Re: [PATCH v3] xen: detect uninitialized xenbus in xenbus_init

[Juergen Gross]

On 22.11.21 23:16, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
>
> If the xenstore page hasn't been allocated properly, reading the value
> of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually return
> error. Instead, it will succeed and return zero. Instead of attempting
> to xen_remap a bad guest physical address, detect this condition and
> return early.
>
> Note that although a guest physical address of zero for
> HVM_PARAM_STORE_PFN is theoretically possible, it is not a good choice
> and zero has never been validly used in that capacity.
>
> Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
>
> For 32-bit Linux, any pfn above ULONG_MAX would get truncated. Pfns
> above ULONG_MAX should never be passed by the Xen tools to HVM guests
> anyway, so check for this condition and return early.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> ---
> Changes in v3:
> - improve in-code comment
> - improve check
>
> Changes in v2:
> - add check for ULLONG_MAX (unitialized)
> - add check for ULONG_MAX #if BITS_PER_LONG == 32 (actual error)
> - add pr_err error message
> ---
>   drivers/xen/xenbus/xenbus_probe.c | 24 ++++++++++++++++++++++++
>   1 file changed, 24 insertions(+)
>
> diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
> index 94405bb3829e..d3ca57d48a73 100644
> --- a/drivers/xen/xenbus/xenbus_probe.c
> +++ b/drivers/xen/xenbus/xenbus_probe.c
> @@ -951,6 +951,30 @@ 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.
> +		 *
> +		 * Also recognize the invalid value of INVALID_PFN which is
> +		 * ULLONG_MAX.

Adjust the comment, e.g. s/ULLONG_MAX/all bits set/ (in the commit
message, too)?

> +		 */
> +		if (!v || !(v + 1)) {

For me "if (!v || !~v)" would be more readable, but I don't really feel
strong here.


Juergen

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