Re: [Linaro-mm-sig] [PATCH] dmabuf: fix use-after-free of dmabuf's file->f_inode

[Sumit Semwal]

Hi Charan,

On Mon, 4 Jan 2021 at 17:22, Christian König
<ckoenig.leichtzumerken@xxxxxxxxx> wrote:
>
> Am 04.01.21 um 12:36 schrieb Charan Teja Reddy:
> > It is observed 'use-after-free' on the dmabuf's file->f_inode with the
> > race between closing the dmabuf file and reading the dmabuf's debug
> > info.
> >
> > Consider the below scenario where P1 is closing the dma_buf file
> > and P2 is reading the dma_buf's debug info in the system:
> >
> > P1                                            P2
> >                                       dma_buf_debug_show()
> > dma_buf_put()
> >    __fput()
> >      file->f_op->release()
> >      dput()
> >      ....
> >        dentry_unlink_inode()
> >          iput(dentry->d_inode)
> >          (where the inode is freed)
> >                                       mutex_lock(&db_list.lock)
> >                                       read 'dma_buf->file->f_inode'
> >                                       (the same inode is freed by P1)
> >                                       mutex_unlock(&db_list.lock)
> >        dentry->d_op->d_release()-->
> >          dma_buf_release()
> >            .....
> >            mutex_lock(&db_list.lock)
> >            removes the dmabuf from the list
> >            mutex_unlock(&db_list.lock)
> >
> > In the above scenario, when dma_buf_put() is called on a dma_buf, it
> > first frees the dma_buf's file->f_inode(=dentry->d_inode) and then
> > removes this dma_buf from the system db_list. In between P2 traversing
> > the db_list tries to access this dma_buf's file->f_inode that was freed
> > by P1 which is a use-after-free case.
> >
> > Since, __fput() calls f_op->release first and then later calls the
> > d_op->d_release, move the dma_buf's db_list removal from d_release() to
> > f_op->release(). This ensures that dma_buf's file->f_inode is not
> > accessed after it is released.
> >
> > Fixes: 4ab59c3c638c ("dma-buf: Move dma_buf_release() from fops to dentry_ops")
> > Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>
>
> Not an expert on the debugfs stuff in DMA-buf, but the explanation
> sounds perfectly correct to me.
>
> Acked-by: Christian König <christian.koenig@xxxxxxx>

Thanks for your fix; I will queue it up in the fixes branch. Can you
please also send it to be queued to 5.4+ stable branches?

>
> > ---
> >   drivers/dma-buf/dma-buf.c | 21 +++++++++++++++++----
> >   1 file changed, 17 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index 0eb80c1..a14dcbb 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -76,10 +76,6 @@ static void dma_buf_release(struct dentry *dentry)
> >
> >       dmabuf->ops->release(dmabuf);
> >
> > -     mutex_lock(&db_list.lock);
> > -     list_del(&dmabuf->list_node);
> > -     mutex_unlock(&db_list.lock);
> > -
> >       if (dmabuf->resv == (struct dma_resv *)&dmabuf[1])
> >               dma_resv_fini(dmabuf->resv);
> >
> > @@ -88,6 +84,22 @@ static void dma_buf_release(struct dentry *dentry)
> >       kfree(dmabuf);
> >   }
> >
> > +static int dma_buf_file_release(struct inode *inode, struct file *file)
> > +{
> > +     struct dma_buf *dmabuf;
> > +
> > +     if (!is_dma_buf_file(file))
> > +             return -EINVAL;
> > +
> > +     dmabuf = file->private_data;
> > +
> > +     mutex_lock(&db_list.lock);
> > +     list_del(&dmabuf->list_node);
> > +     mutex_unlock(&db_list.lock);
> > +
> > +     return 0;
> > +}
> > +
> >   static const struct dentry_operations dma_buf_dentry_ops = {
> >       .d_dname = dmabuffs_dname,
> >       .d_release = dma_buf_release,
> > @@ -413,6 +425,7 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
> >   }
> >
> >   static const struct file_operations dma_buf_fops = {
> > +     .release        = dma_buf_file_release,
> >       .mmap           = dma_buf_mmap_internal,
> >       .llseek         = dma_buf_llseek,
> >       .poll           = dma_buf_poll,
>

Best,
Sumit.