Excerpts from Russell Currey's message of October 27, 2021 5:24 pm:
> The mitigation-patching.sh script in the powerpc selftests toggles
> all mitigations on and off simultaneously, revealing that rfi_flush
> and stf_barrier cannot safely operate at the same time due to races
> in updating the static key.
>
> On some systems, the static key code throws a warning and the kernel
> remains functional. On others, the kernel will hang or crash.
>
> Fix this by slapping on a mutex.
>
> Fixes: 13799748b957 ("powerpc/64: use interrupt restart table to speed up
> return from interrupt")
> Signed-off-by: Russell Currey <rus...@russell.cc>
Doh,
Acked-by: Nicholas Piggin <npig...@gmail.com>
> ---
> arch/powerpc/lib/feature-fixups.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/arch/powerpc/lib/feature-fixups.c
> b/arch/powerpc/lib/feature-fixups.c
> index cda17bee5afe..9956277fbd88 100644
> --- a/arch/powerpc/lib/feature-fixups.c
> +++ b/arch/powerpc/lib/feature-fixups.c
> @@ -228,6 +228,7 @@ static void do_stf_exit_barrier_fixups(enum
> stf_barrier_type types)
>
> static bool stf_exit_reentrant = false;
> static bool rfi_exit_reentrant = false;
> +static DEFINE_MUTEX(exit_flush_lock);
>
> static int __do_stf_barrier_fixups(void *data)
> {
> @@ -253,6 +254,9 @@ void do_stf_barrier_fixups(enum stf_barrier_type types)
> * low level interrupt exit code before patching. After the patching,
> * if allowed, then flip the branch to allow fast exits.
> */
> +
> + // Prevent static key update races with do_rfi_flush_fixups()
> + mutex_lock(&exit_flush_lock);
> static_branch_enable(&interrupt_exit_not_reentrant);
>
> stop_machine(__do_stf_barrier_fixups, &types, NULL);
> @@ -264,6 +268,7 @@ void do_stf_barrier_fixups(enum stf_barrier_type types)
>
> if (stf_exit_reentrant && rfi_exit_reentrant)
> static_branch_disable(&interrupt_exit_not_reentrant);
> + mutex_unlock(&exit_flush_lock);
> }
>
> void do_uaccess_flush_fixups(enum l1d_flush_type types)
> @@ -486,6 +491,9 @@ void do_rfi_flush_fixups(enum l1d_flush_type types)
> * without stop_machine, so this could be achieved with a broadcast
> * IPI instead, but this matches the stf sequence.
> */
> +
> + // Prevent static key update races with do_stf_barrier_fixups()
> + mutex_lock(&exit_flush_lock);
> static_branch_enable(&interrupt_exit_not_reentrant);
>
> stop_machine(__do_rfi_flush_fixups, &types, NULL);
> @@ -497,6 +505,7 @@ void do_rfi_flush_fixups(enum l1d_flush_type types)
>
> if (stf_exit_reentrant && rfi_exit_reentrant)
> static_branch_disable(&interrupt_exit_not_reentrant);
> + mutex_unlock(&exit_flush_lock);
> }
>
> void do_barrier_nospec_fixups_range(bool enable, void *fixup_start, void
> *fixup_end)
> --
> 2.33.1
>
>
