floppy: fix out-of-bounds read in copy_buffer

CVE-2019-14283

This fixes a global out-of-bounds read access in the copy_buffer
function of the floppy driver.

The FDDEFPRM ioctl allows one to set the geometry of a disk.  The sect
and head fields (unsigned int) of the floppy_drive structure are used to
compute the max_sector (int) in the make_raw_rw_request function.  It is
possible to overflow the max_sector.  Next, max_sector is passed to the
copy_buffer function and used in one of the memcpy calls.

An unprivileged user could trigger the bug if the device is accessible,
but requires a floppy disk to be inserted.

The patch adds the check for the .sect * .head multiplication for not
overflowing in the set_geometry function.

The bug was found by syzkaller.
Signed-off-by: Denis Efremov <efremov@ispras.ru>
Tested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit da99466a)
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/block/floppy.c

@@ -3236,8 +3236,10 @@ static int set_geometry(unsigned int cmd, struct floppy_struct *g,
 	int cnt;
 
 	/* sanity checking for parameters. */
-	if (g->sect <= 0 ||
-	    g->head <= 0 ||
+	if ((int)g->sect <= 0 ||
+	    (int)g->head <= 0 ||
+	    /* check for overflow in max_sector */
+	    (int)(g->sect * g->head) <= 0 ||
 	    /* check for zero in F_SECT_PER_TRACK */
 	    (unsigned char)((g->sect << 2) >> FD_SIZECODE(g)) == 0 ||
 	    g->track <= 0 || g->track > UDP->tracks >> STRETCH(g) ||