Re: [PATCH] powerpc/powernv/dump: Fix race while processing OPAL dump

[Michael Ellerman <mpe@ellerman.id.au>]

Vasant Hegde <hegdevasant@linux.vnet.ibm.com> writes:
> diff --git a/arch/powerpc/platforms/powernv/opal-dump.c b/arch/powerpc/platforms/powernv/opal-dump.c
> index 543c816fa99e..7e6eeedec32b 100644
> --- a/arch/powerpc/platforms/powernv/opal-dump.c
> +++ b/arch/powerpc/platforms/powernv/opal-dump.c
> @@ -346,21 +345,39 @@ static struct dump_obj *create_dump_obj(uint32_t id, size_t size,
>  	rc = kobject_add(&dump->kobj, NULL, "0x%x-0x%x", type, id);
>  	if (rc) {
>  		kobject_put(&dump->kobj);
> -		return NULL;
> +		return;
>  	}
>  
> +	/*
> +	 * As soon as the sysfs file for this dump is created/activated there is
> +	 * a chance the opal_errd daemon (or any userspace) might read and
> +	 * acknowledge the dump before kobject_uevent() is called. If that
> +	 * happens then there is a potential race between
> +	 * dump_ack_store->kobject_put() and kobject_uevent() which leads to a
> +	 * use-after-free of a kernfs object resulting in a kernel crash.
> +	 *
> +	 * To avoid that, we need to take a reference on behalf of the bin file,
> +	 * so that our reference remains valid while we call kobject_uevent().
> +	 * We then drop our reference before exiting the function, leaving the
> +	 * bin file to drop the last reference (if it hasn't already).
> +	 */
> +
> +	/* Take a reference for the bin file */
> +	kobject_get(&dump->kobj);
>  	rc = sysfs_create_bin_file(&dump->kobj, &dump->dump_attr);
>  	if (rc) {
>  		kobject_put(&dump->kobj);
> -		return NULL;
> +		/* Drop reference count taken for bin file */
> +		kobject_put(&dump->kobj);
> +		return;
>  	}
>  
>  	pr_info("%s: New platform dump. ID = 0x%x Size %u\n",
>  		__func__, dump->id, dump->size);
>  
>  	kobject_uevent(&dump->kobj, KOBJ_ADD);
> -
> -	return dump;
> +	/* Drop reference count taken for bin file */
> +	kobject_put(&dump->kobj);
>  }

I think this would be better if it was reworked along the lines of:

aea948bb80b4 ("powerpc/powernv/elog: Fix race while processing OPAL error log event.")

cheers