rseq: Rework the TIF_NOTIFY handler

Replace the whole logic with a new implementation, which is shared with
signal delivery and the upcoming exit fast path.

Contrary to the original implementation, this ignores invocations from
KVM/IO-uring, which invoke resume_user_mode_work() with the @regs argument
set to NULL.

The original implementation updated the CPU/Node/MM CID fields, but that
was just a side effect, which was addressing the problem that this
invocation cleared TIF_NOTIFY_RESUME, which in turn could cause an update
on return to user space to be lost.

This problem has been addressed differently, so that it's not longer
required to do that update before entering the guest.

That might be considered a user visible change, when the hosts thread TLS
memory is mapped into the guest, but as this was never intentionally
supported, this abuse of kernel internal implementation details is not
considered an ABI break.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.517640811@linutronix.de

1 files changed, 33 insertions, 43 deletions

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 13faadc737ad..148fb2103023 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -82,12 +82,6 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/rseq.h>
 
-#ifdef CONFIG_MEMBARRIER
-# define RSEQ_EVENT_GUARD	irq
-#else
-# define RSEQ_EVENT_GUARD	preempt
-#endif
-
 DEFINE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
 
 static inline void rseq_control_debug(bool on)
@@ -239,38 +233,15 @@ efault:
 	return false;
 }
 
-/*
- * This resume handler must always be executed between any of:
- * - preemption,
- * - signal delivery,
- * and return to user-space.
- *
- * This is how we can ensure that the entire rseq critical section
- * will issue the commit instruction only if executed atomically with
- * respect to other threads scheduled on the same CPU, and with respect
- * to signal handlers.
- */
-void __rseq_handle_notify_resume(struct pt_regs *regs)
+static void rseq_slowpath_update_usr(struct pt_regs *regs)
 {
+	/* Preserve rseq state and user_irq state for exit to user */
+	const struct rseq_event evt_mask = { .has_rseq = true, .user_irq = true, };
 	struct task_struct *t = current;
 	struct rseq_ids ids;
 	u32 node_id;
 	bool event;
 
-	/*
-	 * If invoked from hypervisors before entering the guest via
-	 * resume_user_mode_work(), then @regs is a NULL pointer.
-	 *
-	 * resume_user_mode_work() clears TIF_NOTIFY_RESUME and re-raises
-	 * it before returning from the ioctl() to user space when
-	 * rseq_event.sched_switch is set.
-	 *
-	 * So it's safe to ignore here instead of pointlessly updating it
-	 * in the vcpu_run() loop.
-	 */
-	if (!regs)
-		return;
-
 	if (unlikely(t->flags & PF_EXITING))
 		return;
 
@@ -294,26 +265,45 @@ void __rseq_handle_notify_resume(struct pt_regs *regs)
 	 * with the result handed in to allow the detection of
 	 * inconsistencies.
 	 */
-	scoped_guard(RSEQ_EVENT_GUARD) {
+	scoped_guard(irq) {
 		event = t->rseq.event.sched_switch;
-		t->rseq.event.sched_switch = false;
+		t->rseq.event.all &= evt_mask.all;
 		ids.cpu_id = task_cpu(t);
 		ids.mm_cid = task_mm_cid(t);
 	}
 
-	if (!IS_ENABLED(CONFIG_DEBUG_RSEQ) && !event)
+	if (!event)
 		return;
 
-	if (!rseq_handle_cs(t, regs))
-		goto error;
-
 	node_id = cpu_to_node(ids.cpu_id);
-	if (!rseq_set_ids(t, &ids, node_id))
-		goto error;
-	return;
 
-error:
-	force_sig(SIGSEGV);
+	if (unlikely(!rseq_update_usr(t, regs, &ids, node_id))) {
+		/*
+		 * Clear the errors just in case this might survive magically, but
+		 * leave the rest intact.
+		 */
+		t->rseq.event.error = 0;
+		force_sig(SIGSEGV);
+	}
+}
+
+void __rseq_handle_notify_resume(struct pt_regs *regs)
+{
+	/*
+	 * If invoked from hypervisors before entering the guest via
+	 * resume_user_mode_work(), then @regs is a NULL pointer.
+	 *
+	 * resume_user_mode_work() clears TIF_NOTIFY_RESUME and re-raises
+	 * it before returning from the ioctl() to user space when
+	 * rseq_event.sched_switch is set.
+	 *
+	 * So it's safe to ignore here instead of pointlessly updating it
+	 * in the vcpu_run() loop.
+	 */
+	if (!regs)
+		return;
+
+	rseq_slowpath_update_usr(regs);
 }
 
 void __rseq_signal_deliver(int sig, struct pt_regs *regs)