linux-hardening - Re: [PATCH v2] scsi: sr: fix unintentional arithmetic wraparound

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Date: Wed, 8 May 2024 10:34:41 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Bill Wendling <morbo@...gle.com>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] scsi: sr: fix unintentional arithmetic wraparound

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@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

Yeah, this looks good. Thanks!

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

-- 
Kees Cook