PATCH: ctdb: buffer write beyond limits
cs at samba.org
Tue Feb 19 16:48:16 UTC 2019
On Tue, Feb 19, 2019 at 12:37:18PM +0100, swen via samba-technical wrote:
> On Tue, 2019-02-19 at 11:53 +0100, Volker Lendecke wrote:
> > On Tue, Feb 19, 2019 at 11:02:18AM +0100, Volker Lendecke via samba-
> > technical wrote:
> > > On Tue, Feb 19, 2019 at 10:53:41AM +0100, swen wrote:
> > > > the code is right if the following facts are taken into account.
> > > > 1. MAX_SIGNED_INT = MAX_UNSIGNED_INT/2 - 1
> > > > 2. length is defined as unsigned int and will get assigned at
> > > > most MAX_SIGNED_INT(num_ready)
> > > > 3. offset is initially zero and will only grow as much as length
> > > > is shrinking (=>code)
> > > > => therefore, offset + length are at most MAX_SIGNED_INT.
> > > > Guaranteed !
> > > > 4. If then another (guaranteed positive) signed integer (here
> > > > num_ready) is added we simply
> > > > cannot overflow an unsigned int because
> > > > MAX_SIGNED_INT + MAX_SIGNED_INT = MAX_UNSIGNED_INT - 2
> > > >
> > > > Taken those simple maths into account, I hope you can agree to
> > > > the code.
> > >
> > > No, I do not agree. There is a simple boiler plate to do a checked
> > > addition, and a simple boiler plate to do checked subtraction.
> > > Please
> > > do it that way.
> > More explanation: I do not agree because it is really simple to get
> > these lines right on their own respect. And having to read a 4-entry
> > proof referencing code elsewhere where it's easy to make to code safe
> > in an isolated manner.
> > The crash was introduced through patches that did not affect this MIN
> > statement at all. This means that the environment can change, and the
> > assumptions that you refer to in bullet point 3 might change, causing
> > the code to crash again. In this kind of code we have to check every
> > individual arithmetic operation in isolation.
> > Attached find something that I did in 10 minutes. Please crush it if
> > you think your approach is better.
Considering the complexity here: Would it be possible to add unit tests
here? First to ensure that the fix addresses the problem at hand and
then also to stress other corner cases. I am not yet sure what the best
strategy here would be. It could be unit testing the function (cmocka?) or
setting up a test client that sends the combinations of requests to ctdb
that we want to test. Either way, we should ensure that this works and
does not regress with future patches.
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