On 18.11.2021 06:32, Juergen Gross wrote:
On 18.11.21 03:37, Stefano Stabellini wrote:
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -951,6 +951,28 @@ static int __init xenbus_init(void)
err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
if (err)
goto out_error;
+ /*
+ * Return error on an invalid value.
+ *
+ * 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) {
Make this "if (v == ULONG_MAX || v== 0)" instead?
This would result in the same err on a new and an old hypervisor
(assuming we switch the hypervisor to init params with ~0UL).
+ err = -ENOENT;
+ goto out_error;
+ }
+ /*
+ * ULONG_MAX is invalid on 64-bit because is INVALID_PFN.
+ * On 32-bit return error to avoid truncation.
+ */
+ if (v >= ULONG_MAX) {
+ err = -EINVAL;
+ goto out_error;
+ }
Does it make sense to continue the system running in case of
truncation? This would be a 32-bit guest with more than 16TB of RAM
and the Xen tools decided to place the Xenstore ring page above the
16TB boundary. This is a completely insane scenario IMO.
A proper panic() in this case would make diagnosis of that much
easier (me having doubts that this will ever be hit, though).
While I agree panic() may be an option here (albeit I'm not sure why
that would be better than trying to cope with 0 and hence without
xenbus), I'd like to point out that the amount of RAM assigned to a
guest is unrelated to the choice of GFNs for the various "magic"
items.
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