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LWN Articles About Linux (Kernel)

Submitted by Roy Schestowitz on Thursday 13th of December 2018 12:29:15 PM Filed under
Linux
  • Bounded loops in BPF programs

    The BPF verifier is charged with ensuring that any given BPF program is safe for the kernel to load and run. Programs that fail to terminate are clearly unsafe, as they present an opportunity for denial-of-service attacks. In current kernels, the verifier uses a heavy-handed technique to block such programs: it disallows any program containing loops. This works, but at the cost of disallowing a wide range of useful programs; if the verifier could determine whether any given loop would terminate within a bounded time, this restriction could be lifted. John Fastabend presented a plan for doing so during the BPF microconference at the 2018 Linux Plumbers Conference.

    Fastabend started by noting that the lack of loops hurts; BPF developers are doing "crazy things" to work around their absence. He is working to enable the use of simple loops that can be modeled by the verifier. There is academic work on ways to verify more complex loops, but that is a problem for later. For now, the objective is to detect simple loops and verify that they will terminate; naturally, it's important that the verifier, too, is able to terminate in a reasonable amount of time.

  • Binary portability for BPF programs

    The BPF virtual machine is the same on all architectures where it is supported; architecture-specific code takes care of translating BPF to something the local processor can understand. So one might be tempted to think that BPF programs would be portable across architectures but, in many cases, that turns out not to be true. During the BPF microconference at the Linux Plumbers Conference, Alexei Starovoitov (assisted by Yonghong Song, who has done much of the work described) explained the problem and the work that has been done toward "compile once, run everywhere" BPF.

    Many BPF programs are indeed portable, in that they will load and execute properly on any type of processor. Packet-filtering programs, in particular, usually just work. But there is a significant class of exceptions in the form of tracing programs, which are one of the biggest growth areas for BPF. Most tracing tools have two components: a user-space program invoked by the user, and a BPF program that is loaded into the kernel to filter, acquire, and possibly boil down the needed data. Both programs are normally written in C.

  • Taming STIBP

    The Spectre class of hardware vulnerabilities was apparently so-named because it can be expected to haunt us for some time. One aspect of that haunting can be seen in the fact that, nearly one year after Spectre was disclosed, the kernel is still unable to prevent one user-space process from attacking another in some situations. An attempt to provide that protection using a new x86 microcode feature called STIBP has run into trouble once its performance impact was understood; now a more nuanced approach may succeed in providing protection where it is needed without slowing down everybody else.

    The Spectre variant 2 vulnerability works by polluting the CPU's branch-prediction buffer (BPB), which is used during speculative execution to make a guess about which branch(es) the code will take; see this article for a refresher on the Spectre vulnerabilities if needed. Closing this hole requires changes at a number of levels, but a fundamental part of the problem is preventing any code that may be targeted from running with a BPB that has been trained by an attacker.

  • The x32 subarchitecture may be removed

    The x32 subarchitecture is a software variant of x86-64; it runs the processor in the 64-bit mode, but uses 32-bit pointers and arithmetic. The idea is to get the advantages of x86-64 without the extra memory usage that goes along with it. It seems, though, that x32 is not much appreciated; few distributions support it and the number of users appears to be small.

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