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Subject: Re: [PATCH v4] x86/split_lock: Sanitize userspace and guest error
 output
To:     linux-kernel@vger.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Tony Luck <tony.luck@intel.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Rahul Tanwar <rahul.tanwar@linux.intel.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
References: <20200616112702.1703-1-prarit@redhat.com>
From:   Prarit Bhargava <prarit@redhat.com>
Message-ID: <f9d8edb6-97f7-13c2-008b-8be8ff69604a@redhat.com>
Date:   Tue, 16 Jun 2020 07:28:31 -0400
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P.


On 6/16/20 7:27 AM, Prarit Bhargava wrote:
> There are two problems with kernel messages in fatal mode that were found
> during testing of guests and userspace programs.
> 
> The first is that no kernel message is output when the split lock detector
> is triggered with a userspace program.  As a result the userspace process
> dies from receiving SIGBUS with no indication to the user of what caused
> the process to die.
> 
> The second problem is that only the first triggering guest causes a kernel
> message to be output because the message is output with pr_warn_once().
> This also results in a loss of information to the user.
> 
> While fixing these I noticed that the same message was being output
> three times so I'm cleaning that up too.
> 
> Fix fatal mode output, and use consistent messages for fatal and
> warn modes for both userspace and guests.
> 
> Co-developed-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: x86@kernel.org
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> Cc: Rahul Tanwar <rahul.tanwar@linux.intel.com>
> Cc: Xiaoyao Li <xiaoyao.li@intel.com>
> Cc: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> ---
> v2: Do not output a message if CPL 3 Alignment Check is turned on (xiaoyao.li)
> v3: refactor code (sean.j.christopherson)
> v4: Fix Sign off (sean.j.christopherson)
> 
>  arch/x86/kernel/cpu/intel.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 63926c94eb5f..3a373f0be674 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -1074,11 +1074,14 @@ static void split_lock_init(void)
>  	split_lock_verify_msr(sld_state != sld_off);
>  }
>  
> -static void split_lock_warn(unsigned long ip)
> +static bool handle_split_lock(unsigned long ip)
>  {
> -	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> +	pr_warn("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
>  			    current->comm, current->pid, ip);
>  
> +	if (sld_state != sld_warn)
> +		return false;
> +
>  	/*
>  	 * Disable the split lock detection for this task so it can make
>  	 * progress and set TIF_SLD so the detection is re-enabled via
> @@ -1086,18 +1089,13 @@ static void split_lock_warn(unsigned long ip)
>  	 */
>  	sld_update_msr(false);
>  	set_tsk_thread_flag(current, TIF_SLD);
> +	return true;
>  }
>  
>  bool handle_guest_split_lock(unsigned long ip)
>  {
> -	if (sld_state == sld_warn) {
> -		split_lock_warn(ip);
> +	if (handle_split_lock(ip))
>  		return true;
> -	}
> -
> -	pr_warn_once("#AC: %s/%d %s split_lock trap at address: 0x%lx\n",
> -		     current->comm, current->pid,
> -		     sld_state == sld_fatal ? "fatal" : "bogus", ip);
>  
>  	current->thread.error_code = 0;
>  	current->thread.trap_nr = X86_TRAP_AC;
> @@ -1108,10 +1106,10 @@ EXPORT_SYMBOL_GPL(handle_guest_split_lock);
>  
>  bool handle_user_split_lock(struct pt_regs *regs, long error_code)
>  {
> -	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> +	if (regs->flags & X86_EFLAGS_AC)
>  		return false;
> -	split_lock_warn(regs->ip);
> -	return true;
> +
> +	return handle_split_lock(regs->ip);
>  }
>  
>  /*
>