Subject: Re: [net-next 01/14] lib: cpu_rmap: Avoid use after free on rmap->obj array entries

From: Jakub Kicinski <[email-masked]>
Date: Tue, 21 Mar 2023 20:38:36 -0700

On Mon, 20 Mar 2023 10:51:31 -0700 Saeed Mahameed wrote:
> From: Eli Cohen <[email-masked]>
> 
> When calling irq_set_affinity_notifier() with NULL at the notify
> argument, it will cause freeing of the glue pointer in the
> corresponding array entry but will leave the pointer in the array. A
> subsequent call to free_irq_cpu_rmap() will try to free this entry again
> leading to possible use after free.
> 
> Fix that by setting NULL to the array entry and checking that we have
> non-zero at the array entry when iterating over the array in
> free_irq_cpu_rmap().

Commit message needs some work. Are you trying to make double
free_irq_cpu_rmap() work fine because of callers? Are there problems
with error path of irq_cpu_rmap_add()? I can tell what you're trying 
to prevent but not why.

> Fixes: c39649c331c7 ("lib: cpu_rmap: CPU affinity reverse-mapping")

What is this Fixes tag doing in a net-next patch :S
If it can be triggered it needs to go to net.

> Signed-off-by: Eli Cohen <[email-masked]>
> Signed-off-by: Saeed Mahameed <[email-masked]>
> ---
>  lib/cpu_rmap.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/cpu_rmap.c b/lib/cpu_rmap.c
> index f08d9c56f712..e77f12bb3c77 100644
> --- a/lib/cpu_rmap.c
> +++ b/lib/cpu_rmap.c
> @@ -232,7 +232,8 @@ void free_irq_cpu_rmap(struct cpu_rmap *rmap)
>  
>  	for (index = 0; index < rmap->used; index++) {

After looking at this code for 10min - isn't the problem that used 
is never decremented on the error path? 

I don't see a way to remove from the map so it can't be sparse.

>  		glue = rmap->obj[index];
> -		irq_set_affinity_notifier(glue->notify.irq, NULL);
> +		if (glue)
> +			irq_set_affinity_notifier(glue->notify.irq, NULL);
>  	}
>  
>  	cpu_rmap_put(rmap);
> @@ -268,6 +269,7 @@ static void irq_cpu_rmap_release(struct kref *ref)
>  		container_of(ref, struct irq_glue, notify.kref);
>  
>  	cpu_rmap_put(glue->rmap);
> +	glue->rmap->obj[glue->index] = NULL;
>  	kfree(glue);
>  }
>  
> @@ -297,6 +299,7 @@ int irq_cpu_rmap_add(struct cpu_rmap *rmap, int irq)
>  	rc = irq_set_affinity_notifier(irq, &glue->notify);
>  	if (rc) {
>  		cpu_rmap_put(glue->rmap);
> +		rmap->obj[glue->index] = NULL;
>  		kfree(glue);
>  	}
>  	return rc;