Re: [PATCH] powerpc/security: Use a mutex for interrupt exit code patching
From: Nicholas Piggin <npiggin@gmail.com>
Date: 2021-10-28 03:04:12
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 <redacted>Doh, Acked-by: Nicholas Piggin <npiggin@gmail.com>
quoted hunk ↗ jump to hunk
--- 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