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

[Xiaoyao Li]

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@xxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > > Cc: x86@xxxxxxxxxx
> > > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > > Cc: Tony Luck <tony.luck@xxxxxxxxx>
> > > Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> > > Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > > Cc: Rahul Tanwar <rahul.tanwar@xxxxxxxxxxxxxxx>
> > > Cc: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > > Cc: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > > ---
> > >  Â arch/x86/kernel/cpu/intel.c | 24 ++++++++++--------------
> > >  Â 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)
> > >  ÂÂÂÂÂ split_lock_verify_msr(sld_state != sld_off);
> > >  Â }
> > >  Â -static void split_lock_warn(unsigned long ip)
> > > +static bool split_lock_warn(unsigned long ip, int fatal)
> > >  Â {
> > > -ÂÂÂ pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ current->comm, current->pid, ip);
> > > +ÂÂÂ pr_warn_ratelimited("#AC: %s/%d %ssplit_lock trap at address: 0x%lx\n",
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ current->comm, current->pid,
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ sld_state == sld_fatal ? "fatal " : "", ip);
> > > +
> > > +ÂÂÂ if (sld_state == sld_fatal || fatal)
> > > +ÂÂÂÂÂÂÂ return false;
> > >  Â ÂÂÂÂÂ /*
> > >  ÂÂÂÂÂÂ * Disable the split lock detection for this task so it can make
> > > @@ -1086,18 +1090,13 @@ static void split_lock_warn(unsigned long ip)
> > >  ÂÂÂÂÂÂ */
> > >  ÂÂÂÂÂ sld_update_msr(false);
> > >  ÂÂÂÂÂ set_tsk_thread_flag(current, TIF_SLD);
> > > +ÂÂÂ return true;
> > >  Â }
> > >  Â Â bool handle_guest_split_lock(unsigned long ip)
> > >  Â {
> > > -ÂÂÂ if (sld_state == sld_warn) {
> > > -ÂÂÂÂÂÂÂ split_lock_warn(ip);
> > > +ÂÂÂ if (split_lock_warn(ip, 0))
> > >  ÂÂÂÂÂÂÂÂÂ return true;
> > > -ÂÂÂ }
> > > -
> > > -ÂÂÂ pr_warn_once("#AC: %s/%d %s split_lock trap at address: 0x%lx\n",
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂ current->comm, current->pid,
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂ sld_state == sld_fatal ? "fatal" : "bogus", ip);
> > >  Â ÂÂÂÂÂ current->thread.error_code = 0;
> > >  ÂÂÂÂÂ current->thread.trap_nr = X86_TRAP_AC;
> > > @@ -1108,10 +1107,7 @@ EXPORT_SYMBOL_GPL(handle_guest_split_lock);
> > >  Â Â bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> > >  Â {
> > > -ÂÂÂ if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> > > -ÂÂÂÂÂÂÂ return false;
> > > -ÂÂÂ split_lock_warn(regs->ip);
> > > -ÂÂÂ return true;
> > > +ÂÂÂ 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".