Re: [PATCH] f2fs: fix to avoid use-after-free in f2fs_stop_gc_thread()

[Jaegeuk Kim]

On 07/27, Jaegeuk Kim wrote:
> On 07/25, Chao Yu wrote:
> > syzbot reports a f2fs bug as below:
> > 
> >  __dump_stack lib/dump_stack.c:88 [inline]
> >  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
> >  print_report+0xe8/0x550 mm/kasan/report.c:491
> >  kasan_report+0x143/0x180 mm/kasan/report.c:601
> >  kasan_check_range+0x282/0x290 mm/kasan/generic.c:189
> >  instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
> >  atomic_fetch_add_relaxed include/linux/atomic/atomic-instrumented.h:252 [inline]
> >  __refcount_add include/linux/refcount.h:184 [inline]
> >  __refcount_inc include/linux/refcount.h:241 [inline]
> >  refcount_inc include/linux/refcount.h:258 [inline]
> >  get_task_struct include/linux/sched/task.h:118 [inline]
> >  kthread_stop+0xca/0x630 kernel/kthread.c:704
> >  f2fs_stop_gc_thread+0x65/0xb0 fs/f2fs/gc.c:210
> >  f2fs_do_shutdown+0x192/0x540 fs/f2fs/file.c:2283
> >  f2fs_ioc_shutdown fs/f2fs/file.c:2325 [inline]
> >  __f2fs_ioctl+0x443a/0xbe60 fs/f2fs/file.c:4325
> >  vfs_ioctl fs/ioctl.c:51 [inline]
> >  __do_sys_ioctl fs/ioctl.c:907 [inline]
> >  __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:893
> >  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > 
> > The root cause is below race condition, it may cause use-after-free
> > issue in sbi->gc_th pointer.
> > 
> > - remount
> >  - f2fs_remount
> >   - f2fs_stop_gc_thread
> >    - kfree(gc_th)
> > 				- f2fs_ioc_shutdown
> > 				 - f2fs_do_shutdown
> > 				  - f2fs_stop_gc_thread
> > 				   - kthread_stop(gc_th->f2fs_gc_task)
> > 
> > We will call f2fs_do_shutdown() in two paths:
> > - for f2fs_ioc_shutdown() path, we should grab sb->s_umount semaphore
> > for fixing.
> > - for f2fs_shutdown() path, it's safe since caller has already grabbed
> > sb->s_umount semaphore.
> > 
> > Reported-by: syzbot+1a8e2b31f2ac9bd3d148@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://lore.kernel.org/linux-f2fs-devel/0000000000005c7ccb061e032b9b@xxxxxxxxxx
> > Fixes: 7950e9ac638e ("f2fs: stop gc/discard thread after fs shutdown")
> > Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
> > ---
> >  fs/f2fs/file.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > index 7a37f2b393b9..62d72da25754 100644
> > --- a/fs/f2fs/file.c
> > +++ b/fs/f2fs/file.c
> > @@ -2388,7 +2388,10 @@ static int f2fs_ioc_shutdown(struct file *filp, unsigned long arg)
> >  		}
> >  	}
> >  
> > +	/* grab sb->s_umount to avoid racing w/ remount() */
> > +	down_read(&sbi->sb->s_umount);
> 
> Is this safe for freeze_bdev()?

bdev_freeze
 -> fs_bdev_freeze
  -> get_bdev_super
   -> bdev_super_lock(bdev, true)
    -> super_lock(sb, true)
     -> __super_lock(sb, true)
      -> down_write(s_umount)

> 
> >  	ret = f2fs_do_shutdown(sbi, in, readonly);
> > +	up_read(&sbi->sb->s_umount);
> >  
> >  	if (need_drop)
> >  		mnt_drop_write_file(filp);
> > -- 
> > 2.40.1