Re: [PATCH v2] acpi: fix a potential inconsistency caused by double-fetch

[Rafael J. Wysocki]

On Wed, Jan 9, 2019 at 9:14 AM Kangjie Lu <kjlu@xxxxxxx> wrote:
>
> "user_buf->length" is in user space, and copied in twice. The second
> copy is after it passes the security check. If a user program races to
> change user_buf->length in user space, the data fetched in the second
> copy may invalidate the security check. The fix avoids the double-fetch
> issue by using the value passing the security check.

AFAICS the patch really does two things: it avoids the issue described
above and avoids using the (redundant) 'table' local variable on the
stack.  Arguably, you could fix the issue without getting rid of the
redundant variable.

>
> Signed-off-by: Kangjie Lu <kjlu@xxxxxxx>
> ---
>  drivers/acpi/custom_method.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
> index 4451877f83b6..f10ee0519033 100644
> --- a/drivers/acpi/custom_method.c
> +++ b/drivers/acpi/custom_method.c
> @@ -26,17 +26,16 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
>         static u32 max_size;
>         static u32 uncopied_bytes;
>
> -       struct acpi_table_header table;
>         acpi_status status;
>
>         if (!(*ppos)) {
>                 /* parse the table header to get the table length */
>                 if (count <= sizeof(struct acpi_table_header))
>                         return -EINVAL;
> -               if (copy_from_user(&table, user_buf,
> -                                  sizeof(struct acpi_table_header)))
> +               if (get_user(max_size,
> +                                       &((struct acpi_table_header *)user_buf)->length))
>                         return -EFAULT;
> -               uncopied_bytes = max_size = table.length;
> +               uncopied_bytes = max_size;
>                 buf = kzalloc(max_size, GFP_KERNEL);
>                 if (!buf)
>                         return -ENOMEM;
> @@ -57,6 +56,9 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
>                 return -EFAULT;
>         }
>
> +       /* Ensure table length is not changed in the second copy */
> +       ((struct acpi_table_header *)(buf + (*ppos)))->length = max_size;

Why don't you return -EFAULT if max_size is different from ->length?
Surely, the table should not be used at all in that case.

Moreover, wouldn't it be even better to compare the entire header with
the one read previously and return -EFAULT if they don't match?

> +
>         uncopied_bytes -= count;
>         *ppos += count;
>
> --