[PATCH v2 1/2] scsi: fix scsi_mode_sense() buffer length handling

[Damien Le Moal <damien.lemoal@wdc.com>]

Several problems exist with scsi_mode_sense() buffer length handling:
1) The allocation length field of the MODE SENSE 10 command is 16-bits,
   occupying bytes 7 and 8 of the CDB. With this command, access to mode
   pages larger than 255 bytes is thus possible. However, the CDB
   allocation length field is set by assigning len to byte 8 only, thus
   truncating buffer length larger than 255.
2) If scsi_mode_sense() is called with len smaller than 8 with
   sdev->use_10_for_ms set, or smaller than 4 otherwise, the buffer
   length is increased to 8 and 4 respectively, and the buffer is zero
   filled with these increased values, thus corrupting the memory
   following the buffer.

Fix these 2 problems by using put_unaligned_be16() to set the allocation
length field of MODE SENSE 10 CDB and by returning an error when len is
too small.

Furthermore, if len is larger than 255B, always try MODE SENSE 10 first,
even if the device driver did not set sdev->use_10_for_ms. In case of
invalid opcode error for MODE SENSE 10, access to mode pages larger
than 255 bytes are not retried using MODE SENSE (6). To avoid buffer
length overflows for the MODE_SENSE 10 case, check that len is smaller
than 65535 bytes

While at it, also fix the folowing:
* use get_unaligned_be16() to retrieve the mode data length and block
  descriptor length fields of the mode sense reply header instead of
  using an open coded calculation.
* Fix the kdoc dbd argument explanation: the DBD bit stands for
  Disable Block Descriptor, which is the opposite of what the dbd
  argument description was.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
---
 drivers/scsi/scsi_lib.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 572673873ddf..327ea74a5e31 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -2075,7 +2075,7 @@ EXPORT_SYMBOL_GPL(scsi_mode_select);
 /**
  *	scsi_mode_sense - issue a mode sense, falling back from 10 to six bytes if necessary.
  *	@sdev:	SCSI device to be queried
- *	@dbd:	set if mode sense will allow block descriptors to be returned
+ *	@dbd:	set to prevent mode sense from returning block descriptors
  *	@modepage: mode page being requested
  *	@buffer: request buffer (may not be smaller than eight bytes)
  *	@len:	length of request buffer.
@@ -2110,18 +2110,18 @@ scsi_mode_sense(struct scsi_device *sdev, int dbd, int modepage,
 		sshdr = &my_sshdr;
 
  retry:
-	use_10_for_ms = sdev->use_10_for_ms;
+	use_10_for_ms = sdev->use_10_for_ms || len > 255;
 
 	if (use_10_for_ms) {
-		if (len < 8)
-			len = 8;
+		if (len < 8 || len > 65535)
+			return -EINVAL;
 
 		cmd[0] = MODE_SENSE_10;
-		cmd[8] = len;
+		put_unaligned_be16(len, &cmd[7]);
 		header_length = 8;
 	} else {
 		if (len < 4)
-			len = 4;
+			return -EINVAL;
 
 		cmd[0] = MODE_SENSE;
 		cmd[4] = len;
@@ -2145,9 +2145,15 @@ scsi_mode_sense(struct scsi_device *sdev, int dbd, int modepage,
 			if ((sshdr->sense_key == ILLEGAL_REQUEST) &&
 			    (sshdr->asc == 0x20) && (sshdr->ascq == 0)) {
 				/*
-				 * Invalid command operation code
+				 * Invalid command operation code: retry using
+				 * MODE SENSE (6) if this was a MODE SENSE 10
+				 * request, except if the request mode page is
+				 * too large for MODE SENSE single byte
+				 * allocation length field.
 				 */
 				if (use_10_for_ms) {
+					if (len > 255)
+						return -EIO;
 					sdev->use_10_for_ms = 0;
 					goto retry;
 				}
@@ -2171,12 +2177,11 @@ scsi_mode_sense(struct scsi_device *sdev, int dbd, int modepage,
 		data->longlba = 0;
 		data->block_descriptor_length = 0;
 	} else if (use_10_for_ms) {
-		data->length = buffer[0]*256 + buffer[1] + 2;
+		data->length = get_unaligned_be16(&buffer[0]) + 2;
 		data->medium_type = buffer[2];
 		data->device_specific = buffer[3];
 		data->longlba = buffer[4] & 0x01;
-		data->block_descriptor_length = buffer[6]*256
-			+ buffer[7];
+		data->block_descriptor_length = get_unaligned_be16(&buffer[6]);
 	} else {
 		data->length = buffer[0] + 1;
 		data->medium_type = buffer[1];
-- 
2.31.1