Re: [PATCH] x86/split_lock: Sanitize userspace and guest error output

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 6/5/2020 7:44 PM, 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.
> 
> Signed-off-by: Prarit Bhargava <prarit@redhat.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>
> ---
>   arch/x86/kernel/cpu/intel.c | 24 ++++++++++--------------
>   1 file changed, 10 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 166d7c355896..463022aa9b7a 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -1074,10 +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 split_lock_warn(unsigned long ip, int fatal)
>   {
> -	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> -			    current->comm, current->pid, ip);
> +	pr_warn_ratelimited("#AC: %s/%d %ssplit_lock trap at address: 0x%lx\n",
> +			    current->comm, current->pid,
> +			    sld_state == sld_fatal ? "fatal " : "", ip);
> +
> +	if (sld_state == sld_fatal || fatal)
> +		return false;
>   
>   	/*
>   	 * Disable the split lock detection for this task so it can make
> @@ -1086,18 +1090,13 @@ 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 (split_lock_warn(ip, 0))
>   		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 +1107,7 @@ 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)
> -		return false;
> -	split_lock_warn(regs->ip);
> -	return true;
> +	return split_lock_warn(regs->ip, regs->flags & X86_EFLAGS_AC);

It's incorrect. You change the behavior that it will print the split 
lock warning even when CPL 3 Alignment Check is turned on.

>   }
>   
>   /*
>