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SubjectRe: [PATCH ebpf] bpf: Disallow unprivileged bpf by default
On 27.10.2021 23:21, Daniel Borkmann wrote:
>Hello Pawan,
>
>On 10/27/21 10:51 PM, Pawan Gupta wrote:
>>Disabling unprivileged BPF by default would help prevent unprivileged
>>users from creating the conditions required for potential speculative
>>execution side-channel attacks on affected hardware as demonstrated by
>>[1][2][3].
>>
>>This will sync mainline with what most distros are currently applying.
>>An admin can enable this at runtime if necessary.
>>
>>Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>>
>>[1] https://access.redhat.com/security/cve/cve-2019-7308
>>[2] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3490
>>[3] https://bugzilla.redhat.com/show_bug.cgi?id=1672355#c5
>
>Some of your above quoted links are just random ?! For example, [2] has really _zero_ to
>do with what you wrote with regards to speculative execution side-channel attacks ...
>
>We recently did a deep dive on our mitigation work we did in BPF here [0]. This also includes
>an appendix with an extract of the main commits related to the different Spectre variants.
>
>I'd suggest to link to that one instead to avoid confusion on what is related and what not.
>
> [0] https://ebpf.io/summit-2021-slides/eBPF_Summit_2021-Keynote-Daniel_Borkmann-BPF_and_Spectre.pdf

Sure, I will add reference to this presentation.

>>---
>> 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
>
>Hm, arm arch has a CPU_SPECTRE Kconfig symbol, see commit c58d237d0852 ("ARM: spectre:
>add Kconfig symbol for CPUs vulnerable to Spectre") that can be selected.
>
>Would be good to generalize it for reuse so archs can select it, and make the above as
>'default y if CPU_SPECTRE'.

Thanks for your feedback, I will send a v2 soon. I guess below is how
you want it to be:

---
diff --git a/arch/Kconfig b/arch/Kconfig
index 8df1c7102643..6aa856d51cb7 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1091,6 +1091,9 @@ config ARCH_SUPPORTS_RT
config CPU_NO_EFFICIENT_FFS
def_bool n

+config CPU_SPECTRE
+ bool
+
config HAVE_ARCH_VMAP_STACK
def_bool n
help
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 8355c3895894..44551465fd03 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -828,9 +828,6 @@ config CPU_BPREDICT_DISABLE
help
Say Y here to disable branch prediction. If unsure, say N.

-config CPU_SPECTRE
- bool
-
config HARDEN_BRANCH_PREDICTOR
bool "Harden the branch predictor against aliasing attacks" if EXPERT
depends on CPU_SPECTRE
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d9830e7e1060..769739da67c6 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -124,6 +124,7 @@ config X86
select CLKEVT_I8253
select CLOCKSOURCE_VALIDATE_LAST_CYCLE
select CLOCKSOURCE_WATCHDOG
+ select CPU_SPECTRE
select DCACHE_WORD_ACCESS
select EDAC_ATOMIC_SCRUB
select EDAC_SUPPORT
diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
index a82d6de86522..510a5a73f9a2 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 if CPU_SPECTRE
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
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