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

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Date:   Wed, 13 Nov 2019 23:43:01 +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 10:32:26 PM CET Kangjie Lu wrote:
> 
> > 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?

Not answered.

> > 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.

OK, that should have been mentioned in the changelog.

> > 
> >> 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.

OK, but does it actually make a practical difference?

> > 
> >> 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 };

Why don't you change obj to a pointer instead?

> >> 	struct acpi_processor_throttling *throttling;
> >> 
> >> 	status = acpi_evaluate_object(pr->handle, "_PTC", NULL, &buffer);
> >>