Re: Potential regression and/or incomplete fix for CVE-2017-12762

[Ibrahim el-sayed <i.elsayed92 () gmail com>]

Hi Brad,
Thank you very much for your reply. This actually clarifies everything :)

Ibrahim

On Tue, Feb 11, 2020 at 9:37 PM Brad Spengler <spender () grsecurity net>
wrote:

> Hi Ibrahim,
>
> > I think it is incomplete and can lead to reading out of bound since it
> does
> > *not* check if the src buffer (p) in this case has 10 bytes at least. The
> > fix assumes p has 10 bytes and copies that into newname. The fix
> > uses strscpy (
> https://github.com/torvalds/linux/blob/cc12071ff39060fc2e47c58b43e249fe0d0061ee/lib/string.c#L180
> )
> > which
> > based on its code it starts copying from count and decrements to zero.
>
> This isn't correct.  There is a 'count' variable that decrements to zero,
> yes,
> but that's not what is used to index the strings.  'res' is used for that,
> and
> it increments from zero as you'd expect.
>
> Regarding OOB, there is the read-by-word trickery, but it's safe and won't
> trip up KASAN for the max 7 bytes it can end up reading past bounds, and
> won't
> in any instance cross a page boundary.
>
> Since 'param' is guaranteed to be NUL-terminated from the fix (the
> isdn_common.c
> change), so is 'p', so the strscpy is fine here, especially since the later
> use of the buffer (coming from the netdev netname) uses strlen as the
> length
> for the buffer copy to userland here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/isdn/i4l/isdn_common.c?id=9f5af546e6acc30f075828cb58c7f09665033967#n1385
> So strscpy_pad() wasn't necessary in this instance, despite it often being
> needed for kernel work (and the better defensive choice, unless performance
> is critical and you can guarantee the remaining part of the buffer never
> gets
> copied to userland or used in any way).
>
> > ## Regression
> > I looked quickly into latest version for the kernel v3.16.81 and it seems
> > that the patch was probably reverted as the code matches exactly to the
> > vulnerable version to the CVE (
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/isdn/i4l/isdn_net.c?id=v3.16.81#n2646
> > )
> > Not sure if the fix was reworked but wanted to surface that issue as well
>
> This wasn't due to a revert, the fix was just never backported to 3.16.
> Happens all the time.  There's never a guarantee that just because a
> security fix is backported to some newer kernel version that it'll be
> backported to all affected versions.  If the patch doesn't apply cleanly
> and no one fixes it up, it just never gets fixed.
>
> For this instance, you can confirm it by looking at the git log for that
> tree:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/log/drivers/isdn/i4l/isdn_net.c?h=linux-3.16.y
>
> The 3.16 kernel has a different maintainer than others listed on
> kernel.org:
> https://www.kernel.org/category/releases.html
> so there may be different critera for what's selected for backporting
> there.
>
> Thanks,
> -Brad


-- 
Regards
Ibrahim M. El-Sayed
Security Engineer
Website: https://www.ibrahim-elsayed.com
@ibrahim_mosaad