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

[quic_zijuhu <quic_zijuhu@quicinc.com>]

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.

do you have suggestion about term to use?

>>
>> Fixes: 9b3fa47d4a76 ("kobject: fix suppressing modalias in uevents delivered over netlink")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
>> ---
>>  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 ?

> thanks,
> 
> greg k-h