[PATCH v3 05/10] Drivers: hv: vmbus: Support >64 VPs for a fully enlightened TDX/SNP VM

Dexuan Cui
2023-08-24T08:11:01Z

Don't set *this_cpu_ptr(hyperv_pcpu_input_arg) before the function
set_memory_decrypted() returns, otherwise we run into this ticky issue:

For a fully enlightened TDX/SNP VM, in hv_common_cpu_init(),
*this_cpu_ptr(hyperv_pcpu_input_arg) is an encrypted page before
the set_memory_decrypted() returns.

When such a VM has more than 64 VPs, if the hyperv_pcpu_input_arg is not
NULL, hv_common_cpu_init() -> set_memory_decrypted() -> ... ->
cpa_flush() -> on_each_cpu() -> ... -> hv_send_ipi_mask() -> ... ->
__send_ipi_mask_ex() tries to call hv_do_rep_hypercall() with the
hyperv_pcpu_input_arg as the hypercall input page, which must be a
decrypted page in such a VM, but the page is still encrypted at this
point, and a fatal fault is triggered.

Fix the issue by setting *this_cpu_ptr(hyperv_pcpu_input_arg) after
set_memory_decrypted(): if the hyperv_pcpu_input_arg is NULL,
__send_ipi_mask_ex() returns HV_STATUS_INVALID_PARAMETER immediately,
and hv_send_ipi_mask() falls back to orig_apic.send_IPI_mask(),
which can use x2apic_send_IPI_all(), which may be slightly slower than
the hypercall but still works correctly in such a VM.

Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Tianyu Lan <tiala@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
---

Changes in v2: None

Changes in v3:
Added Michael's and Tianyu's Reviewed-by
Fixed a typo in the changelog:
hv_do_fast_hypercall16 -> hv_do_rep_hypercall

drivers/hv/hv_common.c | 30 +++++++++++++++++++++++-------
1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
index 897bbb96f411..4c858e1636da 100644
--- a/drivers/hv/hv_common.c
+++ b/drivers/hv/hv_common.c
@@ -360,6 +360,7 @@ int hv_common_cpu_init(unsigned int cpu)
u64 msr_vp_index;
gfp_t flags;
int pgcount = hv_root_partition ? 2 : 1;
+ void *mem;
int ret;

/* hv_cpu_init() can be called with IRQs disabled from hv_resume() */
@@ -372,25 +373,40 @@ int hv_common_cpu_init(unsigned int cpu)
* allocated if this CPU was previously online and then taken offline
*/
if (!*inputarg) {
- *inputarg = kmalloc(pgcount * HV_HYP_PAGE_SIZE, flags);
- if (!(*inputarg))
+ mem = kmalloc(pgcount * HV_HYP_PAGE_SIZE, flags);
+ if (!mem)
return -ENOMEM;

if (hv_root_partition) {
outputarg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
- *outputarg = (char *)(*inputarg) + HV_HYP_PAGE_SIZE;
+ *outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
}

if (hv_isolation_type_en_snp() || hv_isolation_type_tdx()) {
- ret = set_memory_decrypted((unsigned long)*inputarg, pgcount);
+ ret = set_memory_decrypted((unsigned long)mem, pgcount);
if (ret) {
- /* It may be unsafe to free *inputarg */
- *inputarg = NULL;
+ /* It may be unsafe to free 'mem' */
return ret;
}

- memset(*inputarg, 0x00, pgcount * PAGE_SIZE);
+ memset(mem, 0x00, pgcount * HV_HYP_PAGE_SIZE);
}
+
+ /*
+ * In a fully enlightened TDX/SNP VM with more than 64 VPs, if
+ * hyperv_pcpu_input_arg is not NULL, set_memory_decrypted() ->
+ * ... -> cpa_flush()-> ... -> __send_ipi_mask_ex() tries to
+ * use hyperv_pcpu_input_arg as the hypercall input page, which
+ * must be a decrypted page in such a VM, but the page is still
+ * encrypted before set_memory_decrypted() returns. Fix this by
+ * setting *inputarg after the above set_memory_decrypted(): if
+ * hyperv_pcpu_input_arg is NULL, __send_ipi_mask_ex() returns
+ * HV_STATUS_INVALID_PARAMETER immediately, and the function
+ * hv_send_ipi_mask() falls back to orig_apic.send_IPI_mask(),
+ * which may be slightly slower than the hypercall, but still
+ * works correctly in such a VM.
+ */
+ *inputarg = mem;
}

msr_vp_index = hv_get_register(HV_REGISTER_VP_INDEX);
--
2.25.1