On Wed, 10 Jun 2020 at 14:57, Arnd Bergmann arnd@arndb.de wrote:

On Wed, Jun 10, 2020 at 10:33 AM Sumit Semwal sumit.semwal@linaro.org wrote:

...

Charan Teja reported a 'use-after-free' in dmabuffs_dname [1], which happens if the dma_buf_release() is called while the userspace is accessing the dma_buf pseudo fs's dmabuffs_dname() in another process, and dma_buf_release() releases the dmabuf object when the last reference to the struct file goes away.

I discussed with Arnd Bergmann, and he suggested that rather than tying the dma_buf_release() to the file_operations' release(), we can tie it to the dentry_operations' d_release(), which will be called when the last ref to the dentry is removed.

The path exercised by __fput() calls f_op->release() first, and then calls dput, which eventually calls d_op->d_release().

In the 'normal' case, when no userspace access is happening via dma_buf pseudo fs, there should be exactly one fd, file, dentry and inode, so closing the fd will kill of everything right away.

In the presented case, the dentry's d_release() will be called only when the dentry's last ref is released.

Therefore, lets move dma_buf_release() from fops->release() to d_ops->d_release().

Many thanks to Arnd for his FS insights :)

Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls") Reported-by: syzbot+3643a18836bce555bff6@syzkaller.appspotmail.com Cc: stable@vger.kernel.org [5.3+] Cc: Arnd Bergmann arnd@arndb.de Reported-by: Charan Teja Reddy charante@codeaurora.org Signed-off-by: Sumit Semwal sumit.semwal@linaro.org

The patch looks correct to me.

Reviewed-by: Arnd Bergmann arnd@arndb.de

Obviously this should still be verified against the original report if possible.

Thanks, Arnd!

...

drivers/dma-buf/dma-buf.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 01ce125f8e8d..92ba4b6ef3e7 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -54,8 +54,11 @@ static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen) dentry->d_name.name, ret > 0 ? name : ""); }

+static void dma_buf_release(struct dentry *dentry);

static const struct dentry_operations dma_buf_dentry_ops = { .d_dname = dmabuffs_dname,

•   .d_release = dma_buf_release,
  

};

I'd suggest rearranging the file to avoid the forward declaration, even if it makes it a little harder to review the change, the resulting code will remain organized more logically.

Got it, will update it in v2.

...

static struct vfsmount *dma_buf_mnt; @@ -77,14 +80,14 @@ static struct file_system_type dma_buf_fs_type = { .kill_sb = kill_anon_super, };

-static int dma_buf_release(struct inode *inode, struct file *file) +static void dma_buf_release(struct dentry *dentry) { struct dma_buf *dmabuf;

•   if (!is_dma_buf_file(file))
  

•           return -EINVAL;
  

•   if (dentry->d_op != &dma_buf_dentry_ops)
  

•           return;
  

I think the check here is redundant and it's clearer without it.

Ok, will remove.

      Arnd

Best, Sumit.