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Tue, 29 Sep 2026 09:29:49 -0700 (PDT) Received: from x1c ([2405:9800:b670:b64b:8c3c:36b:3374:1c96]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3a497ecef67sm6481678a91.6.2026.09.29.09.29.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 29 Sep 2026 09:29:48 -0700 (PDT) From: Tharit Tangkijwanichakul To: seanjc@google.com, pbonzini@redhat.com, shuah@kernel.org Cc: kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kernel-mentees@lists.linux.dev, skhan@linuxfoundation.org, me@brighamcampbell.com, jkoolstra@xs4all.nl, Tharit Tangkijwanichakul Subject: [PATCH v2] KVM: selftests: Verify KVM_MSR_EXIT_REASON_INVAL on non-canonical FS_BASE write Date: Tue, 29 Sep 2026 23:29:40 +0700 Message-ID: <20260929162940.14610-1-tharitt97@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel-mentees@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The msr_filter_deny test covers the FILTER and UNKNOWN MSR exit reasons, but nothing exercises KVM_MSR_EXIT_REASON_INVAL. Have the guest write a non-canonical value to FS_BASE and verify that the write exits to userspace with KVM_MSR_EXIT_REASON_INVAL along with the attempted value. FS_BASE writes are passed through to hardware, i.e. a plain WRMSR takes #GP directly without exiting, so use forced emulation to route the write through KVM. Signed-off-by: Tharit Tangkijwanichakul --- v2: - Use a write of non-canonical to FS_BASE value instead of reserved EFER bit 63 because writing non-canonical can't become valid without massive architectural changes unlike EFER reserved bit (suggested by Sean). Tested on Intel Core i5-10210U (VMX). Not tested on AMD. --- .../kvm/x86/userspace_msr_exit_test.c | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-) diff --git a/tools/testing/selftests/kvm/x86/userspace_msr_exit_test.c b/tools/testing/selftests/kvm/x86/userspace_msr_exit_test.c index 2808ce727e5f..40b084cba2c0 100644 --- a/tools/testing/selftests/kvm/x86/userspace_msr_exit_test.c +++ b/tools/testing/selftests/kvm/x86/userspace_msr_exit_test.c @@ -309,6 +309,14 @@ static void guest_msr_calls(bool trapped) /* Invalid MSR, should always be handled by user space exit */ GUEST_ASSERT(rdmsr(0xdeadbeef) == 0xdeadbeef); wrmsr(0xdeadbeef, 0x1234); + + /* + * Writing a non-canonical value to FS_BASE is rejected by KVM, but + * FS_BASE writes are passed through to hardware, so force emulation + * to route the write through KVM and get an INVAL exit. + */ + if (is_forced_emulation_enabled) + test_em_wrmsr(MSR_FS_BASE, NONCANONICAL); } static void guest_code_filter_deny(void) @@ -627,6 +635,13 @@ static void handle_wrmsr(struct kvm_run *run) TEST_ASSERT(run->msr.reason == KVM_MSR_EXIT_REASON_UNKNOWN, "deadbeef trap w/o inval fault"); } + + if (run->msr.index == MSR_FS_BASE) { + TEST_ASSERT(run->msr.data == NONCANONICAL, + "MSR_FS_BASE data is not NONCANONICAL"); + TEST_ASSERT(run->msr.reason == KVM_MSR_EXIT_REASON_INVAL, + "MSR_FS_BASE trap w/o inval fault"); + } } KVM_ONE_VCPU_TEST(user_msr, msr_filter_deny, guest_code_filter_deny) @@ -667,7 +682,9 @@ KVM_ONE_VCPU_TEST(user_msr, msr_filter_deny, guest_code_filter_deny) done: TEST_ASSERT(msr_reads == 4, "Handled 4 rdmsr in user space"); - TEST_ASSERT(msr_writes == 3, "Handled 3 wrmsr in user space"); + TEST_ASSERT(msr_writes == (is_forced_emulation_enabled ? 5 : 3), + "Handled %u wrmsr in user space, expected %u", + msr_writes, is_forced_emulation_enabled ? 5 : 3); } KVM_ONE_VCPU_TEST(user_msr, msr_permission_bitmap, guest_code_permission_bitmap) -- 2.53.0