On 18.11.21 09:47, Jan Beulich wrote:
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
I could imagine someone wanting to run a guest without Xenstore access, which BTW will happen in case of a guest created by the hypervisor at boot time.
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.
Yes, but this would still be a major tools problem which probably would render the whole guest rather unusable. Juergen
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