linux-kernel - Re: [PATCH 4.19 41/48] x86/unwind/orc: Prevent unwinding before ORC initialization

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Message-ID: <20200514194457.wipphhvyhzcshcup@treble>
Date:   Thu, 14 May 2020 14:44:57 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Miroslav Benes <mbenes@...e.cz>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>, Dave Jones <dsj@...com>,
        Jann Horn <jannh@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vince Weaver <vincent.weaver@...ne.edu>
Subject: Re: [PATCH 4.19 41/48] x86/unwind/orc: Prevent unwinding before ORC
 initialization

On Wed, May 13, 2020 at 11:52:10PM +0200, Pavel Machek wrote:
> Hi!
> 
> > From: Josh Poimboeuf <jpoimboe@...hat.com>
> > 
> > commit 98d0c8ebf77e0ba7c54a9ae05ea588f0e9e3f46e upstream.
> > 
> > If the unwinder is called before the ORC data has been initialized,
> > orc_find() returns NULL, and it tries to fall back to using frame
> > pointers.  This can cause some unexpected warnings during boot.
> > 
> > Move the 'orc_init' check from orc_find() to __unwind_init(), so that it
> > doesn't even try to unwind from an uninitialized state.
> 
> > @@ -563,6 +560,9 @@ EXPORT_SYMBOL_GPL(unwind_next_frame);
> >  void __unwind_start(struct unwind_state *state, struct task_struct *task,
> >  		    struct pt_regs *regs, unsigned long *first_frame)
> >  {
> > +	if (!orc_init)
> > +		goto done;
> > +
> >  	memset(state, 0, sizeof(*state));
> >  	state->task = task;
> >  
> 
> As this returns the *state to the caller, should the "goto done" move
> below the memset? Otherwise we are returning partialy-initialized
> struct, which is ... weird.

Yeah, it is a little weird.  In most cases it should be fine, but there
is an edge case where if there's a corrupt ORC table and this returns
early, 'arch_stack_walk_reliable() -> unwind_error()' could check an
uninitialized value.

Also the __unwind_start() error handling needs to set that error bit
anyway, in its error cases.  I'll fix it up.

-- 
Josh