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

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Message-ID: <20200514202839.l2ztrqd4zff4e4as@treble>
Date:   Thu, 14 May 2020 15:28:39 -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 Thu, May 14, 2020 at 10:13:40PM +0200, Pavel Machek wrote:
> > > > @@ -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.
> 
> I did this in the mean time. It moves goto around memset, and I
> believe that 8 in get_reg should have been sizeof(long) [not that it
> matters, x86-32 is protected by build bug on.]
> 
> Signed-off-by: Pavel Machek <pavel@....cz>

I already have the same memset patch (along with other error-handling
fixes) which I'll be posting shortly once it runs through my testing.

Since the sizeof(long) thing isn't really a bug, I'll make that change
later, along with some other pending improvements I have.

-- 
Josh