[PATCH] i386/kvm: Set return value after handling KVM_EXIT_HYPERCALL

[Binbin Wu <binbin.wu@linux.intel.com>]

Userspace should set the ret field of hypercall after handling
KVM_EXIT_HYPERCALL.  Otherwise, a stale value could be returned to KVM.

Fixes: 47e76d03b15 ("i386/kvm: Add KVM_EXIT_HYPERCALL handling for KVM_HC_MAP_GPA_RANGE")
Reported-by: Farrah Chen <farrah.chen@intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Tested-by: Farrah Chen <farrah.chen@intel.com>
---
To test the TDX code in kvm-coco-queue, please apply the patch to the QEMU,
otherwise, TDX guest boot could fail.
A matching QEMU tree including this patch is here:
https://github.com/intel-staging/qemu-tdx/releases/tag/tdx-qemu-upstream-v6.1-fix_kvm_hypercall_return_value

Previously, the issue was not triggered because no one would modify the ret
value. But with the refactor patch for __kvm_emulate_hypercall() in KVM,
https://lore.kernel.org/kvm/20241128004344.4072099-7-seanjc@google.com/, the
value could be modified.
---
 target/i386/kvm/kvm.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 8e17942c3b..4bcccb48d1 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -6005,10 +6005,14 @@ static int kvm_handle_hc_map_gpa_range(struct kvm_run *run)
 
 static int kvm_handle_hypercall(struct kvm_run *run)
 {
+    int ret = -EINVAL;
+
     if (run->hypercall.nr == KVM_HC_MAP_GPA_RANGE)
-        return kvm_handle_hc_map_gpa_range(run);
+        ret = kvm_handle_hc_map_gpa_range(run);
+
+    run->hypercall.ret = ret;
 
-    return -EINVAL;
+    return ret;
 }
 
 #define VMX_INVALID_GUEST_STATE 0x80000021

base-commit: ae35f033b874c627d81d51070187fbf55f0bf1a7
-- 
2.46.0