xen: detect uninitialized xenbus in xenbus_init

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 all bits set as an invalid value.

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@vger.kernel.org
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Link: https://lore.kernel.org/r/20211123210748.1910236-1-sstabellini@kernel.org
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
index 5967aa9..fe360c3 100644 (file)
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -949,6 +949,29 @@ 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 all bits set as an invalid value.
+                */
+               if (!v || !~v) {
+                       err = -ENOENT;
+                       goto out_error;
+               }
+               /* Avoid truncation on 32-bit. */
+#if BITS_PER_LONG == 32
+               if (v > ULONG_MAX) {
+                       pr_err("%s: cannot handle HVM_PARAM_STORE_PFN=%llx > ULONG_MAX\n",
+                              __func__, v);
+                       err = -EINVAL;
+                       goto out_error;
+               }
+#endif
                xen_store_gfn = (unsigned long)v;
                xen_store_interface =
                        xen_remap(xen_store_gfn << XEN_PAGE_SHIFT,