Re: [PATCH] acpi: fix potential race conditions bypassing checks

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Monday, October 28, 2019 7:31:14 PM CET Kangjie Lu wrote:
> "obj" is a local variable. Elements are deep-copied from external
> package to obj and security-checked. The original code is
> seemingly fine; however, compilers optimize the deep copies into
> shallow copies, introducing potential race conditions. For
> example, the checks for type and length may be bypassed.

How exactly?

What compiler(s) do such optimizations in this particular case?

> The fix tells compilers to not optimize the deep copy by inserting
> "volatile".

Have you actually analyzed the object code produced by the compiler with and
without the volatile to determine whether or not it has an effect as expected
on code generation?

> Signed-off-by: Kangjie Lu <kjlu@umn.edu>
> ---
>  drivers/acpi/processor_throttling.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/acpi/processor_throttling.c b/drivers/acpi/processor_throttling.c
> index 532a1ae3595a..6f4d86f8a9ce 100644
> --- a/drivers/acpi/processor_throttling.c
> +++ b/drivers/acpi/processor_throttling.c
> @@ -413,7 +413,7 @@ static int acpi_processor_get_throttling_control(struct acpi_processor *pr)
>  	acpi_status status = 0;
>  	struct acpi_buffer buffer = { ACPI_ALLOCATE_BUFFER, NULL };
>  	union acpi_object *ptc = NULL;
> -	union acpi_object obj = { 0 };
> +	volatile union acpi_object obj = { 0 };
>  	struct acpi_processor_throttling *throttling;
>  
>  	status = acpi_evaluate_object(pr->handle, "_PTC", NULL, &buffer);
>