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

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Date:   Mon, 28 Oct 2019 17:32:26 -0400
From:   Kangjie Lu <kjlu@....edu>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
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 Oct 28, 2019, at 4:51 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> 
> 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?

Tested on LLVM. The deep copy is indeed optimized into a shallow copy at optimization level O2.


> 
>> 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?

Yes, with “volatile", the deep copy is preserved, and “obj” is created as a local variable.

> 
>> 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);
>> 
> 
> 
> 
>