Date: Fri, 14 Feb 2020 10:47:16 +0000 From: Ibrahim el-sayed <i.elsayed92@...il.com> To: oss-security@...ts.openwall.com 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 :) Ibrahim On Tue, Feb 11, 2020 at 9:37 PM Brad Spengler <spender@...ecurity.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
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