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

From: Kees Cook
Date: Tue Aug 01 2017 - 22:28:27 EST

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.


Kees Cook
Pixel Security