[PATCH v3 1/2] kernel/sys: add PR_GET_TASK_SIZE option to prctl(2)

[Joel Savitz]

When PR_GET_TASK_SIZE is passed to prctl, the kernel will attempt to
copy the value of TASK_SIZE to the userspace address in arg2.

It is important that we account for the case of the userspace task
running in 32-bit compat mode on a 64-bit kernel. As such, we must be
careful to copy the correct number of bytes to userspace to avoid stack
corruption.

Suggested-by: Yuri Norov <yury.norov@xxxxxxxxx>
Suggested-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Signed-off-by: Joel Savitz <jsavitz@xxxxxxxxxx>
---
 include/uapi/linux/prctl.h |  3 +++
 kernel/sys.c               | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 094bb03b9cc2..2c261c461952 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -229,4 +229,7 @@ struct prctl_mm_map {
 # define PR_PAC_APDBKEY                        (1UL << 3)
 # define PR_PAC_APGAKEY                        (1UL << 4)

+/* Get the process virtual memory size (i.e. the highest usable VM address) */
+#define PR_GET_TASK_SIZE               55
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 12df0e5434b8..709584400070 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2252,6 +2252,26 @@ static int propagate_has_child_subreaper(struct task_struct *p, void *data)
        return 1;
 }

+static int prctl_get_tasksize(void __user *uaddr)
+{
+	unsigned long current_task_size, current_word_size;
+
+	current_task_size = TASK_SIZE;
+	current_word_size = sizeof(unsigned long);
+
+#ifdef CONFIG_64BIT
+	/* On 64-bit architecture, we must check whether the current thread
+	 * is running in 32-bit compat mode. If it is, we can simply cut
+	 * the size in half. This avoids corruption of the userspace stack.
+	 */
+	if (test_thread_flag(TIF_ADDR32))
+		current_word_size >>= 1;
+#endif
+
+	return copy_to_user(uaddr, &current_task_size, current_word_size) ? -EFAULT : 0;
+}
+
 int __weak arch_prctl_spec_ctrl_get(struct task_struct *t, unsigned long which)
 {
        return -EINVAL;
@@ -2486,6 +2506,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
                        return -EINVAL;
                error = PAC_RESET_KEYS(me, arg2);
                break;
+	case PR_GET_TASK_SIZE:
+		error = prctl_get_tasksize((void *)arg2);
+		break;
        default:
                error = -EINVAL;
                break;
--
2.18.1