linux-kernel - Re: [PATCH v2] x86/split_lock: Sanitize userspace and guest error output

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Message-ID: <20200608171552.GB8223@linux.intel.com>
Date:   Mon, 8 Jun 2020 10:15:52 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Prarit Bhargava <prarit@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Tony Luck <tony.luck@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Rahul Tanwar <rahul.tanwar@...ux.intel.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2] x86/split_lock: Sanitize userspace and guest error
 output

On Mon, Jun 08, 2020 at 08:21:14AM -0400, Prarit Bhargava wrote:
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 166d7c355896..e02ec81fe1eb 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -1074,10 +1074,17 @@ 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_no_warn)
>  {
> -	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> -			    current->comm, current->pid, ip);
> +	if (fatal_no_warn)
> +		return false;

This misses the point Xiaoyao was making.  If EFLAGS.AC=1 then the #AC is a
legacy alignment check fault and should not be treated as a split-lock #AC.
The basic premise of the patch makes sense, but the end result is confusing
because incorporating "fatal" and the EFLAGS.AC state into split_lock_warn()
bastardizes both the "split_lock" and "warn" aspects of the function.

E.g. something like this yields the same net effect, it's just organized
differently.  If so desired, the "bogus" message could be dropped via
Xiaoyao's prep patch[*] so that this change would only affect the sld_fatal
messages.

[*] https://lkml.kernel.org/r/20200509110542.8159-3-xiaoyao.li@intel.com


diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 23fd5f319908..1aad0b8e394c 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1071,11 +1071,14 @@ 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",
                            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
@@ -1083,18 +1086,13 @@ 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;
@@ -1105,10 +1103,10 @@ 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);
 }

 /*


> +
> +	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)
> +		return false;
>  
>  	/*
>  	 * Disable the split lock detection for this task so it can make
> @@ -1086,18 +1093,13 @@ 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 +1110,7 @@ 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);
>  }
>  
>  /*
> -- 
> 2.21.3
>