[PATCH 2/2] powerpc/64: Only warn for kuap locked when KCSAN not present

[Rohan McLure]

On Thu, 2024-04-04 at 06:14 +0000, Christophe Leroy wrote:
> 
> 
> Le 04/04/2024 à 06:45, Rohan McLure a écrit :
> > Arbitrary instrumented locations, including syscall handlers, can
> > call
> > arch_local_irq_restore() transitively when KCSAN is enabled, and in
> > turn
> > also replay_soft_interrupts_irqrestore(). The precondition on entry
> > to
> > this routine that is checked is that KUAP is enabled (user access
> > prohibited). Failure to meet this condition only triggers a warning
> > however, and afterwards KUAP is enabled anyway. That is, KUAP being
> > disabled on entry is in fact permissable, but not possible on an
> > uninstrumented kernel.
> > 
> > Disable this assertion only when KCSAN is enabled.
> 
> Please elaborate on that arbitrary call to arch_local_irq_restore() 
> transitively, when does it happen and why, and why only when KCSAN is
> enabled.

The implementation of kcsan depends on this_cpu_* routines, which in
turn need to manage irqs for correctness. This means that the presence
of KCSAN in a uaccess enabled epoch can introduce calls to
arch_local_irq_restore().

For this reason, the warning really only applies in the instance of
uninstrumented code, and so to prevent KCSAN from issuing a false-
positive here, it makes sense to issue it only when KCSAN is not
present.

> 
> I don't understand the reasoning, if it is permissible as you say,
> just 
> drop the warning. If the warning is there, it should stay also with 
> KCSAN. You should fix the root cause instead.

By dropping this assertion when KCSAN is enabled, we open up the
opportunity for KCSAN to warn only when data races are actually
observed, rather than just instances where an unblocked AMR state is
inherited into an IRQ context.

> 
> > 
> > Suggested-by: Nicholas Piggin <npiggin at gmail.com>
> > Signed-off-by: Rohan McLure <rmclure at linux.ibm.com>
> > ---
> >   arch/powerpc/kernel/irq_64.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/powerpc/kernel/irq_64.c
> > b/arch/powerpc/kernel/irq_64.c
> > index d5c48d1b0a31..18b2048389a2 100644
> > --- a/arch/powerpc/kernel/irq_64.c
> > +++ b/arch/powerpc/kernel/irq_64.c
> > @@ -189,7 +189,8 @@ static inline __no_kcsan void
> > replay_soft_interrupts_irqrestore(void)
> >   	 * and re-locking AMR but we shouldn't get here in the
> > first place,
> >   	 * hence the warning.
> >   	 */
> > -	kuap_assert_locked();
> > +	if (!IS_ENABLED(CONFIG_KCSAN))
> > +		kuap_assert_locked();
> >   
> >   	if (kuap_state != AMR_KUAP_BLOCKED)
> >   		set_kuap(AMR_KUAP_BLOCKED);