Re: [PATCH] x86/split_lock: Don't write MSR_TEST_CTRL on CPUs that aren't whitelisted

Sean Christopherson
2020-06-30T05:25:58Z

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@linutronix.de>
> Cc: Xiaoyao Li <xiaoyao.li@intel.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: kvm@vger.kernel.org
> Fixes: dbaba47085b0c ("x86/split_lock: Rework the initialization flow of split lock detection")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> 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
>