cdrom: rearrange last_media_change check to avoid unintentional overflow

[ Upstream commit efb905a ]

When running syzkaller with the newly reintroduced signed integer wrap
sanitizer we encounter this splat:

[  366.015950] UBSAN: signed-integer-overflow in ../drivers/cdrom/cdrom.c:2361:33
[  366.021089] -9223372036854775808 - 346321 cannot be represented in type '__s64' (aka 'long long')
[  366.025894] program syz-executor.4 is using a deprecated SCSI ioctl, please convert it to SG_IO
[  366.027502] CPU: 5 PID: 28472 Comm: syz-executor.7 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1
[  366.027512] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[  366.027518] Call Trace:
[  366.027523]  <TASK>
[  366.027533]  dump_stack_lvl+0x93/0xd0
[  366.027899]  handle_overflow+0x171/0x1b0
[  366.038787] ata1.00: invalid multi_count 32 ignored
[  366.043924]  cdrom_ioctl+0x2c3f/0x2d10
[  366.063932]  ? __pm_runtime_resume+0xe6/0x130
[  366.071923]  sr_block_ioctl+0x15d/0x1d0
[  366.074624]  ? __pfx_sr_block_ioctl+0x10/0x10
[  366.077642]  blkdev_ioctl+0x419/0x500
[  366.080231]  ? __pfx_blkdev_ioctl+0x10/0x10
...

Historically, the signed integer overflow sanitizer did not work in the
kernel due to its interaction with `-fwrapv` but this has since been
changed [1] in the newest version of Clang. It was re-enabled in the
kernel with Commit 557f8c5 ("ubsan: Reintroduce signed overflow
sanitizer").

Let's rearrange the check to not perform any arithmetic, thus not
tripping the sanitizer.

Link: llvm/llvm-project#82432 [1]
Closes: KSPP/linux#354
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/lkml/20240507-b4-sio-ata1-v1-1-810ffac6080a@google.com
Reviewed-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/lkml/ZjqU0fbzHrlnad8D@equinox
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20240507222520.1445-2-phil@philpotter.co.uk
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: JustinStitt

drivers/cdrom/cdrom.c

@@ -2372,7 +2372,7 @@ static int cdrom_ioctl_timed_media_change(struct cdrom_device_info *cdi,
 		return -EFAULT;
 
 	tmp_info.media_flags = 0;
-	if (tmp_info.last_media_change - cdi->last_media_change_ms < 0)
+	if (cdi->last_media_change_ms > tmp_info.last_media_change)
 		tmp_info.media_flags |= MEDIA_CHANGED_FLAG;
 
 	tmp_info.last_media_change = cdi->last_media_change_ms;