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Date: Fri, 14 Feb 2020 10:47:16 +0000
From: Ibrahim el-sayed <>
Subject: Re: Potential regression and/or incomplete fix for CVE-2017-12762

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


On Tue, Feb 11, 2020 at 9:37 PM Brad Spengler <>

> 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 (
> >
> )
> > 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:
> 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 (
> >
> > )
> > 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:
> The 3.16 kernel has a different maintainer than others listed on
> so there may be different critera for what's selected for backporting
> there.
> Thanks,
> -Brad

Ibrahim M. El-Sayed
Security Engineer

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