linux-kernel - Re: [PATCH] xen: detect uninitialized xenbus in xenbus

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Date:   Wed, 17 Nov 2021 08:41:56 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     boris.ostrovsky@...cle.com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano.stabellini@...inx.com>,
        stable@...r.kernel.org, jgross@...e.com
Subject: Re: [PATCH] xen: detect uninitialized xenbus in xenbus_init

On 17.11.2021 03:11, Stefano Stabellini wrote:
> --- a/drivers/xen/xenbus/xenbus_probe.c
> +++ b/drivers/xen/xenbus/xenbus_probe.c
> @@ -951,6 +951,18 @@ static int __init xenbus_init(void)
>  		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
>  		if (err)
>  			goto out_error;
> +		/*
> +		 * Uninitialized hvm_params are zero and return no error.
> +		 * Although it is theoretically possible to have
> +		 * HVM_PARAM_STORE_PFN set to zero on purpose, in reality it is
> +		 * not zero when valid. If zero, it means that Xenstore hasn't
> +		 * been properly initialized. Instead of attempting to map a
> +		 * wrong guest physical address return error.
> +		 */
> +		if (v == 0) {
> +			err = -ENOENT;
> +			goto out_error;
> +		}

If such a check gets added, then I think known-invalid frame numbers
should be covered at even higher a priority than zero. This would,
for example, also mean to ...

>  		xen_store_gfn = (unsigned long)v;

... stop silently truncating a value here.

By covering them we would then have the option to pre-fill PFN params
with, say, ~0 in the hypervisor (to clearly identify them as invalid,
rather than having to guess at the validity of 0). I haven't really
checked yet whether such a change would be compatible with existing
software ...

Jan