scsi_put_cdrom_generic_arg() is potentially copying uninitialized stack
memory to userspace, since the compiler may leave a 3-byte hole in the
middle of `cgc32`. Prevent it by adding a padding field to `struct
compat_cdrom_generic_command`.

Cc: stable@vger.kernel.org
Fixes: f3ee6e63a9df ("compat_ioctl: move CDROM_SEND_PACKET handling into scsi")
Suggested-by: Dan Carpenter <redacted>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peilin Ye <redacted>
---
Change in v3:
    - Improve commit message. scsi_put_cdrom_generic_arg() does not
      *always* leak kernel information. It is compiler dependent.
      Reference: https://www.nccgroup.com/us/about-us/newsroom-and-events/blog/2019/october/padding-the-struct-how-a-compiler-optimization-can-disclose-stack-memory/
    - Base the patch against mainline 5.9-rc4.

Change in v2:
    - Add a padding field to `struct compat_cdrom_generic_command`,
      instead of doing memset() on `cgc32`. (Suggested by Jens Axboe
      [off-list ref])

$ # before
$ pahole -C "compat_cdrom_generic_command" !$
pahole -C "compat_cdrom_generic_command" block/scsi_ioctl.o
struct compat_cdrom_generic_command {
	unsigned char              cmd[12];              /*     0    12 */
	compat_caddr_t             buffer;               /*    12     4 */
	compat_uint_t              buflen;               /*    16     4 */
	compat_int_t               stat;                 /*    20     4 */
	compat_caddr_t             sense;                /*    24     4 */
	unsigned char              data_direction;       /*    28     1 */

	/* XXX 3 bytes hole, try to pack */

	compat_int_t               quiet;                /*    32     4 */
	compat_int_t               timeout;              /*    36     4 */
	compat_caddr_t             reserved[1];          /*    40     4 */

	/* size: 44, cachelines: 1, members: 9 */
	/* sum members: 41, holes: 1, sum holes: 3 */
	/* last cacheline: 44 bytes */
};
$ # after
$ pahole -C "compat_cdrom_generic_command" block/scsi_ioctl.o
struct compat_cdrom_generic_command {
	unsigned char              cmd[12];              /*     0    12 */
	compat_caddr_t             buffer;               /*    12     4 */
	compat_uint_t              buflen;               /*    16     4 */
	compat_int_t               stat;                 /*    20     4 */
	compat_caddr_t             sense;                /*    24     4 */
	unsigned char              data_direction;       /*    28     1 */
	unsigned char              pad[3];               /*    29     3 */
	compat_int_t               quiet;                /*    32     4 */
	compat_int_t               timeout;              /*    36     4 */
	compat_caddr_t             reserved[1];          /*    40     4 */

	/* size: 44, cachelines: 1, members: 10 */
	/* last cacheline: 44 bytes */
};
$ _

 block/scsi_ioctl.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/scsi_ioctl.c b/block/scsi_ioctl.c
index ef722f04f88a..72108404718f 100644
--- a/block/scsi_ioctl.c
+++ b/block/scsi_ioctl.c

@@ -651,6 +651,7 @@ struct compat_cdrom_generic_command {
 	compat_int_t	stat;
 	compat_caddr_t	sense;
 	unsigned char	data_direction;
+	unsigned char	pad[3];
 	compat_int_t	quiet;
 	compat_int_t	timeout;
 	compat_caddr_t	reserved[1];

-- 
2.25.1

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