RE: [PATCH] tty: vt: keyboard: do not copy an extra-byte in copy_to_user

From: Pavel Skripkin

Sent: 06 November 2021 11:24

Hi, Ajay!

On 11/6/21 12:20, Ajay Garg wrote:

quoted

Both (statically-allocated) "user_kdgkb->kb_string" and
(dynamically-allocated) "kbs" are of length "len", so we must
not copy more than "len" bytes.

Signed-off-by: Ajay Garg <redacted>
---
  drivers/tty/vt/keyboard.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index c7fbbcdcc346..dfef7de8a057 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c

@@ -2070,7 +2070,7 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
  		len = strlcpy(kbs, func_table[kb_func] ? : "", len);

		^^^^^^^^^

len is reinitialized here, i.e len passed to kmalloc and len passed to
copy_to_user() can be different.

strlcpy() returns strlen() of source string (2nd argument), that's why
we need +1 here to pass null byte to user.

Am I missing something?

You want strscpy() - returns the number of characters/bytes it copied.

	David

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