Re: [PATCH v2] kobject_uevent: Fix OOB access within zap_modalias_env()

[quic_zijuhu]

On 5/29/2024 6:17 PM, Greg KH wrote:
> On Wed, May 29, 2024 at 06:07:06PM +0800, quic_zijuhu wrote:
>> On 5/29/2024 2:56 AM, Dmitry Torokhov wrote:
>>> On Tue, May 28, 2024 at 11:19:07AM +0800, Zijun Hu wrote:
>>>> zap_modalias_env() wrongly calculates size of memory block to move, so
>>>> will cause OOB memory access issue if variable MODALIAS is not the last
>>>> one within its @env parameter, fixed by correcting size to memmove.
>>>>
>>>> Fixes: 9b3fa47d4a76 ("kobject: fix suppressing modalias in uevents delivered over netlink")
>>>> Cc: stable@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>
>>>> ---
>>>> V1 -> V2: Correct commit messages and add inline comments
>>>>
>>>> V1 discussion link:
>>>> https://lore.kernel.org/lkml/0b916393-eb39-4467-9c99-ac1bc9746512@xxxxxxxxxxx/T/#m8d80165294640dbac72f5c48d14b7ca4f097b5c7
>>>>
>>>>  lib/kobject_uevent.c | 17 ++++++++++++++++-
>>>>  1 file changed, 16 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
>>>> index 03b427e2707e..f22366be020c 100644
>>>> --- a/lib/kobject_uevent.c
>>>> +++ b/lib/kobject_uevent.c
>>>> @@ -433,8 +433,23 @@ static void zap_modalias_env(struct kobj_uevent_env *env)
>>>>  		len = strlen(env->envp[i]) + 1;
>>>>  
>>>>  		if (i != env->envp_idx - 1) {
>>>> +			/* @env->envp[] contains pointers to @env->buf[]
>>>> +			 * with @env->buflen elements, and we want to
>>>> +			 * remove variable MODALIAS pointed by
>>>> +			 * @env->envp[i] with length @len as shown below:
>>>> +			 *
>>>> +			 * 0          @env->buf[]      @env->buflen
>>>> +			 * ----------------------------------------
>>>> +			 *      ^              ^                  ^
>>>> +			 *      |->   @len   <-|   target block   |
>>>> +			 * @env->envp[i]  @env->envp[i+1]
>>>> +			 *
>>>> +			 * so the "target block" indicated above is moved
>>>> +			 * backward by @len, and its right size is
>>>> +			 * (@env->buf + @env->buflen - @env->envp[i + 1]).
>>>> +			 */
>>>>  			memmove(env->envp[i], env->envp[i + 1],
>>>> -				env->buflen - len);
>>>> +				env->buf + env->buflen - env->envp[i + 1]);
>>>
>>> Thank you for noticing this, it is indeed a bug.
>>>
>>> I wonder if this would not be expressed better as:
>>>
>>> 			tail_len = env->buflen - (env->envp[i + 1] - env->envp[0]);
>>> 			memmove(env->envp[i], env->envp[i + 1], tail_len);
>>>
>>> and we would not need the large comment.
>>>
>> Greg KH suggests add inline comments since my fix is not obvious with
>> first glance, let us wait for his comments within 2 days about below
>> question:
>> is it okay to remove those inline comments if block size to move is
>> changed to env->buflen - (env->envp[i + 1] - env->envp[0]) ?
> 
> I'm all for making this simpler, please do so as Dmitry's response looks
> better and easier to understand, don't you think?
> 
> And add comments, they are always good here for stuff like this :)
> 
okay, will send v3 to take Dmitry's suggestion and keep inline comments.
> thanks,
> 
> greg k-h