Re: [PATCH] kobject_uevent: Fix OOB access within zap_modalias_env()

[quic_zijuhu]

On 5/24/2024 1:21 PM, Greg KH wrote:
> On Fri, May 24, 2024 at 01:15:01PM +0800, quic_zijuhu wrote:
>> On 5/24/2024 12:33 PM, Greg KH wrote:
>>> On Fri, May 24, 2024 at 12:20:03PM +0800, Zijun Hu wrote:
>>>> zap_modalias_env() wrongly calculates size of memory block
>>>> to move, so maybe cause OOB memory access issue, fixed by
>>>> correcting size to memmove.
>>>
>>> "maybe" or "does"?  That's a big difference :)
>>>
>> i found this issue by reading code instead of really meeting this issue.
>> this issue should be prone to happen if there are more than 1 other
>> environment vars.
> 
> But does it?  Given that we have loads of memory checkers, and I haven't
> ever seen any report of any overrun, it would be nice to be sure.
> 
yes. if @env includes env vairable MODALIAS and  more than one other env
vairables. then (env->buflen - len) must be greater that actual size of
"target block" shown previously, so the OOB issue must happen.
>> do you have suggestion about term to use?
> 
> Some confirmation that this really is the case would be nice :)
> 
>>>> Fixes: 9b3fa47d4a76 ("kobject: fix suppressing modalias in uevents delivered over netlink")
>>>> Cc: stable@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>
>>>> ---
>>>>  lib/kobject_uevent.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
>>>> index 03b427e2707e..f153b4f9d4d9 100644
>>>> --- a/lib/kobject_uevent.c
>>>> +++ b/lib/kobject_uevent.c
>>>> @@ -434,7 +434,7 @@ static void zap_modalias_env(struct kobj_uevent_env *env)
>>>>  
>>>>  		if (i != env->envp_idx - 1) {
>>>>  			memmove(env->envp[i], env->envp[i + 1],
>>>> -				env->buflen - len);
>>>> +				env->buf + env->buflen - env->envp[i + 1]);
>>>
>>> How is this "more correct"?  Please explain it better, this logic is not
>>> obvious at all.
>>>
>> env->envp[] contains pointers to env->buf[] with length env->buflen,
>> we want to delete environment variable pointed by env->envp[i] with
>> length @len as shown below.
>>
>> env->buf[]            |-> target block <-|
>> 0-----------------------------------------env->buflen
>>         ^             ^
>> 	| ->  @len <- |
>>   env->envp[i]   env->envp[i+1]
>>
>> so move "target block" forward by @len, so size of target block is
>> env->buf + env->buflen - env->envp[i+1] instead of env->buflen
>> -len.
>>
>> do you suggest add inline comments to explain it ?
> 
> Yes please.
> 
> thanks,
> 
> greg k-h