On 12/12/2024 3:09 PM, Xiaoyao Li wrote:
On 12/12/2024 1:18 PM, Binbin Wu wrote:
On 12/12/2024 11:44 AM, Xiaoyao Li wrote:
On 12/12/2024 11:26 AM, Binbin Wu wrote:
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@xxxxxxxxx>
Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
Tested-by: Farrah Chen <farrah.chen@xxxxxxxxx>
---
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@xxxxxxxxxx/, 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;
Updating run->hypercall.ret is useful only when QEMU needs to re-enter the guest. For the case of ret < 0, QEMU will stop the vcpu.
IMHO, assign run->hypercall.ret anyway should be OK, no need to add a
per-condition on ret, although the value is not used when ret < 0.
Currently, since QEMU will stop the vcpu when ret < 0, this patch doesn't
convert ret to -Exxx that the ABI expects.
I was thinking if it is better to let each specific handling function to update run->hypercall.ret with its own logic. E.g., for this case, let kvm_handle_hc_map_gpa_range() to update the run->hypercall.ret.
I think it makes sense.
Also, each handling function can decide whether the vcpu should continue if the handling failed.
- Return 0 and set the error code ( 0 or -Exxx) to run->hypercall.ret if it want to continue.
- Return negative value if it want to stop the vcpu thread.
Reusing the return value of the handling function to update
run->hypercall.ret seems not logically correct to me.
I think we might need re-think on the handling of KVM_EXIT_HYPERCALL. E.g., in what error case should QEMU stop the vcpu, and in what case can QEMU return the error back to the guest via run->hypercall.ret.
Actually, I had the similar question before.
https://lore.kernel.org/kvm/ d25cc62c-0f56-4be2-968a-63c8b1d63b5a@xxxxxxxxxxxxxxx/
It might depends on the hypercall number?
Another option is QEMU always sets run->hypercall.ret appropriately and continues the vcpu thread.
- return -EINVAL;
+ return ret;
}
#define VMX_INVALID_GUEST_STATE 0x80000021
base-commit: ae35f033b874c627d81d51070187fbf55f0bf1a7
