| Message ID | 20240123002814.1396804-75-keescook@chromium.org (mailing list archive) |
|---|---|
| State | Changes Requested |
| Headers | show |
| Series | overflow: Refactor open-coded arithmetic wrap-around | expand |
On Mon, Jan 22, 2024 at 4:36 PM 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: John Stultz <jstultz@google.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Stephen Boyd <sboyd@kernel.org> > Signed-off-by: Kees Cook <keescook@chromium.org> > --- > kernel/time/timekeeping.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c > index 266d02809dbb..2fc7cf16584c 100644 > --- a/kernel/time/timekeeping.c > +++ b/kernel/time/timekeeping.c > @@ -1984,7 +1984,7 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk, > * Which simplifies to: > * xtime_nsec -= offset > */ > - if ((mult_adj > 0) && (tk->tkr_mono.mult + mult_adj < mult_adj)) { > + if ((mult_adj > 0) && (add_would_overflow(mult_adj, tk->tkr_mono.mult))) { > /* NTP adjustment caused clocksource mult overflow */ > WARN_ON_ONCE(1); > return; Acked-by: John Stultz <jstultz@google.com>
On Mon, Jan 22 2024 at 16:27, Kees Cook 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: John Stultz <jstultz@google.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Stephen Boyd <sboyd@kernel.org> > Signed-off-by: Kees Cook <keescook@chromium.org> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 266d02809dbb..2fc7cf16584c 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -1984,7 +1984,7 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk, * Which simplifies to: * xtime_nsec -= offset */ - if ((mult_adj > 0) && (tk->tkr_mono.mult + mult_adj < mult_adj)) { + if ((mult_adj > 0) && (add_would_overflow(mult_adj, tk->tkr_mono.mult))) { /* NTP adjustment caused clocksource mult overflow */ WARN_ON_ONCE(1); return;
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: John Stultz <jstultz@google.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Stephen Boyd <sboyd@kernel.org> Signed-off-by: Kees Cook <keescook@chromium.org> --- kernel/time/timekeeping.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)