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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 10 Jun 2020 11:26:53 +0200
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Subject: Re: [PATCH] dma-buf: Move dma_buf_release() from fops to dentry_ops
To:     Sumit Semwal <sumit.semwal@linaro.org>
Cc:     Linux Media Mailing List <linux-media@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Chenbo Feng <fengc@google.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linaro-mm-sig@lists.linaro.org,
        Charan Teja Reddy <charante@codeaurora.org>,
        syzbot+3643a18836bce555bff6@syzkaller.appspotmail.com,
        "# 3.4.x" <stable@vger.kernel.org>
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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 :)
>
> [1]: https://lore.kernel.org/patchwork/patch/1238278/
>
> 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.

>  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.

>  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.

          Arnd