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

[Eric Biggers]

On Mon, Mar 11, 2019 at 11:25:41PM -0700, Andrew Morton 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!
> 

I don't understand what happened here.  First, the syzbot report doesn't say
which linux-next version was tested (which it should), but I get:

$ git tag --contains 29a4b8e275d1f10c51c7891362877ef6cffae9e7
next-20190110
next-20190111
next-20190114
next-20190115
next-20190116

That's almost 2 months old, yet this bug was just reported now.  Why?

- Eric