linux-kernel - Re: [PATCH] scs: Fix a wrong parameter in __scs

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Message-ID: <8ee8f1d2-feb6-4b96-af17-c3a690e01a46@vivo.com>
Date: Mon, 13 Oct 2025 08:19:36 +0000
From: 林芝驰 <zhichi.lin@...o.com>
To: Andrew Morton <akpm@...ux-foundation.org>
CC: "elver@...gle.com" <elver@...gle.com>, "will@...nel.org"
	<will@...nel.org>, "andreyknvl@...il.com" <andreyknvl@...il.com>,
	"samitolvanen@...gle.com" <samitolvanen@...gle.com>, "yee.lee@...iatek.com"
	<yee.lee@...iatek.com>, "keescook@...omium.org" <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	谢纪元 <xiejiyuan@...o.com>
Subject: Re: [PATCH] scs: Fix a wrong parameter in __scs_magic

On 10/12/2025 1:11 AM, Andrew Morton wrote:
> On Sat, 11 Oct 2025 16:22:22 +0800 Zhichi Lin <zhichi.lin@...o.com> wrote:
> 
>> __scs_magic() needs a 'void *' variable, but a 'struct task_struct *'
>> is given. 'task_scs(tsk)' is the starting address of the task's shadow
>> call stack, and '__scs_magic(task_scs(tsk))' is the end address of the
>> task's shadow call stack.
>> Here should be '__scs_magic(task_scs(tsk))'.
> 
> What are the userspace-visible runtime effects of this bug?  Please
> always describe this when fixing something.
> 

The user-visible effect of this bug is that when 
CONFIG_DEBUG_STACK_USAGE is enabled, the shadow call stack usage 
checking function (scs_check_usage) would scan an incorrect memory 
range. This could lead to:

1. **Inaccurate stack usage reporting**: The function would calculate 
wrong usage statistics for the shadow call stack, potentially showing 
incorrect value in kmsg.

2. **Potential kernel crash**: If the value of __scs_magic(tsk)is 
greater than that of __scs_magic(task_scs(tsk)), the for loop may access 
unmapped memory, potentially causing a kernel panic. However, this 
scenario is unlikely because task_struct is allocated via the slab 
allocator (which typically returns lower addresses), while the shadow 
call stack returned by task_scs(tsk) is allocated via vmalloc(which 
typically returns higher addresses).

However, since this is purely a debugging feature 
(CONFIG_DEBUG_STACK_USAGE), normal production systems should be not 
unaffected. The bug only impacts developers and testers who are
actively debugging stack usage with this configuration enabled.

Additionally, this issue was discovered during code reading - our 
project does not actually use this feature. For a more accurate 
assessment of real-world impact and backporting necessity, I think Will 
and Sami would be better positioned to evaluate, as they have deeper 
knowledge of how this debugging feature is actually used.

>> Fixes: 5bbaf9d1fcb9 ("scs: Add support for stack usage debugging")
> 
> This might need backporting into -stable kernels, that depends on the
> answer to the above question.
> 
>> --- a/kernel/scs.c
>> +++ b/kernel/scs.c
>> @@ -135,7 +135,7 @@ static void scs_check_usage(struct task_struct *tsk)
>>   	if (!IS_ENABLED(CONFIG_DEBUG_STACK_USAGE))
>>   		return;
>>   
>> -	for (p = task_scs(tsk); p < __scs_magic(tsk); ++p) {
>> +	for (p = task_scs(tsk); p < __scs_magic(task_scs(tsk)); ++p) {
>>   		if (!READ_ONCE_NOCHECK(*p))
>>   			break;
>>   		used += sizeof(*p);
> 
> Thanks, I'll grab the patch for now, maybe Will would prefer to take it?