Re: [PATCH v2] dma-buf: Move dma_buf_release() from fops to dentry_ops

[Sumit Semwal]

Hello,

If there are no objections to this, I will plan to merge it soon.

Daniel, Chris, Chenbo?

On Thu, 11 Jun 2020 at 17:14, Sumit Semwal <sumit.semwal@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: <stable@xxxxxxxxxxxxxxx> [5.3+]
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Reported-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>
> Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
>
> ---
> v2: per Arnd: Moved dma_buf_release() above to avoid forward declaration;
>      removed dentry_ops check.
> ---
>  drivers/dma-buf/dma-buf.c | 54 ++++++++++++++++++---------------------
>  1 file changed, 25 insertions(+), 29 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 01ce125f8e8d..412629601ad3 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -54,37 +54,11 @@ static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen)
>                              dentry->d_name.name, ret > 0 ? name : "");
>  }
>
> -static const struct dentry_operations dma_buf_dentry_ops = {
> -       .d_dname = dmabuffs_dname,
> -};
> -
> -static struct vfsmount *dma_buf_mnt;
> -
> -static int dma_buf_fs_init_context(struct fs_context *fc)
> -{
> -       struct pseudo_fs_context *ctx;
> -
> -       ctx = init_pseudo(fc, DMA_BUF_MAGIC);
> -       if (!ctx)
> -               return -ENOMEM;
> -       ctx->dops = &dma_buf_dentry_ops;
> -       return 0;
> -}
> -
> -static struct file_system_type dma_buf_fs_type = {
> -       .name = "dmabuf",
> -       .init_fs_context = dma_buf_fs_init_context,
> -       .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;
> -
> -       dmabuf = file->private_data;
> +       dmabuf = dentry->d_fsdata;
>
>         BUG_ON(dmabuf->vmapping_counter);
>
> @@ -110,9 +84,32 @@ static int dma_buf_release(struct inode *inode, struct file *file)
>         module_put(dmabuf->owner);
>         kfree(dmabuf->name);
>         kfree(dmabuf);
> +}
> +
> +static const struct dentry_operations dma_buf_dentry_ops = {
> +       .d_dname = dmabuffs_dname,
> +       .d_release = dma_buf_release,
> +};
> +
> +static struct vfsmount *dma_buf_mnt;
> +
> +static int dma_buf_fs_init_context(struct fs_context *fc)
> +{
> +       struct pseudo_fs_context *ctx;
> +
> +       ctx = init_pseudo(fc, DMA_BUF_MAGIC);
> +       if (!ctx)
> +               return -ENOMEM;
> +       ctx->dops = &dma_buf_dentry_ops;
>         return 0;
>  }
>
> +static struct file_system_type dma_buf_fs_type = {
> +       .name = "dmabuf",
> +       .init_fs_context = dma_buf_fs_init_context,
> +       .kill_sb = kill_anon_super,
> +};
> +
>  static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
>  {
>         struct dma_buf *dmabuf;
> @@ -412,7 +409,6 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
>  }
>
>  static const struct file_operations dma_buf_fops = {
> -       .release        = dma_buf_release,
>         .mmap           = dma_buf_mmap_internal,
>         .llseek         = dma_buf_llseek,
>         .poll           = dma_buf_poll,
> --
> 2.27.0
>

Best,
Sumit.