Merge tag 'x86_bugs_for_v5.17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 spectre fixes from Borislav Petkov:

 - Mitigate Spectre v2-type Branch History Buffer attacks on machines
   which support eIBRS, i.e., the hardware-assisted speculation
   restriction after it has been shown that such machines are vulnerable
   even with the hardware mitigation.

 - Do not use the default LFENCE-based Spectre v2 mitigation on AMD as
   it is insufficient to mitigate such attacks. Instead, switch to
   retpolines on all AMD by default.

 - Update the docs and add some warnings for the obviously vulnerable
   cmdline configurations.

* tag 'x86_bugs_for_v5.17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/speculation: Warn about eIBRS + LFENCE + Unprivileged eBPF + SMT
  x86/speculation: Warn about Spectre v2 LFENCE mitigation
  x86/speculation: Update link to AMD speculation whitepaper
  x86/speculation: Use generic retpoline by default on AMD
  x86/speculation: Include unprivileged eBPF status in Spectre v2 mitigation reporting
  Documentation/hw-vuln: Update spectre doc
  x86/speculation: Add eIBRS + Retpoline options
  x86/speculation: Rename RETPOLINE_AMD to RETPOLINE_LFENCE

2 files changed, 39 insertions, 19 deletions

diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index a2b22d5640ec..9e9556826450 100644
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -60,8 +60,8 @@ privileged data touched during the speculative execution.
 Spectre variant 1 attacks take advantage of speculative execution of
 conditional branches, while Spectre variant 2 attacks use speculative
 execution of indirect branches to leak privileged memory.
-See :ref:`[1] <spec_ref1>` :ref:`[5] <spec_ref5>` :ref:`[7] <spec_ref7>`
-:ref:`[10] <spec_ref10>` :ref:`[11] <spec_ref11>`.
+See :ref:`[1] <spec_ref1>` :ref:`[5] <spec_ref5>` :ref:`[6] <spec_ref6>`
+:ref:`[7] <spec_ref7>` :ref:`[10] <spec_ref10>` :ref:`[11] <spec_ref11>`.
 
 Spectre variant 1 (Bounds Check Bypass)
 ---------------------------------------
@@ -131,6 +131,19 @@ steer its indirect branch speculations to gadget code, and measure the
 speculative execution's side effects left in level 1 cache to infer the
 victim's data.
 
+Yet another variant 2 attack vector is for the attacker to poison the
+Branch History Buffer (BHB) to speculatively steer an indirect branch
+to a specific Branch Target Buffer (BTB) entry, even if the entry isn't
+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).
+
 Attack scenarios
 ----------------
 
@@ -364,13 +377,15 @@ The possible values in this file are:
 
   - Kernel status:
 
-  ====================================  =================================
-  'Not affected'                        The processor is not vulnerable
-  'Vulnerable'                          Vulnerable, no mitigation
-  'Mitigation: Full generic retpoline'  Software-focused mitigation
-  'Mitigation: Full AMD retpoline'      AMD-specific software mitigation
-  'Mitigation: Enhanced IBRS'           Hardware-focused mitigation
-  ====================================  =================================
+  ========================================  =================================
+  'Not affected'                            The processor is not vulnerable
+  'Mitigation: None'                        Vulnerable, no mitigation
+  'Mitigation: Retpolines'                  Use Retpoline thunks
+  'Mitigation: LFENCE'                      Use LFENCE instructions
+  'Mitigation: Enhanced IBRS'               Hardware-focused mitigation
+  'Mitigation: Enhanced IBRS + Retpolines'  Hardware-focused + Retpolines
+  'Mitigation: Enhanced IBRS + LFENCE'      Hardware-focused + LFENCE
+  ========================================  =================================
 
   - Firmware status: Show if Indirect Branch Restricted Speculation (IBRS) is
     used to protect against Spectre variant 2 attacks when calling firmware (x86 only).
@@ -583,12 +598,13 @@ kernel command line.
 
 		Specific mitigations can also be selected manually:
 
-		retpoline
-					replace indirect branches
-		retpoline,generic
-					google's original retpoline
-		retpoline,amd
-					AMD-specific minimal thunk
+                retpoline               auto pick between generic,lfence
+                retpoline,generic       Retpolines
+                retpoline,lfence        LFENCE; indirect branch
+                retpoline,amd           alias for retpoline,lfence
+                eibrs                   enhanced IBRS
+                eibrs,retpoline         enhanced IBRS + Retpolines
+                eibrs,lfence            enhanced IBRS + LFENCE
 
 		Not specifying this option is equivalent to
 		spectre_v2=auto.
@@ -599,7 +615,7 @@ kernel command line.
 		spectre_v2=off. Spectre variant 1 mitigations
 		cannot be disabled.
 
-For spectre_v2_user see :doc:`/admin-guide/kernel-parameters`.
+For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt
 
 Mitigation selection guide
 --------------------------
@@ -681,7 +697,7 @@ AMD white papers:
 
 .. _spec_ref6:
 
-[6] `Software techniques for managing speculation on AMD processors <https://developer.amd.com/wp-content/resources/90343-B_SoftwareTechniquesforManagingSpeculation_WP_7-18Update_FNL.pdf>`_.
+[6] `Software techniques for managing speculation on AMD processors <https://developer.amd.com/wp-content/resources/Managing-Speculation-on-AMD-Processors.pdf>`_.
 
 ARM white papers:
 
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f5a27f067db9..7123524a86b8 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5361,8 +5361,12 @@
 			Specific mitigations can also be selected manually:
 
 			retpoline	  - replace indirect branches
-			retpoline,generic - google's original retpoline
-			retpoline,amd     - AMD-specific minimal thunk
+			retpoline,generic - Retpolines
+			retpoline,lfence  - LFENCE; indirect branch
+			retpoline,amd     - alias for retpoline,lfence
+			eibrs		  - enhanced IBRS
+			eibrs,retpoline   - enhanced IBRS + Retpolines
+			eibrs,lfence      - enhanced IBRS + LFENCE
 
 			Not specifying this option is equivalent to
 			spectre_v2=auto.