floppy: disable FDRAWCMD by default

Minh Yuan reported a concurrency use-after-free issue in the floppy code
between raw_cmd_ioctl and seek_interrupt.

[ It turns out this has been around, and that others have reported the
  KASAN splats over the years, but Minh Yuan had a reproducer for it and
  so gets primary credit for reporting it for this fix   - Linus ]

The problem is, this driver tends to break very easily and nowadays,
nobody is expected to use FDRAWCMD anyway since it was used to
manipulate non-standard formats.  The risk of breaking the driver is
higher than the risk presented by this race, and accessing the device
requires privileges anyway.

Let's just add a config option to completely disable this ioctl and
leave it disabled by default.  Distros shouldn't use it, and only those
running on antique hardware might need to enable it.

Link: https://lore.kernel.org/all/000000000000b71cdd05d703f6bf@google.com/
Link: https://lore.kernel.org/lkml/CAKcFiNC=MfYVW-Jt9A3=FPJpTwCD2PL_ULNCpsCVE5s8ZeBQgQ@mail.gmail.com
Link: https://lore.kernel.org/all/CAEAjamu1FRhz6StCe_55XY5s389ZP_xmCF69k987En+1z53=eg@mail.gmail.com
Reported-by: Minh Yuan <yuanmingbuaa@gmail.com>
Reported-by: syzbot+8e8958586909d62b6840@syzkaller.appspotmail.com
Reported-by: cruise k <cruise4k@gmail.com>
Reported-by: Kyungtae Kim <kt0755@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Tested-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: wtarreau

drivers/block/Kconfig

@@ -33,6 +33,22 @@ config BLK_DEV_FD
 	  To compile this driver as a module, choose M here: the
 	  module will be called floppy.
 
+config BLK_DEV_FD_RAWCMD
+	bool "Support for raw floppy disk commands (DEPRECATED)"
+	depends on BLK_DEV_FD
+	help
+	  If you want to use actual physical floppies and expect to do
+	  special low-level hardware accesses to them (access and use
+	  non-standard formats, for example), then enable this.
+
+	  Note that the code enabled by this option is rarely used and
+	  might be unstable or insecure, and distros should not enable it.
+
+	  Note: FDRAWCMD is deprecated and will be removed from the kernel
+	  in the near future.
+
+	  If unsure, say N.
+
 config AMIGA_FLOPPY
 	tristate "Amiga floppy support"
 	depends on AMIGA

drivers/block/floppy.c

@@ -2982,6 +2982,8 @@ static const char *drive_name(int type, int drive)
 		return "(null)";
 }
 
+#ifdef CONFIG_BLK_DEV_FD_RAWCMD
+
 /* raw commands */
 static void raw_cmd_done(int flag)
 {
@@ -3181,6 +3183,35 @@ static int raw_cmd_ioctl(int cmd, void __user *param)
 	return ret;
 }
 
+static int floppy_raw_cmd_ioctl(int type, int drive, int cmd,
+				void __user *param)
+{
+	int ret;
+
+	pr_warn_once("Note: FDRAWCMD is deprecated and will be removed from the kernel in the near future.\n");
+
+	if (type)
+		return -EINVAL;
+	if (lock_fdc(drive))
+		return -EINTR;
+	set_floppy(drive);
+	ret = raw_cmd_ioctl(cmd, param);
+	if (ret == -EINTR)
+		return -EINTR;
+	process_fd_request();
+	return ret;
+}
+
+#else /* CONFIG_BLK_DEV_FD_RAWCMD */
+
+static int floppy_raw_cmd_ioctl(int type, int drive, int cmd,
+				void __user *param)
+{
+	return -EOPNOTSUPP;
+}
+
+#endif
+
 static int invalidate_drive(struct block_device *bdev)
 {
 	/* invalidate the buffer track to force a reread */
@@ -3369,7 +3400,6 @@ static int fd_locked_ioctl(struct block_device *bdev, fmode_t mode, unsigned int
 {
 	int drive = (long)bdev->bd_disk->private_data;
 	int type = ITYPE(drive_state[drive].fd_device);
-	int i;
 	int ret;
 	int size;
 	union inparam {
@@ -3520,16 +3550,7 @@ static int fd_locked_ioctl(struct block_device *bdev, fmode_t mode, unsigned int
 		outparam = &write_errors[drive];
 		break;
 	case FDRAWCMD:
-		if (type)
-			return -EINVAL;
-		if (lock_fdc(drive))
-			return -EINTR;
-		set_floppy(drive);
-		i = raw_cmd_ioctl(cmd, (void __user *)param);
-		if (i == -EINTR)
-			return -EINTR;
-		process_fd_request();
-		return i;
+		return floppy_raw_cmd_ioctl(type, drive, cmd, (void __user *)param);
 	case FDTWADDLE:
 		if (lock_fdc(drive))
 			return -EINTR;