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

[quic_zijuhu]

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]) ?
> Otherwise:
> 
> Acked-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
> 
>>  
>>  			for (j = i; j < env->envp_idx - 1; j++)
>>  				env->envp[j] = env->envp[j + 1] - len;
>> -- 
>> 2.7.4
>>
> 
> Thanks.
>