Ping. This would ideally get into 5.8, the bad behavior is quite nasty. On Fri, Jun 05, 2020 at 12:26:05PM -0700, Sean Christopherson wrote: > 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 >