linux-kernel - Re: [PATCH] kobject: Fix global-out-of-bounds in kobject_action

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Date:   Tue, 7 Mar 2023 17:08:02 +0800
From:   Xia Fukun <xiafukun@...wei.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     <prajnoha@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kobject: Fix global-out-of-bounds in
 kobject_action_type()

Thanks for your reply.
Your understanding is correct."offline\0\0\0\0\0\0\0\0\0\0" is indeed blocked
from matching "offline" and returns a failed result.

I'm not sure whether to relax the restrictions to make it match successfully.
After all, the incoming count is too large and not the actual length of
"offline".

在 2023/3/7 16:16, Greg KH 写道:
> On Tue, Mar 07, 2023 at 02:37:57PM +0800, Xia Fukun wrote:
>> The following c language code can trigger KASAN's global variable
>> out-of-bounds access error in kobject_action_type():
>>
>>
>> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
>> index 7c44b7ae4c5c..668346bd28fa 100644
>> --- a/lib/kobject_uevent.c
>> +++ b/lib/kobject_uevent.c
>> @@ -84,7 +84,7 @@ static int kobject_action_type(const char *buf, size_t count,
>>  	for (action = 0; action < ARRAY_SIZE(kobject_actions); action++) {
>>  		if (strncmp(kobject_actions[action], buf, count_first) != 0)
>>  			continue;
>> -		if (kobject_actions[action][count_first] != '\0')
>> +		if (strlen(kobject_actions[action]) != count_first)
>>  			continue;
>>  		if (args)
>>  			*args = args_start;
> 
> Yes, this will stop us from reading a read-only location somewhere in
> the kernel outside of the string array, but is it still doing the same
> functional logic here?
> 
> In your change, this call to strlen will cause the length check to fail,
> so the loop will continue on, and the type will never be set properly.
> Is that correct in your testing?  You just prevented a string of
> "offline\0\0\0\0\0\0\0\0\0\0" from properly being parsed as an offline
> event, which I don't think is what you meant to do, right?
> 
> Or am I reading this code incorrectly?  It really could be cleaned up,
> it's not obvious at all.  Parsing strings in C is a mess...
> 
> thanks,
> 
> greg k-h
> 
>