Hi Charan,

On Mon, 4 Jan 2021 at 17:22, Christian König ckoenig.leichtzumerken@gmail.com 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@codeaurora.org

Not an expert on the debugfs stuff in DMA-buf, but the explanation sounds perfectly correct to me.

Acked-by: Christian König christian.koenig@amd.com

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.