linux-kernel - Re: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer dereference at 000001f2

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Date:   Tue, 3 Oct 2017 10:05:38 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Fengguang Wu <fengguang.wu@...el.com>
Cc:     Byungchul Park <byungchul.park@....com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, LKP <lkp@...org>
Subject: Re: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer
 dereference at 000001f2

On Tue, Oct 03, 2017 at 09:41:36AM -0500, Josh Poimboeuf wrote:
> On Tue, Oct 03, 2017 at 09:31:47AM -0500, Josh Poimboeuf wrote:
> > On Tue, Oct 03, 2017 at 10:06:34PM +0800, Fengguang Wu wrote:
> > > Hi Byungchul,
> > > 
> > > This patch triggers a NULL-dereference bug at update_stack_state().
> > > Although its parent commit also has a NULL-dereference bug, however
> > > the call stack looks rather different. Both dmesg files are attached.
> > > 
> > > It also triggers this warning, which is being discussed in another
> > > thread, so CC Josh. The full dmesg attached, too.
> > > 
> > >         Please press Enter to activate this console.
> > >         [  138.605622] WARNING: kernel stack regs at be299c9a in procd:340 has bad 'bp' value 000001be
> > >         [  138.605627] unwind stack type:0 next_sp:  (null) mask:0x2 graph_idx:0
> > >         [  138.605631] be299c9a: 299ceb00 (0x299ceb00)
> > >         [  138.605633] be299c9e: 2281f1be (0x2281f1be)
> > >         [  138.605634] be299ca2: 299cebb6 (0x299cebb6)
> > 
> > I suspect the bug is in:
> > 
> >   ce07a9415f26 ("locking/lockdep: Make check_prev_add() able to handle external stack_trace")
> > 
> > It converts the stack-allocated stack_trace struct from static to
> > non-static, yet still adds it to a list.  Does this fix it?
> 
> Actually, I spoke too soon.  It's not actually adding the pointer to the
> list, it's copying its contents.  So never mind...

I don't know the lockdep code, but one more comment from the peanut
gallery.  This code looks suspect to me:


			/*
			 * Stop saving stack_trace if save_trace() was
			 * called at least once:
			 */
			if (save && ret == 2)
				save = NULL;


>From looking at check_prev_add(), a return value of 2 doesn't
necessarily imply that save_trace() was called.  If the
check_redundant() call returns 0, then check_prev_add() can return 2,
and the trace will still be uninitialized, but save will be set to NULL
even though save_trace() hasn't been called.  Then a subsequent call to
check_prev_add() could add an uninitialized stack_trace struct to the
dependency list.

I could be wrong, but it's at least something the lockdep folks might
want to look at.

-- 
Josh