Merge tag 'nativebhi' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 mitigations from Thomas Gleixner:
 "Mitigations for the native BHI hardware vulnerabilty:

  Branch History Injection (BHI) attacks may allow a malicious
  application to influence indirect branch prediction in kernel by
  poisoning the branch history. eIBRS isolates indirect branch targets
  in ring0. 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.

  Add mitigations against it either with the help of microcode or with
  software sequences for the affected CPUs"

[ This also ends up enabling the full mitigation by default despite the
  system call hardening, because apparently there are other indirect
  calls that are still sufficiently reachable, and the 'auto' case just
  isn't hardened enough.

  We'll have some more inevitable tweaking in the future    - Linus ]

* tag 'nativebhi' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  KVM: x86: Add BHI_NO
  x86/bhi: Mitigate KVM by default
  x86/bhi: Add BHI mitigation knob
  x86/bhi: Enumerate Branch History Injection (BHI) bug
  x86/bhi: Define SPEC_CTRL_BHI_DIS_S
  x86/bhi: Add support for clearing branch history at syscall entry
  x86/syscall: Don't force use of indirect calls for system calls
  x86/bugs: Change commas to semicolons in 'spectre_v2' sysfs file

1 files changed, 12 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 623fce7d5fcd..70046a019d42 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6063,6 +6063,18 @@
 	sonypi.*=	[HW] Sony Programmable I/O Control Device driver
 			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
+			deployment of the HW BHI control and the SW BHB
+			clearing sequence.
+
+			on   - unconditionally enable.
+			off  - unconditionally disable.
+			auto - (default) enable hardware mitigation
+			       (BHI_DIS_S) if available, otherwise enable
+			       alternate mitigation in KVM.
+
 	spectre_v2=	[X86,EARLY] Control mitigation of Spectre variant 2
 			(indirect branch speculation) vulnerability.
 			The default operation protects the kernel from