rseq: Update kernel fields in lockstep with CONFIG_DEBUG_RSEQ=y

With CONFIG_DEBUG_RSEQ=y, an in-kernel copy of the read-only fields is
kept synchronized with the user-space fields. Ensure the updates are
done in lockstep in case we error out on a write to user-space.

Fixes: 7d5265f ("rseq: Validate read-only fields under DEBUG_RSEQ config")
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/r/20250225202500.731245-1-mjeanson@efficios.com

Author: mjeanson

kernel/rseq.c

@@ -78,24 +78,24 @@ static int rseq_validate_ro_fields(struct task_struct *t)
 	return -EFAULT;
 }
 
-static void rseq_set_ro_fields(struct task_struct *t, u32 cpu_id_start, u32 cpu_id,
-			       u32 node_id, u32 mm_cid)
-{
-	rseq_kernel_fields(t)->cpu_id_start = cpu_id;
-	rseq_kernel_fields(t)->cpu_id = cpu_id;
-	rseq_kernel_fields(t)->node_id = node_id;
-	rseq_kernel_fields(t)->mm_cid = mm_cid;
-}
+/*
+ * Update an rseq field and its in-kernel copy in lock-step to keep a coherent
+ * state.
+ */
+#define rseq_unsafe_put_user(t, value, field, error_label)		\
+	do {								\
+		unsafe_put_user(value, &t->rseq->field, error_label);	\
+		rseq_kernel_fields(t)->field = value;			\
+	} while (0)
+
 #else
 static int rseq_validate_ro_fields(struct task_struct *t)
 {
 	return 0;
 }
 
-static void rseq_set_ro_fields(struct task_struct *t, u32 cpu_id_start, u32 cpu_id,
-			       u32 node_id, u32 mm_cid)
-{
-}
+#define rseq_unsafe_put_user(t, value, field, error_label)		\
+	unsafe_put_user(value, &t->rseq->field, error_label)
 #endif
 
 /*
@@ -173,17 +173,18 @@ static int rseq_update_cpu_node_id(struct task_struct *t)
 	WARN_ON_ONCE((int) mm_cid < 0);
 	if (!user_write_access_begin(rseq, t->rseq_len))
 		goto efault;
-	unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
-	unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
-	unsafe_put_user(node_id, &rseq->node_id, efault_end);
-	unsafe_put_user(mm_cid, &rseq->mm_cid, efault_end);
+
+	rseq_unsafe_put_user(t, cpu_id, cpu_id_start, efault_end);
+	rseq_unsafe_put_user(t, cpu_id, cpu_id, efault_end);
+	rseq_unsafe_put_user(t, node_id, node_id, efault_end);
+	rseq_unsafe_put_user(t, mm_cid, mm_cid, efault_end);
+
 	/*
 	 * Additional feature fields added after ORIG_RSEQ_SIZE
 	 * need to be conditionally updated only if
 	 * t->rseq_len != ORIG_RSEQ_SIZE.
 	 */
 	user_write_access_end();
-	rseq_set_ro_fields(t, cpu_id, cpu_id, node_id, mm_cid);
 	trace_rseq_update(t);
 	return 0;
 
@@ -195,45 +196,44 @@ static int rseq_update_cpu_node_id(struct task_struct *t)
 
 static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
 {
+	struct rseq __user *rseq = t->rseq;
 	u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED, node_id = 0,
 	    mm_cid = 0;
 
 	/*
 	 * Validate read-only rseq fields.
 	 */
 	if (rseq_validate_ro_fields(t))
-		return -EFAULT;
-	/*
-	 * Reset cpu_id_start to its initial state (0).
-	 */
-	if (put_user(cpu_id_start, &t->rseq->cpu_id_start))
-		return -EFAULT;
-	/*
-	 * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
-	 * in after unregistration can figure out that rseq needs to be
-	 * registered again.
-	 */
-	if (put_user(cpu_id, &t->rseq->cpu_id))
-		return -EFAULT;
-	/*
-	 * Reset node_id to its initial state (0).
-	 */
-	if (put_user(node_id, &t->rseq->node_id))
-		return -EFAULT;
+		goto efault;
+
+	if (!user_write_access_begin(rseq, t->rseq_len))
+		goto efault;
+
 	/*
-	 * Reset mm_cid to its initial state (0).
+	 * Reset all fields to their initial state.
+	 *
+	 * All fields have an initial state of 0 except cpu_id which is set to
+	 * RSEQ_CPU_ID_UNINITIALIZED, so that any user coming in after
+	 * unregistration can figure out that rseq needs to be registered
+	 * again.
 	 */
-	if (put_user(mm_cid, &t->rseq->mm_cid))
-		return -EFAULT;
-
-	rseq_set_ro_fields(t, cpu_id_start, cpu_id, node_id, mm_cid);
+	rseq_unsafe_put_user(t, cpu_id_start, cpu_id_start, efault_end);
+	rseq_unsafe_put_user(t, cpu_id, cpu_id, efault_end);
+	rseq_unsafe_put_user(t, node_id, node_id, efault_end);
+	rseq_unsafe_put_user(t, mm_cid, mm_cid, efault_end);
 
 	/*
 	 * Additional feature fields added after ORIG_RSEQ_SIZE
 	 * need to be conditionally reset only if
 	 * t->rseq_len != ORIG_RSEQ_SIZE.
 	 */
+	user_write_access_end();
 	return 0;
+
+efault_end:
+	user_write_access_end();
+efault:
+	return -EFAULT;
 }
 
 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)