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Re: [PATCH] x86-64 memcmp: Use unsigned Jcc instructions on size
On Sat, Feb 2, 2019 at 7:32 AM Florian Weimer <email@example.com> wrote:
> * H. J. Lu:
> > On Sat, Feb 2, 2019 at 6:57 AM Florian Weimer <firstname.lastname@example.org> wrote:
> >> * H. J. Lu:
> >> > Since the size argument is unsigned. we should use unsigned Jcc
> >> > instructions, instead of signed to check size.
> >> >
> >> > Tested on x86-64 and x32, with and without --disable-multi-arch.
> >> Does this impact x86-64 at all (technically), consider that an object
> >> size larger than SSIZE_MAX would be undefined anyway?
> > I don't think we will hit it on x86-64.
> >> It seems that on x32, it can give incorrect results if the sign bit on
> >> the 64-bit register is set. In this sense, it is similar to
> >> CVE-2019-6488 in impact, right? If we decide to treat this as a
> > For x32, there is no invalid memory access. It just gives the wrong
> > result.
> Well, that could be problematic as well.
> Does the comparison stop early, only checking a prefix?
On x32, memcmp always returns 0 when the most significant bit of RDX is set
since it treats size as 0, like memcmp (a, b, 0).
> >> security vulnerability, we need a new CVE ID because the version range
> >> is different (bug 24155 was not fixed in the 2.29 release).
> > Since the wrong result from memcmp may lead to security vulnerability, we
> > should apply for CVE.
> Sure, I'll take care of it.