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KVM: x86: clear vcpu->run->hypercall.ret before exiting for KVM_EXIT_HYPERCALL

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QEMU up to 9.2.0 is assuming that vcpu->run->hypercall.ret is 0 on exit and
it never modifies it when processing KVM_EXIT_HYPERCALL.  Make this explicit
in the code, to avoid breakage when KVM starts modifying that field.

This in principle is not a good idea... It would have been much better if
KVM had set the field to -KVM_ENOSYS from the beginning, so that a dumb
userspace that does nothing on KVM_EXIT_HYPERCALL would tell the guest it
does not support KVM_HC_MAP_GPA_RANGE.  However, breaking userspace is
a Very Bad Thing, as everybody should know.

Reported-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2024-12-13 14:36:25 -05:00
parent 10b2c8a67c
commit 9a1dfeff44
2 changed files with 21 additions and 0 deletions
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14
arch/x86/kvm/svm/sev.c
View File

@ -3634,6 +3634,13 @@ static int snp_begin_psc_msr(struct vcpu_svm *svm, u64 ghcb_msr)
vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
vcpu->run->hypercall.nr = KVM_HC_MAP_GPA_RANGE;
/*
* In principle this should have been -KVM_ENOSYS, but userspace (QEMU <=9.2)
* assumed that vcpu->run->hypercall.ret is never changed by KVM and thus that
* it was always zero on KVM_EXIT_HYPERCALL. Since KVM is now overwriting
* vcpu->run->hypercall.ret, ensuring that it is zero to not break QEMU.
*/
vcpu->run->hypercall.ret = 0;
vcpu->run->hypercall.args[0] = gpa;
vcpu->run->hypercall.args[1] = 1;
vcpu->run->hypercall.args[2] = (op == SNP_PAGE_STATE_PRIVATE)
@ -3797,6 +3804,13 @@ static int snp_begin_psc(struct vcpu_svm *svm, struct psc_buffer *psc)
case VMGEXIT_PSC_OP_SHARED:
vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
vcpu->run->hypercall.nr = KVM_HC_MAP_GPA_RANGE;
/*
* In principle this should have been -KVM_ENOSYS, but userspace (QEMU <=9.2)
* assumed that vcpu->run->hypercall.ret is never changed by KVM and thus that
* it was always zero on KVM_EXIT_HYPERCALL. Since KVM is now overwriting
* vcpu->run->hypercall.ret, ensuring that it is zero to not break QEMU.
*/
vcpu->run->hypercall.ret = 0;
vcpu->run->hypercall.args[0] = gfn_to_gpa(gfn);
vcpu->run->hypercall.args[1] = npages;
vcpu->run->hypercall.args[2] = entry_start.operation == VMGEXIT_PSC_OP_PRIVATE

7
arch/x86/kvm/x86.c
View File

@ -10052,6 +10052,13 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
vcpu->run->hypercall.nr = KVM_HC_MAP_GPA_RANGE;
/*
* In principle this should have been -KVM_ENOSYS, but userspace (QEMU <=9.2)
* assumed that vcpu->run->hypercall.ret is never changed by KVM and thus that
* it was always zero on KVM_EXIT_HYPERCALL. Since KVM is now overwriting
* vcpu->run->hypercall.ret, ensuring that it is zero to not break QEMU.
*/
vcpu->run->hypercall.ret = 0;
vcpu->run->hypercall.args[0] = gpa;
vcpu->run->hypercall.args[1] = npages;
vcpu->run->hypercall.args[2] = attrs;