[RFC PATCH 02/10] entry: snapshot thread flags

From: Mark Rutland
Date: Wed Jun 09 2021 - 08:20:25 EST


Some thread flags can be set remotely, and so even when IRQs are
disabled, the flags can change under our feet. Generally this is
unlikely to cause a problem in practice, but it is somewhat unsound, and
KCSAN will legitimately warn that there is a data race.

To avoid such issues, we should snapshot the flags prior to using them.
Let's use the new helpers to do so in the common entry code.

Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
include/linux/entry-kvm.h | 2 +-
kernel/entry/common.c | 4 ++--
kernel/entry/kvm.c | 4 ++--
3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/linux/entry-kvm.h b/include/linux/entry-kvm.h
index 8b2b1d68b954..bc487dffb803 100644
--- a/include/linux/entry-kvm.h
+++ b/include/linux/entry-kvm.h
@@ -70,7 +70,7 @@ static inline void xfer_to_guest_mode_prepare(void)
*/
static inline bool __xfer_to_guest_mode_work_pending(void)
{
- unsigned long ti_work = READ_ONCE(current_thread_info()->flags);
+ unsigned long ti_work = read_thread_flags();

return !!(ti_work & XFER_TO_GUEST_MODE_WORK);
}
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index a0b3b04fb596..3147a1f2ed74 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -188,7 +188,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
/* Check if any of the above work has queued a deferred wakeup */
rcu_nocb_flush_deferred_wakeup();

- ti_work = READ_ONCE(current_thread_info()->flags);
+ ti_work = read_thread_flags();
}

/* Return the latest work state for arch_exit_to_user_mode() */
@@ -197,7 +197,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,

static void exit_to_user_mode_prepare(struct pt_regs *regs)
{
- unsigned long ti_work = READ_ONCE(current_thread_info()->flags);
+ unsigned long ti_work = read_thread_flags();

lockdep_assert_irqs_disabled();

diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
index 49972ee99aff..96d476e06c77 100644
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -26,7 +26,7 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
if (ret)
return ret;

- ti_work = READ_ONCE(current_thread_info()->flags);
+ ti_work = read_thread_flags();
} while (ti_work & XFER_TO_GUEST_MODE_WORK || need_resched());
return 0;
}
@@ -43,7 +43,7 @@ int xfer_to_guest_mode_handle_work(struct kvm_vcpu *vcpu)
* disabled in the inner loop before going into guest mode. No need
* to disable interrupts here.
*/
- ti_work = READ_ONCE(current_thread_info()->flags);
+ ti_work = read_thread_flags();
if (!(ti_work & XFER_TO_GUEST_MODE_WORK))
return 0;

--
2.11.0