Re: [RFC PATCH] exec: Avoid recursive modprobe for binary format handlers

[Luis R. Rodriguez]

On Mon, Aug 07, 2017 at 11:26:09AM +0100, Matt Redfearn wrote:
> Hi Luis,
> On 03/08/17 00:23, Luis R. Rodriguez wrote:
> > On Tue, Aug 01, 2017 at 07:28:20PM -0700, Kees Cook wrote:
> > > On Tue, Aug 1, 2017 at 5:12 PM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx> wrote:
> > > > On Fri, Jul 21, 2017 at 03:05:20PM +0100, Matt Redfearn wrote:
> > > > > Commit 6d7964a722af ("kmod: throttle kmod thread limit") which was
> > > > > merged in v4.13-rc1 broke this behaviour since the recursive modprobe is
> > > > > no longer caught, it just ends up waiting indefinitely for the kmod_wq
> > > > > wait queue. Hence the kernel appears to hang silently when starting
> > > > > userspace.
> > > > Indeed, the recursive issue were no longer expected to exist.
> > > Errr, yeah, recursive binfmt loads can still happen.
> > > 
> > > > The *old* implementation would also prevent a set of binaries to daisy chain
> > > > a set of 50 different binaries which require different binfmt loaders. The
> > > > current implementation enables this and we'd just wait. There's a bound to
> > > > the number of binfmd loaders though, so this would be bounded. If however
> > > > a 2nd loader loaded the first binary we'd run into the same issue I think.
> > > > 
> > > > If we can't think of a good way to resolve this we'll just have to revert
> > > > 6d7964a722af for now.
> > > The weird but "normal" recursive case is usually a script calling a
> > > script calling a misc format. Getting a chain of modprobes running,
> > > though, seems unlikely. I *think* Matt's patch is okay, but I agree,
> > > it'd be better for the request_module() to fail.
> > In that case how about we just have each waiter only wait max X seconds,
> > if the number of concurrent ongoing modprobe calls hasn't reduced by
> > a single digit in X seconds we give up on request_module() for the
> > module and clearly indicate what happened.
> > 
> > Matt, can you test?
> 
> Sure - I've tested patch this on Cavium Octeon under the same conditions as
> before (64 bit kernel with 32bit userspace & no binfmt handler builtin).
> 
> The failing modprobe is now caught and rather than silence we get the
> expected kernel panic, albeit all the warnings look quite noisy.

Thanks for testing! I agree its all too verbose, I'll clean that up and
resubmit with a cleaner log.

I tried to devise a test case for this but for the life of me I could not. If
you happen to come up with something please feel free to submit one for
lib/test_kmod.c!

> In any case, this is better than the 4.13-rc1 behavior, so
> 
> Tested-by: Matt Redfearn <matt.redfearn@xxxxxxxxxx>

 Luis