[PATCH ebpf] bpf: Disallow unprivileged bpf by default

[PATCH ebpf] bpf: Disallow unprivileged bpf by default

[Pawan Gupta]

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
---
 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

Re: [PATCH ebpf] bpf: Disallow unprivileged bpf by default

[Daniel Borkmann]

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

> ---
>   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'.

>   	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
> 

Thanks,
Daniel

Re: [PATCH ebpf] bpf: Disallow unprivileged bpf by default

[Pawan Gupta]

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