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Date:   Wed, 5 Sep 2018 10:02:41 +0200 (CEST)
From:   Jiri Kosina <jikos@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Woodhouse, David" <dwmw@amazon.co.uk>,
        Oleg Nesterov <oleg@redhat.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH v3 2/3] x86/speculation: apply IBPB more strictly to
 avoid cross-process data leak
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On Wed, 5 Sep 2018, Peter Zijlstra wrote:

> On Tue, Sep 04, 2018 at 06:18:55PM +0200, Thomas Gleixner wrote:
> > On Tue, 4 Sep 2018, Jiri Kosina wrote:
> > >  		if (tsk && tsk->mm &&
> > >  		    tsk->mm->context.ctx_id != last_ctx_id &&
> > > -		    get_dumpable(tsk->mm) != SUID_DUMP_USER)
> > > +		    ___ptrace_may_access(current, tsk, PTRACE_MODE_IBPB))
> > 
> > Uurgh. If X86_FEATURE_USE_IBPB is not enabled, then the whole
> > __ptrace_may_access() overhead is just done for nothing.
> > 
> > >  			indirect_branch_prediction_barrier();
> > 
> > This really wants to be runtime patched:
> > 
> > 		if (static_cpu_has(X86_FEATURE_USE_IBPB))
> > 			stop_speculation(tsk, last_ctx_id);
> > 
> > and have an inline for that:
> > 
> > static inline void stop_speculation(struct task_struct *tsk, u64 last_ctx_id)
> > {
> > 	if (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id &&
> > 		___ptrace_may_access(current, tsk, PTRACE_MODE_IBPB))
> > 		indirect_branch_prediction_barrier();
> > }
> > 
> > which also makes the whole mess readable.
> 
> How about something like:
> 
> 	if (static_cpu_has(X86_FEATURE_USE_IBPB) && need_ibpb(tsk, last_ctx_id))
> 		indirect_branch_predictor_barrier();
> 
> where:
> 
> static inline bool need_ibpb(struct task_struct *next, u64 last_ctx_id)
> {
> 	return next && next->mm && next->mm->context.ctx_id != last_ctx_id &&
> 		__ptrace_may_access(next, PTRACE_MODE_IBPB));
> }
> 
> I don't much like "stop_speculation" for a name here.

Yeah, I did more or less that earlier today; my series currently has

+static bool ibpb_needed(struct task_struct *tsk, u64 last_ctx_id)
+{
+       return (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id &&
+                       __ptrace_may_access(tsk, PTRACE_MODE_IBPB));
+}
+
[ ... ]
-               if (tsk && tsk->mm &&
-                   tsk->mm->context.ctx_id != last_ctx_id &&
-                   get_dumpable(tsk->mm) != SUID_DUMP_USER)
+               if (static_cpu_has(X86_FEATURE_USE_IBPB) &&
+                               ibpb_needed(tsk, last_ctx_id))
                        indirect_branch_prediction_barrier();

Thanks,

-- 
Jiri Kosina
SUSE Labs