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authorLinus Torvalds <torvalds@linux-foundation.org>2022-03-08 04:29:47 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2022-03-08 04:29:47 +0300
commit4a01e748a51cdc0527fdc913546dd46e822aa00d (patch)
tree8dbdba3aa96fb2323ba8194e8838dddf917752e5 /Documentation
parentea4424be16887a37735d6550cfd0611528dbe5d9 (diff)
parent0de05d056afdb00eca8c7bbb0c79a3438daf700c (diff)
downloadlinux-4a01e748a51cdc0527fdc913546dd46e822aa00d.tar.xz
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
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/admin-guide/hw-vuln/spectre.rst50
-rw-r--r--Documentation/admin-guide/kernel-parameters.txt8
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.