[PATCH 6.1 62/69] x86/bugs: Clarify that syscall hardening isnt a BHI mitigation

Greg Kroah-Hartman
2024-04-15T14:41:26Z

From: Josh Poimboeuf <jpoimboe@kernel.org>

commit 5f882f3b0a8bf0788d5a0ee44b1191de5319bb8a upstream.

While syscall hardening helps prevent some BHI attacks, there's still
other low-hanging fruit remaining. Don't classify it as a mitigation
and make it clear that the system may still be vulnerable if it doesn't
have a HW or SW mitigation enabled.

Fixes: ec9404e40e8f ("x86/bhi: Add BHI mitigation knob")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/b5951dae3fdee7f1520d5136a27be3bdfe95f88b.1712813475.git.jpoimboe@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/admin-guide/hw-vuln/spectre.rst | 11 +++++------
Documentation/admin-guide/kernel-parameters.txt | 3 +--
arch/x86/kernel/cpu/bugs.c | 6 +++---
3 files changed, 9 insertions(+), 11 deletions(-)

--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -441,10 +441,10 @@ The possible values in this file are:
- System is protected by BHI_DIS_S
* - BHI: SW loop, KVM SW loop
- System is protected by software clearing sequence
- * - BHI: Syscall hardening
- - Syscalls are hardened against BHI
- * - BHI: Syscall hardening, KVM: SW loop
- - System is protected from userspace attacks by syscall hardening; KVM is protected by software clearing sequence
+ * - BHI: Vulnerable
+ - System is vulnerable to BHI
+ * - BHI: Vulnerable, KVM: SW loop
+ - System is vulnerable; KVM is protected by software clearing sequence

Full mitigation might require a microcode update from the CPU
vendor. When the necessary microcode is not available, the kernel will
@@ -661,8 +661,7 @@ kernel command line.
spectre_bhi=

[X86] Control mitigation of Branch History Injection
- (BHI) vulnerability. Syscalls are hardened against BHI
- regardless of this setting. This setting affects the deployment
+ (BHI) vulnerability. This setting affects the deployment
of the HW BHI control and the SW BHB clearing sequence.

on
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5735,8 +5735,7 @@
See Documentation/admin-guide/laptops/sonypi.rst

spectre_bhi= [X86] Control mitigation of Branch History Injection
- (BHI) vulnerability. Syscalls are hardened against BHI
- reglardless of this setting. This setting affects the
+ (BHI) vulnerability. This setting affects the
deployment of the HW BHI control and the SW BHB
clearing sequence.

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2797,10 +2797,10 @@ static const char *spectre_bhi_state(voi
return "; BHI: SW loop, KVM: SW loop";
else if (boot_cpu_has(X86_FEATURE_RETPOLINE) && rrsba_disabled)
return "; BHI: Retpoline";
- else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT))
- return "; BHI: Syscall hardening, KVM: SW loop";
+ else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT))
+ return "; BHI: Vulnerable, KVM: SW loop";

- return "; BHI: Vulnerable (Syscall hardening enabled)";
+ return "; BHI: Vulnerable";
}

static ssize_t spectre_v2_show_state(char *buf)