Date:   Mon, 29 Jun 2020 22:25:55 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH] x86/split_lock: Don't write MSR_TEST_CTRL on CPUs that
 aren't whitelisted

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

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