[PATCH 35/82] ACPI: custom_method: Refactor intentional wrap-around test

[PATCH 35/82] ACPI: custom_method: Refactor intentional wrap-around test

[Kees Cook]

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded wrap-around addition test to use add_would_overflow().
This paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Len Brown <lenb@kernel.org>
Cc: linux-acpi@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 drivers/acpi/custom_method.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
index d39a9b474727..0789317f4a1a 100644
--- a/drivers/acpi/custom_method.c
+++ b/drivers/acpi/custom_method.c
@@ -54,7 +54,7 @@ static ssize_t cm_write(struct file *file, const char __user *user_buf,
 
 	if ((*ppos > max_size) ||
 	    (*ppos + count > max_size) ||
-	    (*ppos + count < count) ||
+	    (add_would_overflow(count, *ppos)) ||
 	    (count > uncopied_bytes)) {
 		kfree(buf);
 		buf = NULL;
-- 
2.34.1

Re: [PATCH 35/82] ACPI: custom_method: Refactor intentional wrap-around test

[Rafael J. Wysocki]

On Tue, Jan 23, 2024 at 2:03 AM Kees Cook <keescook@chromium.org> wrote:
>
> In an effort to separate intentional arithmetic wrap-around from
> unexpected wrap-around, we need to refactor places that depend on this
> kind of math. One of the most common code patterns of this is:
>
>         VAR + value < VAR
>
> Notably, this is considered "undefined behavior" for signed and pointer
> types, which the kernel works around by using the -fno-strict-overflow
> option in the build[1] (which used to just be -fwrapv). Regardless, we
> want to get the kernel source to the position where we can meaningfully
> instrument arithmetic wrap-around conditions and catch them when they
> are unexpected, regardless of whether they are signed[2], unsigned[3],
> or pointer[4] types.
>
> Refactor open-coded wrap-around addition test to use add_would_overflow().
> This paves the way to enabling the wrap-around sanitizers in the future.
>
> Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
> Link: https://github.com/KSPP/linux/issues/26 [2]
> Link: https://github.com/KSPP/linux/issues/27 [3]
> Link: https://github.com/KSPP/linux/issues/344 [4]
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Len Brown <lenb@kernel.org>
> Cc: linux-acpi@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  drivers/acpi/custom_method.c | 2 +-

I may attempt to drop custom_method.c in this cycle, is there a
problem if I take this into my tree for now?

>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
> index d39a9b474727..0789317f4a1a 100644
> --- a/drivers/acpi/custom_method.c
> +++ b/drivers/acpi/custom_method.c
> @@ -54,7 +54,7 @@ static ssize_t cm_write(struct file *file, const char __user *user_buf,
>
>         if ((*ppos > max_size) ||
>             (*ppos + count > max_size) ||
> -           (*ppos + count < count) ||
> +           (add_would_overflow(count, *ppos)) ||
>             (count > uncopied_bytes)) {
>                 kfree(buf);
>                 buf = NULL;
> --
> 2.34.1
>

Re: [PATCH 35/82] ACPI: custom_method: Refactor intentional wrap-around test

[Kees Cook]

On Wed, Jan 24, 2024 at 08:52:48PM +0100, Rafael J. Wysocki wrote:
> On Tue, Jan 23, 2024 at 2:03 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > In an effort to separate intentional arithmetic wrap-around from
> > unexpected wrap-around, we need to refactor places that depend on this
> > kind of math. One of the most common code patterns of this is:
> >
> >         VAR + value < VAR
> >
> > Notably, this is considered "undefined behavior" for signed and pointer
> > types, which the kernel works around by using the -fno-strict-overflow
> > option in the build[1] (which used to just be -fwrapv). Regardless, we
> > want to get the kernel source to the position where we can meaningfully
> > instrument arithmetic wrap-around conditions and catch them when they
> > are unexpected, regardless of whether they are signed[2], unsigned[3],
> > or pointer[4] types.
> >
> > Refactor open-coded wrap-around addition test to use add_would_overflow().
> > This paves the way to enabling the wrap-around sanitizers in the future.
> >
> > Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
> > Link: https://github.com/KSPP/linux/issues/26 [2]
> > Link: https://github.com/KSPP/linux/issues/27 [3]
> > Link: https://github.com/KSPP/linux/issues/344 [4]
> > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > Cc: Len Brown <lenb@kernel.org>
> > Cc: linux-acpi@vger.kernel.org
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >  drivers/acpi/custom_method.c | 2 +-
> 
> I may attempt to drop custom_method.c in this cycle, is there a
> problem if I take this into my tree for now?

The helper doesn't exist in tree yet, but it may be a bit before these
refactors land, so if custom_method vanishes before then, that's great!
:)

-Kees

-- 
Kees Cook