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-rw-r--r--Documentation/admin-guide/hw-vuln/spectre.rst44
1 files changed, 38 insertions, 6 deletions
diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index 32a8893e56..e0a1be97fa 100644
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -138,11 +138,10 @@ associated with the source address of the indirect branch. Specifically,
the BHB might be shared across privilege levels even in the presence of
Enhanced IBRS.
-Currently the only known real-world BHB attack vector is via
-unprivileged eBPF. Therefore, it's highly recommended to not enable
-unprivileged eBPF, especially when eIBRS is used (without retpolines).
-For a full mitigation against BHB attacks, it's recommended to use
-retpolines (or eIBRS combined with retpolines).
+Previously the only known real-world BHB attack vector was via unprivileged
+eBPF. Further research has found attacks that don't require unprivileged eBPF.
+For a full mitigation against BHB attacks it is recommended to set BHI_DIS_S or
+use the BHB clearing sequence.
Attack scenarios
----------------
@@ -430,6 +429,23 @@ The possible values in this file are:
'PBRSB-eIBRS: Not affected' CPU is not affected by PBRSB
=========================== =======================================================
+ - Branch History Injection (BHI) protection status:
+
+.. list-table::
+
+ * - BHI: Not affected
+ - System is not affected
+ * - BHI: Retpoline
+ - System is protected by retpoline
+ * - BHI: BHI_DIS_S
+ - System is protected by BHI_DIS_S
+ * - BHI: SW loop, KVM SW loop
+ - System 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
report vulnerability.
@@ -484,7 +500,11 @@ Spectre variant 2
Systems which support enhanced IBRS (eIBRS) enable IBRS protection once at
boot, by setting the IBRS bit, and they're automatically protected against
- Spectre v2 variant attacks.
+ some Spectre v2 variant attacks. The BHB can still influence the choice of
+ indirect branch predictor entry, and although branch predictor entries are
+ isolated between modes when eIBRS is enabled, the BHB itself is not isolated
+ between modes. Systems which support BHI_DIS_S will set it to protect against
+ BHI attacks.
On Intel's enhanced IBRS systems, this includes cross-thread branch target
injections on SMT systems (STIBP). In other words, Intel eIBRS enables
@@ -638,6 +658,18 @@ kernel command line.
spectre_v2=off. Spectre variant 1 mitigations
cannot be disabled.
+ spectre_bhi=
+
+ [X86] Control mitigation of Branch History Injection
+ (BHI) vulnerability. This setting affects the deployment
+ of the HW BHI control and the SW BHB clearing sequence.
+
+ on
+ (default) Enable the HW or SW mitigation as
+ needed.
+ off
+ Disable the mitigation.
+
For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt
Mitigation selection guide