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Date: Sat, 6 Jun 2020 11:02:17 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Prarit Bhargava <prarit@...hat.com>, linux-kernel@...r.kernel.org
Cc: 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>,
Sean Christopherson <sean.j.christopherson@...el.com>,
Rahul Tanwar <rahul.tanwar@...ux.intel.com>,
Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH] x86/split_lock: Sanitize userspace and guest error output
On 6/6/2020 12:42 AM, Prarit Bhargava wrote:
>
>
> On 6/5/20 11:29 AM, Xiaoyao Li wrote:
>> 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@...hat.com>
>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>> Cc: Ingo Molnar <mingo@...hat.com>
>>> Cc: Borislav Petkov <bp@...en8.de>
>>> Cc: x86@...nel.org
>>> Cc: "H. Peter Anvin" <hpa@...or.com>
>>> Cc: Tony Luck <tony.luck@...el.com>
>>> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
>>> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
>>> Cc: Rahul Tanwar <rahul.tanwar@...ux.intel.com>
>>> Cc: Xiaoyao Li <xiaoyao.li@...el.com>
>>> Cc: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
>>> Cc: Dave Hansen <dave.hansen@...ux.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.
>
> Do you want the message to be displayed in the fatal case of CPL 3 Alignment check?
>
No. It should never be displayed if #AC happens in CPL 3 and
X86_EFLAGS_AC is set. In this case, an unaligned access triggers #AC
regardless of #LOCK prefix. What's more, even there is a #LOCK prefix,
we still cannot tell the cause because we don't know the priority of
legacy alignment check #AC and split lock #AC.
If you do want a message, we can only say "unaligned access at address xxx".
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