rseq: Fix segfault on registration when rseq_cs is non-zero

The rseq_cs field is documented as being set to 0 by user-space prior to
registration, however this is not currently enforced by the kernel. This
can result in a segfault on return to user-space if the value stored in
the rseq_cs field doesn't point to a valid struct rseq_cs.

The correct solution to this would be to fail the rseq registration when
the rseq_cs field is non-zero. However, some older versions of glibc
will reuse the rseq area of previous threads without clearing the
rseq_cs field and will also terminate the process if the rseq
registration fails in a secondary thread. This wasn't caught in testing
because in this case the leftover rseq_cs does point to a valid struct
rseq_cs.

What we can do is clear the rseq_cs field on registration when it's
non-zero which will prevent segfaults on registration and won't break
the glibc versions that reuse rseq areas on thread creation.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250306211223.109455-1-mjeanson@efficios.com

Author: mjeanson

kernel/rseq.c

@@ -236,6 +236,29 @@ static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
 	return -EFAULT;
 }
 
+/*
+ * Get the user-space pointer value stored in the 'rseq_cs' field.
+ */
+static int rseq_get_rseq_cs_ptr_val(struct rseq __user *rseq, u64 *rseq_cs)
+{
+	if (!rseq_cs)
+		return -EFAULT;
+
+#ifdef CONFIG_64BIT
+	if (get_user(*rseq_cs, &rseq->rseq_cs))
+		return -EFAULT;
+#else
+	if (copy_from_user(rseq_cs, &rseq->rseq_cs, sizeof(*rseq_cs)))
+		return -EFAULT;
+#endif
+
+	return 0;
+}
+
+/*
+ * If the rseq_cs field of 'struct rseq' contains a valid pointer to
+ * user-space, copy 'struct rseq_cs' from user-space and validate its fields.
+ */
 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
 {
 	struct rseq_cs __user *urseq_cs;
@@ -244,17 +267,16 @@ static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
 	u32 sig;
 	int ret;
 
-#ifdef CONFIG_64BIT
-	if (get_user(ptr, &t->rseq->rseq_cs))
-		return -EFAULT;
-#else
-	if (copy_from_user(&ptr, &t->rseq->rseq_cs, sizeof(ptr)))
-		return -EFAULT;
-#endif
+	ret = rseq_get_rseq_cs_ptr_val(t->rseq, &ptr);
+	if (ret)
+		return ret;
+
+	/* If the rseq_cs pointer is NULL, return a cleared struct rseq_cs. */
 	if (!ptr) {
 		memset(rseq_cs, 0, sizeof(*rseq_cs));
 		return 0;
 	}
+	/* Check that the pointer value fits in the user-space process space. */
 	if (ptr >= TASK_SIZE)
 		return -EINVAL;
 	urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
@@ -330,7 +352,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
 	return !!event_mask;
 }
 
-static int clear_rseq_cs(struct task_struct *t)
+static int clear_rseq_cs(struct rseq __user *rseq)
 {
 	/*
 	 * The rseq_cs field is set to NULL on preemption or signal
@@ -341,9 +363,9 @@ static int clear_rseq_cs(struct task_struct *t)
 	 * Set rseq_cs to NULL.
 	 */
 #ifdef CONFIG_64BIT
-	return put_user(0UL, &t->rseq->rseq_cs);
+	return put_user(0UL, &rseq->rseq_cs);
 #else
-	if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
+	if (clear_user(&rseq->rseq_cs, sizeof(rseq->rseq_cs)))
 		return -EFAULT;
 	return 0;
 #endif
@@ -375,11 +397,11 @@ static int rseq_ip_fixup(struct pt_regs *regs)
 	 * Clear the rseq_cs pointer and return.
 	 */
 	if (!in_rseq_cs(ip, &rseq_cs))
-		return clear_rseq_cs(t);
+		return clear_rseq_cs(t->rseq);
 	ret = rseq_need_restart(t, rseq_cs.flags);
 	if (ret <= 0)
 		return ret;
-	ret = clear_rseq_cs(t);
+	ret = clear_rseq_cs(t->rseq);
 	if (ret)
 		return ret;
 	trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
@@ -453,6 +475,7 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 		int, flags, u32, sig)
 {
 	int ret;
+	u64 rseq_cs;
 
 	if (flags & RSEQ_FLAG_UNREGISTER) {
 		if (flags & ~RSEQ_FLAG_UNREGISTER)
@@ -507,6 +530,19 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 		return -EINVAL;
 	if (!access_ok(rseq, rseq_len))
 		return -EFAULT;
+
+	/*
+	 * If the rseq_cs pointer is non-NULL on registration, clear it to
+	 * avoid a potential segfault on return to user-space. The proper thing
+	 * to do would have been to fail the registration but this would break
+	 * older libcs that reuse the rseq area for new threads without
+	 * clearing the fields.
+	 */
+	if (rseq_get_rseq_cs_ptr_val(rseq, &rseq_cs))
+	        return -EFAULT;
+	if (rseq_cs && clear_rseq_cs(rseq))
+		return -EFAULT;
+
 #ifdef CONFIG_DEBUG_RSEQ
 	/*
 	 * Initialize the in-kernel rseq fields copy for validation of