[Linux-kernel-mentees] [PATCH v2] block/floppy: Prevent kernel-infoleak in raw_cmd_copyout()

[Peilin Ye <yepeilin.cs@gmail.com>]

raw_cmd_copyout() is potentially copying uninitialized kernel stack memory
since it is initializing `cmd` by assignment, which may cause the compiler
to leave uninitialized holes in this structure. Fix it by using memcpy()
instead.

Cc: stable@vger.kernel.org
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
---
Change in v2:
    - Remove inappropriate "Fixes:" tag. (Suggested by: Denis Efremov
      <efremov@linux.com>)

Reference: https://lwn.net/Articles/417989/

$ pahole -C "floppy_raw_cmd" drivers/block/floppy.o
struct floppy_raw_cmd {
	unsigned int               flags;                /*     0     4 */

	/* XXX 4 bytes hole, try to pack */

	void *                     data;                 /*     8     8 */
	char *                     kernel_data;          /*    16     8 */
	struct floppy_raw_cmd *    next;                 /*    24     8 */
	long int                   length;               /*    32     8 */
	long int                   phys_length;          /*    40     8 */
	int                        buffer_length;        /*    48     4 */
	unsigned char              rate;                 /*    52     1 */
	unsigned char              cmd_count;            /*    53     1 */
	union {
		struct {
			unsigned char cmd[16];           /*    54    16 */
			/* --- cacheline 1 boundary (64 bytes) was 6 bytes ago --- */
			unsigned char reply_count;       /*    70     1 */
			unsigned char reply[16];         /*    71    16 */
		};                                       /*    54    33 */
		unsigned char      fullcmd[33];          /*    54    33 */
	};                                               /*    54    33 */

	/* XXX 1 byte hole, try to pack */

	/* --- cacheline 1 boundary (64 bytes) was 24 bytes ago --- */
	int                        track;                /*    88     4 */
	int                        resultcode;           /*    92     4 */
	int                        reserved1;            /*    96     4 */
	int                        reserved2;            /*   100     4 */

	/* size: 104, cachelines: 2, members: 14 */
	/* sum members: 99, holes: 2, sum holes: 5 */
	/* last cacheline: 40 bytes */
};

 drivers/block/floppy.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
index 09079aee8dc4..b8ea98f7a9cb 100644
--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -3126,7 +3126,9 @@ static int raw_cmd_copyout(int cmd, void __user *param,
 	int ret;
 
 	while (ptr) {
-		struct floppy_raw_cmd cmd = *ptr;
+		struct floppy_raw_cmd cmd;
+
+		memcpy(&cmd, ptr, sizeof(cmd));
 		cmd.next = NULL;
 		cmd.kernel_data = NULL;
 		ret = copy_to_user(param, &cmd, sizeof(cmd));
-- 
2.25.1