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Date:   Fri, 29 Oct 2021 12:43:54 -0700
From:   Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
Cc:     Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, netdev@vger.kernel.org,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
        antonio.gomez.iglesias@intel.com, tony.luck@intel.com,
        dave.hansen@linux.intel.com, gregkh@linuxfoundation.org,
        mark.rutland@arm.com, linux@armlinux.org.uk
Subject: [PATCH ebpf v3] bpf: Disallow unprivileged bpf by default
Message-ID: <0ace9ce3f97656d5f62d11093ad7ee81190c3c25.1635535215.git.pawan.kumar.gupta@linux.intel.com>
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Disabling unprivileged BPF would help prevent unprivileged users from
creating the conditions required for potential speculative execution
side-channel attacks on affected hardware. A deep dive on such attacks
and mitigation is available here [1].

Sync with what many distros are currently applying, disable unprivileged
BPF by default. An admin can enable this at runtime, if necessary.

[1] https://ebpf.io/summit-2021-slides/eBPF_Summit_2021-Keynote-Daniel_Borkmann-BPF_and_Spectre.pdf

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
v3:
- Drop the conditional default for CONFIG_BPF_UNPRIV_DEFAULT_OFF until
  we have an arch generic way to determine arch-common spectre type bugs.
  [Mark Rutland, Daniel Borkmann].
- Also drop the patch to Generalize ARM's CONFIG_CPU_SPECTRE.
- Minor changes to commit message.

v2: https://lore.kernel.org/lkml/cover.1635383031.git.pawan.kumar.gupta@linux.intel.com/
- Generalize ARM's CONFIG_CPU_SPECTRE to be available for all architectures.
- Make CONFIG_BPF_UNPRIV_DEFAULT_OFF depend on CONFIG_CPU_SPECTRE.
- Updated commit message to reflect the dependency on CONFIG_CPU_SPECTRE.
- Add reference to BPF spectre presentation in commit message.

v1: https://lore.kernel.org/all/d37b01e70e65dced2659561ed5bc4b2ed1a50711.1635367330.git.pawan.kumar.gupta@linux.intel.com/

 kernel/bpf/Kconfig | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
index a82d6de86522..73d446294455 100644
--- a/kernel/bpf/Kconfig
+++ b/kernel/bpf/Kconfig
@@ -64,6 +64,7 @@ config BPF_JIT_DEFAULT_ON
 
 config BPF_UNPRIV_DEFAULT_OFF
 	bool "Disable unprivileged BPF by default"
+	default y
 	depends on BPF_SYSCALL
 	help
 	  Disables unprivileged BPF by default by setting the corresponding
@@ -72,6 +73,10 @@ config BPF_UNPRIV_DEFAULT_OFF
 	  disable it by setting it to 1 (from which no other transition to
 	  0 is possible anymore).
 
+	  Unprivileged BPF can be used to exploit potential speculative
+	  execution side-channel vulnerabilities on affected hardware. If you
+	  are concerned about it, answer Y.
+
 source "kernel/bpf/preload/Kconfig"
 
 config BPF_LSM
-- 
2.31.1