Fiasco.OC: null-pointer dereference?

Matthias Lange matthias.lange at kernkonzept.com
Mon May 8 15:36:58 CEST 2017


Hi Stefan,

On 05/08/2017 09:09 AM, Stefan Kalkowski wrote:
> Dear L4-Hackers,
> 
> recently, I started to upgrade the Fiasco.OC kernel version that is used
> by the Genode OS framework to the lastly released version (r72). I took
> the opportunity to upgrade, because the upcoming Genode release uses a
> fresh compiler toolchain that refused to build the very old Fiasco.OC
> kernel version that was used until now (r56).
> Everything went quite smoothly, and I'm glad to see how the kernel
> develops further. Thanks to all developers at this point!
> 
> Unfortunately, I stumbled across an issue when it comes to thread
> destruction. In our system all threads are constructed and destructed by
> the roottask that is called 'core'. In some cases, not always but quite
> often, the Ram_quota pointer of the thread object is zero during the
> call of the Thread_oject's delete operator, which leads to a page-fault
> within the kernel-code. A simple check[1] before dereferencing the
> pointer solves the problem, but I wonder whether we will leak quota or
> memory then, or in general cover some more serious problem.

Thank you for reporting this issue. I will forward this to our kernel
maintainer.

Could you elaborate a little bit more on the circumstances leading to
this issue? I wonder whether we can come up with a simple test case
triggering the page fault.

Best,
Matthias.

> Obviously, we have different usage patterns of syscalls, e.g.: the order
> of destructing IPC-gates, threads, IRQs, and tasks. Moreover, we still
> have some very few patches[2] so that the kernel meets our requirements.
> But none of them explains the thread's Ram_quota pointer getting zero.
> The page-fault triggers across all x86 and arm platforms that we use.
> 
> Any hint would be very much appreciated, all the best!
> Stefan
> 
> [1]
> https://github.com/skalk/foc/commit/2b01c9d16fd8e29e6af18fe750be2c8a312b4762
> [2] https://github.com/skalk/foc/commits/r72



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