vt: Replace strlcpy with strscpy

strlcpy() reads the entire source buffer first and returns the size of
the source string, not the destination string, which can be accidentally
misused [1].

The copy_to_user() call uses @len returned from strlcpy() directly
without checking its value. This could potentially lead to read
overflow. There is no existing bug since @len is always guaranteed to be
greater than hardcoded strings in @func_table[kb_func]. But as written
it is very fragile and specifically uses a strlcpy() result without sanity
checking and using it to copy to userspace.

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 <azeems@google.com>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230919192156.121503-1-azeems@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 358f216..6e8eeff 100644 (file)
--- 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 = -ENOSPC;
+                       break;
+               }
                ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ?
                        -EFAULT : 0;
-
                break;
        }
        case KDSKBSENT: