On Wed, Aug 30, 2023 at 04:04:10PM +0000, Azeem Shaikh wrote: > strlcpy() reads the entire source buffer first. Perhaps add: "... and returns the size of the source string, not the destination string, which can be accidentally misused." > 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. Since the code as written today is "accidentally correct", it's worth clarifying this: there is no existing bug, but as written it is very fragile and specifically uses a strlcpy() result without sanity checking and using it to copy to userspace. (This is the exact anti-pattern for strlcpy(), and only because the source strings are known good is this safe.) > 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@xxxxxxxxx> > --- > 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; I think this should be -ENOSPC as EFAULT implies an actual memory fault. > + break; > + } > ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ? > -EFAULT : 0; > - > break; > } > case KDSKBSENT: > -- > 2.42.0.rc2.253.gd59a3bf2b4-goog > > Thanks for sticking with these refactorings; we're almost free from strlcpy. :) -Kees -- Kees Cook
