[PATCH] cdrom: rearrange last_media_change check to avoid unintentional overflow

[Justin Stitt <justinstitt@google.com>]

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 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow
sanitizer").

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

Link: https://github.com/llvm/llvm-project/pull/82432 [1]
Closes: https://github.com/KSPP/linux/issues/354
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
 drivers/cdrom/cdrom.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/cdrom/cdrom.c b/drivers/cdrom/cdrom.c
index a5e07270e0d4..20c90ebb3a3f 100644
--- a/drivers/cdrom/cdrom.c
+++ b/drivers/cdrom/cdrom.c
@@ -2358,7 +2358,7 @@ 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;

---
base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc
change-id: 20240507-b4-sio-ata1-9d64089f91c0

Best regards,
--
Justin Stitt <justinstitt@google.com>