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

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Message-ID: <3605712.hsAW26GfSv@kreacher>
Date:   Mon, 28 Oct 2019 21:51:22 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Kangjie Lu <kjlu@....edu>
Cc:     Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] acpi: fix potential race conditions bypassing checks

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@....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);
>