		if (cmd_in == MTSETBLK || cmd_in == SET_DENS_AND_BLK) {
-			int old_block_size = STp->block_size;
 			STp->block_size = arg & MT_ST_BLKSIZE_MASK;
 			if (STp->block_size != 0) {
-				if (old_block_size == 0)
-					normalize_buffer(STp->buffer);
 				(STp->buffer)->buffer_blocks =
 				    (STp->buffer)->buffer_size / STp->block_size;
 			}