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

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

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@vger.kernel.org
> Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
> ---
> V1 -> V2: Correct commit messages and add inline comments
> 
> V1 discussion link:
> https://lore.kernel.org/lkml/0b916393-eb39-4467-9c99-ac1bc9746512@quicinc.com/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.

Otherwise:

Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

>  
>  			for (j = i; j < env->envp_idx - 1; j++)
>  				env->envp[j] = env->envp[j + 1] - len;
> -- 
> 2.7.4
> 

Thanks.

-- 
Dmitry