[v2] scsi: sr: fix unintentional arithmetic wraparound

Running syzkaller with the newly reintroduced signed integer overflow
sanitizer produces this report:

[   65.194362] ------------[ cut here ]------------
[   65.197752] UBSAN: signed-integer-overflow in ../drivers/scsi/sr_ioctl.c:436:9
[   65.203607] -2147483648 * 177 cannot be represented in type 'int'
[   65.207911] CPU: 2 PID: 10416 Comm: syz-executor.1 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1
[   65.213585] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[   65.219923] Call Trace:
[   65.221556]  <TASK>
[   65.223029]  dump_stack_lvl+0x93/0xd0
[   65.225573]  handle_overflow+0x171/0x1b0
[   65.228219]  sr_select_speed+0xeb/0xf0
[   65.230786]  ? __pm_runtime_resume+0xe6/0x130
[   65.233606]  sr_block_ioctl+0x15d/0x1d0
...

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").

Firstly, let's change the type of "speed" to unsigned long as
sr_select_speed()'s only caller passes in an unsigned long anyways.

$ git grep '\.select_speed'
|	drivers/scsi/sr.c:      .select_speed           = sr_select_speed,
...
|	static int cdrom_ioctl_select_speed(struct cdrom_device_info *cdi,
|	                unsigned long arg)
|	{
|	        ...
|	        return cdi->ops->select_speed(cdi, arg);
|	}

Next, let's add an extra check to make sure we don't exceed 0xffff/177
(350) since 0xffff is the max speed. This has two benefits: 1) we deal
with integer overflow before it happens and 2) we properly respect the
max speed of 0xffff. There are some "magic" numbers here but I did not
want to change more than what was necessary.

Link: https://github.com/llvm/llvm-project/pull/82432 [1]
Closes: https://github.com/KSPP/linux/issues/357
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Changes in v2:
- change @speed type from int to unsigned long and use a clamp (thanks Kees)
- also update documentation to avoid confusion
- Link to v1: https://lore.kernel.org/r/20240508-b4-b4-sio-sr_select_speed-v1-1-968906b908b7@google.com
---
Here's the syzkaller reproducer:
r0 = openat$cdrom(0xffffffffffffff9c, &(0x7f0000000140), 0x800, 0x0)
ioctl$CDROM_SELECT_SPEED(r0, 0x5322, 0x7ee9f7c1)

... which was used against Kees' tree here (v6.8rc2):
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=wip/v6.9-rc2/unsigned-overflow-sanitizer

... with this config:
https://gist.github.com/JustinStitt/824976568b0f228ccbcbe49f3dee9bf4
---
 Documentation/cdrom/cdrom-standard.rst | 4 ++--
 drivers/scsi/sr.h                      | 2 +-
 drivers/scsi/sr_ioctl.c                | 5 ++++-
 include/linux/cdrom.h                  | 2 +-
 4 files changed, 8 insertions(+), 5 deletions(-)


---
base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc
change-id: 20240507-b4-b4-sio-sr_select_speed-e68c0d426891

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

On Wed, May 08, 2024 at 05:22:51PM +0000, Justin Stitt wrote:
> Running syzkaller with the newly reintroduced signed integer overflow
> sanitizer produces this report:
> 
> [   65.194362] ------------[ cut here ]------------
> [   65.197752] UBSAN: signed-integer-overflow in ../drivers/scsi/sr_ioctl.c:436:9
> [   65.203607] -2147483648 * 177 cannot be represented in type 'int'
> [   65.207911] CPU: 2 PID: 10416 Comm: syz-executor.1 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1
> [   65.213585] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [   65.219923] Call Trace:
> [   65.221556]  <TASK>
> [   65.223029]  dump_stack_lvl+0x93/0xd0
> [   65.225573]  handle_overflow+0x171/0x1b0
> [   65.228219]  sr_select_speed+0xeb/0xf0
> [   65.230786]  ? __pm_runtime_resume+0xe6/0x130
> [   65.233606]  sr_block_ioctl+0x15d/0x1d0
> ...
> 
> 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").
> 
> Firstly, let's change the type of "speed" to unsigned long as
> sr_select_speed()'s only caller passes in an unsigned long anyways.
> 
> $ git grep '\.select_speed'
> |	drivers/scsi/sr.c:      .select_speed           = sr_select_speed,
> ...
> |	static int cdrom_ioctl_select_speed(struct cdrom_device_info *cdi,
> |	                unsigned long arg)
> |	{
> |	        ...
> |	        return cdi->ops->select_speed(cdi, arg);
> |	}
> 
> Next, let's add an extra check to make sure we don't exceed 0xffff/177
> (350) since 0xffff is the max speed. This has two benefits: 1) we deal
> with integer overflow before it happens and 2) we properly respect the
> max speed of 0xffff. There are some "magic" numbers here but I did not
> want to change more than what was necessary.
> 
> Link: https://github.com/llvm/llvm-project/pull/82432 [1]
> Closes: https://github.com/KSPP/linux/issues/357
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@google.com>

Yeah, this looks good. Thanks!

Reviewed-by: Kees Cook <keescook@chromium.org>

Justin,

> Running syzkaller with the newly reintroduced signed integer overflow
> sanitizer produces this report:
>
> [   65.194362] ------------[ cut here ]------------
> [   65.197752] UBSAN: signed-integer-overflow in ../drivers/scsi/sr_ioctl.c:436:9
> [   65.203607] -2147483648 * 177 cannot be represented in type 'int'
> [   65.207911] CPU: 2 PID: 10416 Comm: syz-executor.1 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1
> [   65.213585] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [   65.219923] Call Trace:
> [   65.221556]  <TASK>
> [   65.223029]  dump_stack_lvl+0x93/0xd0
> [   65.225573]  handle_overflow+0x171/0x1b0
> [   65.228219]  sr_select_speed+0xeb/0xf0
> [   65.230786]  ? __pm_runtime_resume+0xe6/0x130
> [   65.233606]  sr_block_ioctl+0x15d/0x1d0
> ...
>

Applied to 6.10/scsi-staging, thanks!