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

[David Laight <David.Laight@ACULAB.COM>]

From: Pavel Skripkin
> Sent: 06 November 2021 11:24
> 
> Hi, Ajay!
> 
> On 11/6/21 12:20, Ajay Garg wrote:
> > 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 <ajaygargnsit@gmail.com>
> > ---
> >   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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