Choo! Choo! All aboard the Split Lock Express, with direct service to Wreckage! Skip split_lock_verify_msr() if the CPU isn't whitelisted as a possible SLD-enabled CPU model to avoid writing MSR_TEST_CTRL. MSR_TEST_CTRL exists, and is writable, on many generations of CPUs. Writing the MSR, even with '0', can result in bizarre, undocumented behavior. This fixes a crash on Haswell when resuming from suspend with a live KVM guest. Because APs use the standard SMP boot flow for resume, they will go through split_lock_init() and the subsequent RDMSR/WRMSR sequence, which runs even when sld_state==sld_off to ensure SLD is disabled. On Haswell (at least, my Haswell), writing MSR_TEST_CTRL with '0' will succeed and _may_ take the SMT _sibling_ out of VMX root mode. When KVM has an active guest, KVM performs VMXON as part of CPU onlining (see kvm_starting_cpu()). Because SMP boot is serialized, the resulting flow is effectively: on_each_ap_cpu() { WRMSR(MSR_TEST_CTRL, 0) VMXON } As a result, the WRMSR can disable VMX on a different CPU that has already done VMXON. This ultimately results in a #UD on VMPTRLD when KVM regains control and attempt run its vCPUs. The above voodoo was confirmed by reworking KVM's VMXON flow to write MSR_TEST_CTRL prior to VMXON, and to serialize the sequence as above. Further verification of the insanity was done by redoing VMXON on all APs after the initial WRMSR->VMXON sequence. The additional VMXON, which should VM-Fail, occasionally succeeded, and also eliminated the unexpected #UD on VMPTRLD. The damage done by writing MSR_TEST_CTRL doesn't appear to be limited to VMX, e.g. after suspend with an active KVM guest, subsequent reboots almost always hang (even when fudging VMXON), a #UD on a random Jcc was observed, suspend/resume stability is qualitatively poor, and so on and so forth. kernel BUG at arch/x86/kvm/x86.c:386! invalid opcode: 0000 [#7] SMP CPU: 1 PID: 2592 Comm: CPU 6/KVM Tainted: G D Hardware name: ASUS Q87M-E/Q87M-E, BIOS 1102 03/03/2014 RIP: 0010:kvm_spurious_fault+0xf/0x20 Code: <0f> 0b 0f 1f 44 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 RSP: 0018:ffffc0bcc1677b78 EFLAGS: 00010246 RAX: 0000617640000000 RBX: ffff9e8d01d80000 RCX: ffff9e8d4fa40000 RDX: ffff9e8d03360000 RSI: 00000003c3360000 RDI: ffff9e8d03360000 RBP: 0000000000000001 R08: ffff9e8d046d9d40 R09: 0000000000000018 R10: ffffc0bcc1677b80 R11: 0000000000000008 R12: 0000000000000006 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 FS: 00007fe16c9f9700(0000) GS:ffff9e8d4fa40000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000d7a418 CR3: 00000003c47b1006 CR4: 00000000001626e0 Call Trace: vmx_vcpu_load_vmcs+0x1fb/0x2b0 vmx_vcpu_load+0x3e/0x160 kvm_arch_vcpu_load+0x48/0x260 finish_task_switch+0x140/0x260 __schedule+0x460/0x720 _cond_resched+0x2d/0x40 kvm_arch_vcpu_ioctl_run+0x82e/0x1ca0 kvm_vcpu_ioctl+0x363/0x5c0 ksys_ioctl+0x88/0xa0 __x64_sys_ioctl+0x16/0x20 do_syscall_64+0x4c/0x170 entry_SYSCALL_64_after_hwframe+0x44/0xa9 Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Xiaoyao Li <xiaoyao.li@xxxxxxxxx> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> Cc: kvm@xxxxxxxxxxxxxxx Fixes: dbaba47085b0c ("x86/split_lock: Rework the initialization flow of split lock detection") Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> --- arch/x86/kernel/cpu/intel.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c index a19a680542ce..19b6c42739fc 100644 --- a/arch/x86/kernel/cpu/intel.c +++ b/arch/x86/kernel/cpu/intel.c @@ -48,6 +48,13 @@ enum split_lock_detect_state { static enum split_lock_detect_state sld_state __ro_after_init = sld_off; static u64 msr_test_ctrl_cache __ro_after_init; +/* + * With a name like MSR_TEST_CTL it should go without saying, but don't touch + * MSR_TEST_CTL unless the CPU is one of the whitelisted models. Writing it + * on CPUs that do not support SLD can cause fireworks, even when writing '0'. + */ +static bool cpu_model_supports_sld __ro_after_init; + /* * Processors which have self-snooping capability can handle conflicting * memory type across CPUs by snooping its own cache. However, there exists @@ -1064,7 +1071,8 @@ static void sld_update_msr(bool on) static void split_lock_init(void) { - split_lock_verify_msr(sld_state != sld_off); + if (cpu_model_supports_sld) + split_lock_verify_msr(sld_state != sld_off); } static void split_lock_warn(unsigned long ip) @@ -1167,5 +1175,6 @@ void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) return; } + cpu_model_supports_sld = true; split_lock_setup(); } -- 2.26.0