[PATCH 7/9] Drivers: hv: vmbus: Bring the post_msg_page back for TDX VMs with the paravisor

Dexuan Cui
2023-08-11T22:20:22Z

The post_msg_page was removed in
commit 9a6b1a170ca8 ("Drivers: hv: vmbus: Remove the per-CPU post_msg_page")

However, it turns out that we need to bring it back, but only for a TDX VM
with the paravisor: in such a VM, the hyperv_pcpu_input_arg is not decrypted,
but the HVCALL_POST_MESSAGE in such a VM needs a decrypted page as the
hypercall input page: see the comments in hyperv_init() for a detailed
explanation.

Except for HVCALL_POST_MESSAGE and HVCALL_SIGNAL_EVENT, the other hypercalls
in a TDX VM with the paravisor still use hv_hypercall_pg and must use the
hyperv_pcpu_input_arg (which is encrypted in such a VM), when a hypercall
input page is used.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
---
arch/x86/hyperv/hv_init.c | 20 +++++++++++--
drivers/hv/hv.c | 63 ++++++++++++++++++++++++++++++++++-----
drivers/hv/hyperv_vmbus.h | 11 +++++++
3 files changed, 85 insertions(+), 9 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index e67e2430fba35..9c12a199ea62c 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -485,6 +485,22 @@ void __init hyperv_init(void)
* Setup the hypercall page and enable hypercalls.
* 1. Register the guest ID
* 2. Enable the hypercall and register the hypercall page
+ *
+ * A TDX VM with no paravisor only uses TDX GHCI rather than hv_hypercall_pg:
+ * when the hypercall input is a page, such a VM must pass a decrypted
+ * page to Hyper-V, e.g. hv_post_message() uses the per-CPU page
+ * hyperv_pcpu_input_arg, which is decrypted if no paravisor is present.
+ *
+ * A TDX VM with the paravisor uses hv_hypercall_pg for most hypercalls,
+ * which are handled by the paravisor and the VM must use an encrypted
+ * input page: in such a VM, the hyperv_pcpu_input_arg is encrypted and
+ * used in the hypercalls, e.g. see hv_mark_gpa_visibility() and
+ * hv_arch_irq_unmask(). Such a VM uses TDX GHCI for two hypercalls:
+ * 1. HVCALL_SIGNAL_EVENT: see vmbus_set_event() and _hv_do_fast_hypercall8().
+ * 2. HVCALL_POST_MESSAGE: the input page must be a decrypted page, i.e.
+ * hv_post_message() in such a VM can't use the encrypted hyperv_pcpu_input_arg;
+ * instead, hv_post_message() uses the post_msg_page, which is decrypted
+ * in such a VM and is only used in such a VM.
*/
guest_id = hv_generate_guest_id(LINUX_VERSION_CODE);
wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id);
@@ -492,8 +508,8 @@ void __init hyperv_init(void)
/* Hyper-V requires to write guest os id via ghcb in SNP IVM. */
hv_ghcb_msr_write(HV_X64_MSR_GUEST_OS_ID, guest_id);

- /* A TDX guest uses the GHCI call rather than hv_hypercall_pg. */
- if (hv_isolation_type_tdx())
+ /* A TDX VM with no paravisor only uses TDX GHCI rather than hv_hypercall_pg */
+ if (hv_isolation_type_tdx() && !hyperv_paravisor_present)
goto skip_hypercall_pg_init;

hv_hypercall_pg = __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START,
diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index 20bc44923e4f0..6b5f1805d4749 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -57,20 +57,39 @@ int hv_post_message(union hv_connection_id connection_id,

local_irq_save(flags);

- aligned_msg = *this_cpu_ptr(hyperv_pcpu_input_arg);
+ /*
+ * A TDX VM with the paravisor must use the decrypted post_msg_page: see
+ * the comment in struct hv_per_cpu_context. A SNP VM with the paravisor
+ * can use the encrypted hyperv_pcpu_input_arg because it copies the
+ * input into the GHCB page, which has been decrypted by the paravisor.
+ */
+ if (hv_isolation_type_tdx() && hyperv_paravisor_present)
+ aligned_msg = this_cpu_ptr(hv_context.cpu_context)->post_msg_page;
+ else
+ aligned_msg = *this_cpu_ptr(hyperv_pcpu_input_arg);
+
aligned_msg->connectionid = connection_id;
aligned_msg->reserved = 0;
aligned_msg->message_type = message_type;
aligned_msg->payload_size = payload_size;
memcpy((void *)aligned_msg->payload, payload, payload_size);

- if (hv_isolation_type_snp())
- status = hv_ghcb_hypercall(HVCALL_POST_MESSAGE,
- (void *)aligned_msg, NULL,
- sizeof(*aligned_msg));
- else
+ if (hyperv_paravisor_present) {
+ if (hv_isolation_type_tdx()) {
+ u64 gpa_boundary = ms_hyperv.shared_gpa_boundary;
+ u64 in = virt_to_phys(aligned_msg) | gpa_boundary;
+
+ status = hv_tdx_hypercall(HVCALL_POST_MESSAGE, in, 0);
+ } else if (hv_isolation_type_snp())
+ status = hv_ghcb_hypercall(HVCALL_POST_MESSAGE,
+ aligned_msg, NULL,
+ sizeof(*aligned_msg));
+ else
+ status = HV_STATUS_INVALID_PARAMETER;
+ } else {
status = hv_do_hypercall(HVCALL_POST_MESSAGE,
aligned_msg, NULL);
+ }

local_irq_restore(flags);

@@ -105,6 +124,24 @@ int hv_synic_alloc(void)
tasklet_init(&hv_cpu->msg_dpc,
vmbus_on_msg_dpc, (unsigned long) hv_cpu);

+ if (hyperv_paravisor_present && hv_isolation_type_tdx()) {
+ hv_cpu->post_msg_page = (void *)get_zeroed_page(GFP_ATOMIC);
+ if (hv_cpu->post_msg_page == NULL) {
+ pr_err("Unable to allocate post msg page\n");
+ goto err;
+ }
+
+ ret = set_memory_decrypted((unsigned long)hv_cpu->post_msg_page, 1);
+ if (ret) {
+ pr_err("Failed to decrypt post msg page: %d\n", ret);
+ /* Just leak the page, as it's unsafe to free the page. */
+ hv_cpu->post_msg_page = NULL;
+ goto err;
+ }
+
+ memset(hv_cpu->post_msg_page, 0, PAGE_SIZE);
+ }
+
/*
* Synic message and event pages are allocated by paravisor.
* Skip these pages allocation here.
@@ -178,7 +215,18 @@ void hv_synic_free(void)
= per_cpu_ptr(hv_context.cpu_context, cpu);

/* It's better to leak the page if the encryption fails. */
- if (!ms_hyperv.paravisor_present &&
+ if (hyperv_paravisor_present && hv_isolation_type_tdx()) {
+ if (hv_cpu->post_msg_page) {
+ ret = set_memory_encrypted((unsigned long)
+ hv_cpu->post_msg_page, 1);
+ if (ret) {
+ pr_err("Failed to encrypt post msg page: %d\n", ret);
+ hv_cpu->post_msg_page = NULL;
+ }
+ }
+ }
+
+ if (!hyperv_paravisor_present &&
(hv_isolation_type_en_snp() || hv_isolation_type_tdx())) {
if (hv_cpu->synic_message_page) {
ret = set_memory_encrypted((unsigned long)
@@ -199,6 +247,7 @@ void hv_synic_free(void)
}
}

+ free_page((unsigned long)hv_cpu->post_msg_page);
free_page((unsigned long)hv_cpu->synic_event_page);
free_page((unsigned long)hv_cpu->synic_message_page);
}
diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
index 55f2086841ae4..f6b1e710f8055 100644
--- a/drivers/hv/hyperv_vmbus.h
+++ b/drivers/hv/hyperv_vmbus.h
@@ -123,6 +123,17 @@ struct hv_per_cpu_context {
void *synic_message_page;
void *synic_event_page;

+ /*
+ * The page is only used in hv_post_message() for a TDX VM (with the
+ * paravisor) to post a messages to Hyper-V: when such a VM calls
+ * HVCALL_POST_MESSAGE, it can't use the hyperv_pcpu_input_arg (which
+ * is encrypted in such a VM) as the hypercall input page, because
+ * the input page for HVCALL_POST_MESSAGE must be decrypted in such a
+ * VM, so post_msg_page (which is decrypted in hv_synic_alloc()) is
+ * introduced for this purpose. See hyperv_init() for more comments.
+ */
+ void *post_msg_page;
+
/*
* Starting with win8, we can take channel interrupts on any CPU;
* we will manage the tasklet that handles events messages on a per CPU
--
2.25.1