[PATCH] vt: Fix potential read overflow of kernel memory

[Azeem Shaikh <azeemshaikh38@gmail.com>]

strlcpy() reads the entire source buffer first.
This read may exceed the destination size limit if
a source string is not NUL-terminated [1].

The copy_to_user() call uses @len returned from strlcpy() directly
without checking its value. This could potentially lead to read
overflow.

In an effort to remove strlcpy() completely [2], replace
strlcpy() here with strscpy().

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
[2] https://github.com/KSPP/linux/issues/89

Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
---
 drivers/tty/vt/keyboard.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 358f216c6cd6..15359c328a23 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -2079,12 +2079,15 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
 			return -ENOMEM;

 		spin_lock_irqsave(&func_buf_lock, flags);
-		len = strlcpy(kbs, func_table[kb_func] ? : "", len);
+		len = strscpy(kbs, func_table[kb_func] ? : "", len);
 		spin_unlock_irqrestore(&func_buf_lock, flags);

+		if (len < 0) {
+			ret = -EFAULT;
+			break;
+		}
 		ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ?
 			-EFAULT : 0;
-
 		break;
 	}
 	case KDSKBSENT:
--
2.42.0.rc2.253.gd59a3bf2b4-goog