On Wed 03-06-15 09:50:30, Martin Schwidefsky wrote:
> On Tue, 2 Jun 2015 17:07:33 +0200
> Jan Kara <jack@xxxxxxx> wrote:
>
> > strnlen_user() returns the length of the string including terminating 0.
> > So avoid counting it again and unnecessarily reducing maximum string
> > size by 1.
> >
> > CC: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> > Signed-off-by: Jan Kara <jack@xxxxxxx>
> > ---
> > drivers/s390/char/keyboard.c | 8 +++-----
> > 1 file changed, 3 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/s390/char/keyboard.c b/drivers/s390/char/keyboard.c
> > index 01463b052ae7..e45541915a73 100644
> > --- a/drivers/s390/char/keyboard.c
> > +++ b/drivers/s390/char/keyboard.c
> > @@ -433,20 +433,18 @@ do_kdgkb_ioctl(struct kbd_data *kbd, struct kbsentry __user *u_kbs,
> > case KDSKBSENT:
> > if (!perm)
> > return -EPERM;
> > - len = strnlen_user(u_kbs->kb_string,
> > - sizeof(u_kbs->kb_string) - 1);
> > + len = strnlen_user(u_kbs->kb_string, sizeof(u_kbs->kb_string));
> > if (!len)
> > return -EFAULT;
> > - if (len > sizeof(u_kbs->kb_string) - 1)
> > + if (len > sizeof(u_kbs->kb_string))
> > return -EINVAL;
> > - p = kmalloc(len + 1, GFP_KERNEL);
> > + p = kmalloc(len, GFP_KERNEL);
> > if (!p)
> > return -ENOMEM;
> > if (copy_from_user(p, u_kbs->kb_string, len)) {
> > kfree(p);
> > return -EFAULT;
> > }
> > - p[len] = 0;
> > kfree(kbd->func_table[kb_func]);
> > kbd->func_table[kb_func] = p;
> > break;
>
> The simplification with the string length is nice but removing
> the explicit NUL termination is imho a mistake. Who guarantees
> you that the string in user space is still the same after the
> initial strnlen_user? It might have changed before the
> copy_from_user call and then we end up with an unterminated
> string in the kernel. Not good.
Ah, that's a good point. Thanks for catching this. But it would deserve
a comment in the code. Attached is an updated patch.
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
>From b73200d6924504e6250abc979331de94f8e31d9b Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@xxxxxxx>
Date: Tue, 2 Jun 2015 12:38:39 +0200
Subject: [PATCH] s390 keyboard: Avoid off-by-one when using strnlen_user()
strnlen_user() returns the length of the string including terminating 0.
So avoid counting it again and unnecessarily reducing maximum string
size by 1.
CC: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
drivers/s390/char/keyboard.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)
diff --git a/drivers/s390/char/keyboard.c b/drivers/s390/char/keyboard.c
index 01463b052ae7..cdb180ea44f9 100644
--- a/drivers/s390/char/keyboard.c
+++ b/drivers/s390/char/keyboard.c
@@ -433,20 +433,23 @@ do_kdgkb_ioctl(struct kbd_data *kbd, struct kbsentry __user *u_kbs,
case KDSKBSENT:
if (!perm)
return -EPERM;
- len = strnlen_user(u_kbs->kb_string,
- sizeof(u_kbs->kb_string) - 1);
+ len = strnlen_user(u_kbs->kb_string, sizeof(u_kbs->kb_string));
if (!len)
return -EFAULT;
- if (len > sizeof(u_kbs->kb_string) - 1)
+ if (len > sizeof(u_kbs->kb_string))
return -EINVAL;
- p = kmalloc(len + 1, GFP_KERNEL);
+ p = kmalloc(len, GFP_KERNEL);
if (!p)
return -ENOMEM;
if (copy_from_user(p, u_kbs->kb_string, len)) {
kfree(p);
return -EFAULT;
}
- p[len] = 0;
+ /*
+ * Make sure the string is terminated by 0. User could have
+ * modified it between us running strnlen_user() and copying it.
+ */
+ p[len-1] = 0;
kfree(kbd->func_table[kb_func]);
kbd->func_table[kb_func] = p;
break;
--
2.1.4
