netdev - Re: sysctl: setting key "net.core.bpf_jit

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Date:   Sun, 11 Apr 2021 18:23:44 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Paul Menzel <pmenzel@...gen.mpg.de>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Sandipan Das <sandipan@...ux.ibm.com>
Cc:     it+linux-bpf@...gen.mpg.de, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: sysctl: setting key "net.core.bpf_jit_enable": Invalid argument



Le 11/04/2021 à 13:09, Paul Menzel a écrit :
> Dear Linux folks,
> 
> 
> Related to * [CVE-2021-29154] Linux kernel incorrect computation of branch displacements in BPF JIT 
> compiler can be abused to execute arbitrary code in Kernel mode* [1], on the POWER8 system IBM 
> S822LC with self-built Linux 5.12.0-rc5+, I am unable to disable `bpf_jit_enable`.
> 
>     $ /sbin/sysctl net.core.bpf_jit_enable
>     net.core.bpf_jit_enable = 1
>     $ sudo /sbin/sysctl -w net.core.bpf_jit_enable=0
>     sysctl: setting key "net.core.bpf_jit_enable": Invalid argument
> 
> It works on an x86 with Debian sid/unstable and Linux 5.10.26-1.

Maybe you have selected CONFIG_BPF_JIT_ALWAYS_ON in your self-built kernel ?

config BPF_JIT_ALWAYS_ON
	bool "Permanently enable BPF JIT and remove BPF interpreter"
	depends on BPF_SYSCALL && HAVE_EBPF_JIT && BPF_JIT
	help
	  Enables BPF JIT and removes BPF interpreter to avoid
	  speculative execution of BPF instructions by the interpreter


Christophe