Re: kernel, mm: NULL deref in copy_process while OOMing

[Michal Hocko <mhocko@kernel.org>]

On Sun 19-06-16 12:06:53, Tetsuo Handa wrote:
> On 2016/06/16 18:39, Michal Hocko wrote:
> > On Wed 15-06-16 12:50:43, Sasha Levin wrote:
> >> Hi all,
> >>
> >> I'm seeing the following NULL ptr deref in copy_process right after a bunch
> >> of OOM killing activity on -next kernels:
> >>
> >> Out of memory (oom_kill_allocating_task): Kill process 3477 (trinity-c159) score 0 or sacrifice child
> >> Killed process 3477 (trinity-c159) total-vm:3226820kB, anon-rss:36832kB, file-rss:1640kB, shmem-rss:444kB
> >> oom_reaper: reaped process 3477 (trinity-c159), now anon-rss:0kB, file-rss:0kB, shmem-rss:444kB
> >> Out of memory (oom_kill_allocating_task): Kill process 3450 (trinity-c156) score 0 or sacrifice child
> >> Killed process 3450 (trinity-c156) total-vm:3769768kB, anon-rss:36832kB, file-rss:1652kB, shmem-rss:508kB
> >> oom_reaper: reaped process 3450 (trinity-c156), now anon-rss:0kB, file-rss:0kB, shmem-rss:572kB
> >> BUG: unable to handle kernel NULL pointer dereference at 0000000000000150
> >> IP: copy_process (./arch/x86/include/asm/atomic.h:103 kernel/fork.c:484 kernel/fork.c:964 kernel/fork.c:1018 kernel/fork.c:1484)
> >> PGD 1ff944067 PUD 1ff929067 PMD 0
> >> Oops: 0002 [#1] PREEMPT SMP KASAN
> >> Modules linked in:
> >> CPU: 18 PID: 8761 Comm: trinity-main Not tainted 4.7.0-rc3-sasha-02101-g1e1b9fa #3108
> > 
> > Is this a common parent of the oom killed children?
> > 
> >> task: ffff880165564000 ti: ffff880337ad0000 task.ti: ffff880337ad0000
> >> RIP: copy_process (./arch/x86/include/asm/atomic.h:103 kernel/fork.c:484 kernel/fork.c:964 kernel/fork.c:1018 kernel/fork.c:1484)
> > 
> > IIUC this should be:
> > _do_fork
> >   copy_process
> >     copy_mm
> >       dup_mm
> >         dup_mmap
> > 	  if (tmp->vm_flags & VM_DENYWRITE)
> > 	    atomic_dec(&inode->i_writecount);
> > 
> > I am not really sure how f->f_inode can become NULL when file should pin
> > the inode AFAIR, and VMA should pin the file. Anyway this shouldn't be
> > directly related to the OOM killer or at least the recent changes
> > in that area because the oom reaper doesn't touch VMAs file.
> 
> These OOM messages say that oom_kill_allocating_task != 0 is used.
> That is, a __GFP_FS allocation by a child process which is trying to
> duplicate the parent's mm_struct was killed by the OOM killer and
> reaped by the OOM reaper.

The whole copy_process is done on behalf of the parent. The child
is not running yet so it cannot allocate thus get killed with
oom_kill_allocating_task. The parent hasn't been killed though, at least
the log doesn't indicate that.

> I guess that mmap related stuff are not
> fully initialized (or consistent) yet while the OOM reaper assumed
> that it is safe to access such child's mmap related stuff.

The task gets visible to the system/oom killer after it has been fully
initialized AFAICS.
-- 
Michal Hocko
SUSE Labs

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