libata: reject passthrough WRITE SAME requests

commit c6ade20f upstream.

The WRITE SAME to TRIM translation rewrites the DATA OUT buffer.  While
the SCSI code accomodates for this by passing a read-writable buffer
userspace applications don't cater for this behavior.  In fact it can
be used to rewrite e.g. a readonly file through mmap and should be
considered as a security fix.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
[bwh: Backported to 3.16:
 - Open-code blk_rq_is_passthrough()
 - We don't distinguish which field is invaid so goto invalid_fld
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/ata/libata-scsi.c

@@ -3097,6 +3097,14 @@ static unsigned int ata_scsi_write_same_xlat(struct ata_queued_cmd *qc)
 	if (unlikely(!dev->dma_mode))
 		goto invalid_fld;
 
+	/*
+	 * We only allow sending this command through the block layer,
+	 * as it modifies the DATA OUT buffer, which would corrupt user
+	 * memory for SG_IO commands.
+	 */
+	if (unlikely(scmd->request->cmd_type != REQ_TYPE_FS))
+		goto invalid_fld;
+
 	if (unlikely(scmd->cmd_len < 16))
 		goto invalid_fld;
 	scsi_16_lba_len(cdb, &block, &n_block);