KVM: SVM: Properly check RAX on #GP intercept of SVM instructions

When KVM intercepts #GP on an SVM instruction, it re-injects the #GP if
the instruction was executed with a mis-algined RAX. However, a #GP
should also be reinjected if RAX contains an illegal GPA, according to
the APM, one of #GP conditions is:

  rAX referenced a physical address above the maximum
  supported physical address.

Replace the PAGE_MASK check with page_address_valid(), which checks both
page-alignment as well as the legality of the GPA based on the vCPU's
MAXPHYADDR. Use kvm_register_read() to read RAX to so that bits 63:32 are
dropped when the vCPU is in 32-bit mode, i.e. to avoid a false positive
when checking the validity of the address.

Note that this is currently only a problem if KVM is running an L2 guest
and ends up synthesizing a #VMEXIT to L1, as the RAX check takes
precedence over the intercept. Otherwise, if KVM emulates the
instruction, kvm_vcpu_map() should fail on illegal GPAs and inject a #GP
anyway. However, following patches will change the failure behavior of
kvm_vcpu_map(), so make sure the #GP interception handler does this
appropriately.

Opportunistically drop a teaser FIXME about the SVM instructions
handling on #GP belonging in the emulator.

Fixes: 82a11e9c6fa2 ("KVM: SVM: Add emulation support for #GP triggered by SVM instructions")
Fixes: d1cba6c92237 ("KVM: x86: nSVM: test eax for 4K alignment for GP errata workaround")
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260316202732.3164936-4-yosry@kernel.org
[sean: massage wording with respect to kvm_register_read()]
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 927764894b89ab206f0ea78de9c1fefbd7a1fed0..f68958447e585b9e8c9393992eabe38e718c0c72 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2282,10 +2282,10 @@ static int gp_interception(struct kvm_vcpu *vcpu)
        if (x86_decode_emulated_instruction(vcpu, 0, NULL, 0) != EMULATION_OK)
                goto reinject;
 
+       /* FIXME: Handle SVM instructions through the emulator */
        svm_exit_code = svm_get_decoded_instr_exit_code(vcpu);
        if (svm_exit_code) {
-               /* All SVM instructions expect page aligned RAX */
-               if (svm->vmcb->save.rax & ~PAGE_MASK)
+               if (!page_address_valid(vcpu, kvm_register_read(vcpu, VCPU_REGS_RAX)))
                        goto reinject;
 
                if (!is_guest_mode(vcpu))