linux-kernel - Re: [PATCH] stm class: fix a missing-check bug

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Date:   Wed, 3 Oct 2018 09:54:34 -0500
From:   Wenwen Wang <wang6495@....edu>
To:     alexander.shishkin@...ux.intel.com
Cc:     Kangjie Lu <kjlu@....edu>,
        open list <linux-kernel@...r.kernel.org>,
        Wenwen Wang <wang6495@....edu>
Subject: Re: [PATCH] stm class: fix a missing-check bug

On Wed, Oct 3, 2018 at 2:57 AM Alexander Shishkin
<alexander.shishkin@...ux.intel.com> wrote:
>
> Wenwen Wang <wang6495@....edu> writes:
>
> > In stm_char_policy_set_ioctl(), the 'size' field of the struct
> > 'stp_polic_id' is firstly copied from the user space and then checked,
> > because the length of the 'id' field in this struct, which represents an
> > identification string, is not fixed. If the 'size' field cannot pass the
> > check, an error code EINVAL will be returned. However, after the check, the
> > whole struct is copied again from the user space. Given that the user data
> > resides in the user space, a malicious user-space process can race to
> > change the size between the two copies. By doing so, the attacker can
> > bypass the check on the 'size' field and inject malicious data.
>
> How? The id->size is not used for anything.
>
> And even if there was a problem, this:
>
> > -     if (copy_from_user(id, arg, size)) {
> > +     if (copy_from_user(&id->master, (char __user *)arg + sizeof(size),
> > +                             size - sizeof(size))) {
>
> is completely pointless.

Given that id->size is not used, it should not be copied from the user
space. This code is used to remove such redundant copy.

>
> >               ret = -EFAULT;
> >               goto err_free;
> >       }
> >
> > +     id->size = size;
>
> So, if we did use id->size after the copying, we'd indeed have this line
> in the code. But since we don't, it's also pointless, so it's not there.
>
> Thanks,
> --
> Alex