Re: KASAN: null-ptr-deref Read in reclaim_high

[Dmitry Vyukov]

On Tue, Mar 12, 2019 at 7:25 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 12 Mar 2019 07:08:38 +0100 Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>
> > On Tue, Mar 12, 2019 at 12:37 AM Andrew Morton
> > <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, 11 Mar 2019 06:08:01 -0700 syzbot <syzbot+fa11f9da42b46cea3b4a@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > > syzbot has bisected this bug to:
> > > >
> > > > commit 29a4b8e275d1f10c51c7891362877ef6cffae9e7
> > > > Author: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > > > Date:   Wed Jan 9 22:02:21 2019 +0000
> > > >
> > > >      memcg: schedule high reclaim for remote memcgs on high_work
> > > >
> > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=155bf5db200000
> > > > start commit:   29a4b8e2 memcg: schedule high reclaim for remote memcgs on..
> > > > git tree:       linux-next
> > > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=175bf5db200000
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=135bf5db200000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=611f89e5b6868db
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=fa11f9da42b46cea3b4a
> > > > userspace arch: amd64
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14259017400000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=141630a0c00000
> > > >
> > > > Reported-by: syzbot+fa11f9da42b46cea3b4a@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > Fixes: 29a4b8e2 ("memcg: schedule high reclaim for remote memcgs on
> > > > high_work")
> > >
> > > The following patch
> > > memcg-schedule-high-reclaim-for-remote-memcgs-on-high_work-v3.patch
> > > might have fixed this.  Was it applied?
> >
> > Hi Andrew,
> >
> > You mean if the patch was applied during the bisection?
> > No, it wasn't. Bisection is very specifically done on the same tree
> > where the bug was hit. There are already too many factors that make
> > the result flaky/wrong/inconclusive without changing the tree state.
> > Now, if syzbot would know about any pending fix for this bug, then it
> > would not do the bisection at all. But it have not seen any patch in
> > upstream/linux-next with the Reported-by tag, nor it received any syz
> > fix commands for this bugs. Should have been it aware of the fix? How?
>
> memcg-schedule-high-reclaim-for-remote-memcgs-on-high_work-v3.patch was
> added to linux-next on Jan 10.  I take it that this bug was hit when
> testing the entire linux-next tree, so we can assume that
> memcg-schedule-high-reclaim-for-remote-memcgs-on-high_work-v3.patch
> does not fix it, correct?
> In which case, over to Shakeel!

Jan 10 is exactly when this bug was reported:
https://groups.google.com/forum/#!msg/syzkaller-bugs/5YkhNUg2PFY/4-B5M7bDCAAJ
https://syzkaller.appspot.com/bug?extid=fa11f9da42b46cea3b4a

We don't know if that patch fixed the bug or not because nobody tested
the reproducer with that patch.

It seems that the problem here is that nobody associated the fix with
the bug report. So people looking at open bug reports will spend time
again and again debugging this just to find that this was fixed months
ago. syzbot also doesn't have a chance to realize that this is fixed
and bisection is not necessary anymore. It also won't confirm/disprove
that the fix actually fixes the bug because even if the crash will
continue to happen it will look like the old crash just continues to
happen, so nothing to notify about.

Associating fixes with bug reports solves all these problems for
humans and bots.