[v4] efi: capsule-loader: Fix use-after-free in efi_capsule_write

A race condition may occur if the user calls close() on another
thread during a write() operation on the device node of the efi capsule.

This is a race condition that occurs between the efi_capsule_write()
and efi_capsule_flush() functions of efi_capsule_fops,
which ultimately results in UAF.

So, the page freeing process is modified to be done in
efi_capsule_release() instead of efi_capsule_flush().

Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
 drivers/firmware/efi/capsule-loader.c | 29 ++++++---------------------
 1 file changed, 6 insertions(+), 23 deletions(-)

On Wed, 7 Sept 2022 at 17:35, Hyunwoo Kim <imv4bel@gmail.com> wrote:
>
> A race condition may occur if the user calls close() on another
> thread during a write() operation on the device node of the efi capsule.
>
> This is a race condition that occurs between the efi_capsule_write()
> and efi_capsule_flush() functions of efi_capsule_fops,
> which ultimately results in UAF.
>
> So, the page freeing process is modified to be done in
> efi_capsule_release() instead of efi_capsule_flush().
>
> Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
> ---
>  drivers/firmware/efi/capsule-loader.c | 29 ++++++---------------------
>  1 file changed, 6 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/firmware/efi/capsule-loader.c b/drivers/firmware/efi/capsule-loader.c
> index 4dde8edd53b6..788e860304ae 100644
> --- a/drivers/firmware/efi/capsule-loader.c
> +++ b/drivers/firmware/efi/capsule-loader.c
> @@ -243,18 +243,17 @@ static ssize_t efi_capsule_write(struct file *file, const char __user *buff,
>  }
>
>  /**
> - * efi_capsule_flush - called by file close or file flush
> + * efi_capsule_release - called by file close
> + * @inode: not used
>   * @file: file pointer
> - * @id: not used
>   *
> - *     If a capsule is being partially uploaded then calling this function
> - *     will be treated as upload termination and will free those completed
> - *     buffer pages and -ECANCELED will be returned.
> + *     We will not free successfully submitted pages since efi update
> + *     requires data to be maintained across system reboot.
>   **/
> -static int efi_capsule_flush(struct file *file, fl_owner_t id)
> +static int efi_capsule_release(struct inode *inode, struct file *file)
>  {
> -       int ret = 0;
>         struct capsule_info *cap_info = file->private_data;
> +       int ret = 0;
>
>         if (cap_info->index > 0) {

OK, so what happens if the /dev/efi_capsule_loader is close() after a
successful write and capsule update? AIUI, the pages should not be
freed in that case, as the firmware will perform the capsule update
based on their contents.

So I assume this condition should become something like

       if (cap_info->index > 0 &&
           (cap_info->header.headersize == 0 ||
            cap_info->count < cap_info->total_size)) {

so the inverse condition of what triggers the actual call to
efi_capsule_submit_update() is used to decide whether the capsule
update happened, and whether the pages need to be preserved.


>                 pr_err("capsule upload not complete\n");
> @@ -262,21 +261,6 @@ static int efi_capsule_flush(struct file *file, fl_owner_t id)
>                 ret = -ECANCELED;

The return value of the release() hook doesn't seem to be used
anywhere so I'm not sure this is worth keeping.


>         }
>
> -       return ret;
> -}
> -
> -/**
> - * efi_capsule_release - called by file close
> - * @inode: not used
> - * @file: file pointer
> - *
> - *     We will not free successfully submitted pages since efi update
> - *     requires data to be maintained across system reboot.
> - **/
> -static int efi_capsule_release(struct inode *inode, struct file *file)
> -{
> -       struct capsule_info *cap_info = file->private_data;
> -
>         kfree(cap_info->pages);
>         kfree(cap_info->phys);
>         kfree(file->private_data);
> @@ -324,7 +308,6 @@ static const struct file_operations efi_capsule_fops = {
>         .owner = THIS_MODULE,
>         .open = efi_capsule_open,
>         .write = efi_capsule_write,
> -       .flush = efi_capsule_flush,
>         .release = efi_capsule_release,
>         .llseek = no_llseek,
>  };
> --
> 2.25.1
>