Re: [PATCH] bdev: fix NULL pointer dereference in sync()/close() race

[Vegard Nossum]

On 08/29/2016 09:55 PM, Tejun Heo wrote:
> Hello,
>
> On Sat, Aug 27, 2016 at 11:30:22AM +0200, Vegard Nossum wrote:
> > > Don't know what's the right fix, but I posted a slightly different one
> > > for the same crash some months ago:
> > > https://patchwork.kernel.org/patch/8556941/
> >
> > Ah, I'm sorry, I didn't see that.
> >
> > Your patch is 100% identical to my first attempt at a fix and I can
> > confirm that it also fixes the problem for me.
> >
> > If people who are more savvy in block/fs code could ack the locking bits
> > I think we should apply the patch ASAP because it's an easy local DOS if
> > you have (open/read) access to any block device.
>
> I think the right thing to do there is doing blkdev_get() /
> blkdev_put() around func() invocation in iterate_bdevs() rather than
> holding bd_mutex across the callback.  Can you please verify whether
> that works?

Hrmph, I tried this patch first:

    diff --git a/fs/block_dev.c b/fs/block_dev.c
    index e17bdbd..489473d 100644
    --- a/fs/block_dev.c
    +++ b/fs/block_dev.c
    @@ -1884,6 +1884,7 @@ void iterate_bdevs(void (*func)(struct 
block_device *, void *), void *arg)
            spin_lock(&blockdev_superblock->s_inode_list_lock);
            list_for_each_entry(inode, &blockdev_superblock->s_inodes, 
i_sb_list) {
                    struct address_space *mapping = inode->i_mapping;
    +               struct block_device *bdev;

                    spin_lock(&inode->i_lock);
                    if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW) ||
    @@ -1905,7 +1906,11 @@ void iterate_bdevs(void (*func)(struct 
block_device *, void *), void *arg)
                    iput(old_inode);
                    old_inode = inode;

    -               func(I_BDEV(inode), arg);
    +               bdev = bd_acquire(inode);
    +               if (bdev) {
    +                       func(bdev, arg);
    +                       bdput(bdev);
    +               }

                    spin_lock(&blockdev_superblock->s_inode_list_lock);
            }

That didn't work at all. I guess bd_acquire() would just do a bdgrab()
and not touch ->bd_holders, whereas blkdev_get() would increment
bd_holders and therefore prevent __blkdev_put() from freeing the block
device? Too confusing...

I'll give your suggestion a try.


Vegard