Re: [PATCH v2] xen: detect uninitialized xenbus in xenbus_init

From: Boris Ostrovsky
Date: Fri Nov 19 2021 - 17:32:34 EST

On 11/19/21 3:29 PM, 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>

Reviewed-by: Boris Ostrovsky <boris.ostrvsky@xxxxxxxxxx>