[Xen-devel] [PATCH v3 3/7] xen/nospec: Introduce CONFIG_SPECULATIVE_HARDEN_BRANCH

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Just as with CONFIG_SPECULATIVE_HARDEN_ARRAY, branch hardening should be
configurable at compile time.

The previous CONFIG_HVM was a consequence of what could be discussed publicly
at the time the patches were submitted, and wasn't actually correct.  Later
patches will make further corrections.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>

v3:
 * Reduce to just the Kconfig option.  Split other changes out into separate
   patches.

v2:
 * Expand the commit message to describe how the generated code is broken.
 * Rename to CONFIG_SPECULATIVE_HARDEN_BRANCH
 * Switch alternative() to asm()
 * Fix a comment typo
---
 xen/common/Kconfig           | 23 +++++++++++++++++++++++
 xen/include/asm-x86/nospec.h |  2 +-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index 7b5dd9d495..c9e671869e 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -102,6 +102,29 @@ config SPECULATIVE_HARDEN_ARRAY
 
          If unsure, say Y.
 
+config SPECULATIVE_HARDEN_BRANCH
+       bool "Speculative Branch Hardening"
+       default y
+       depends on X86
+        ---help---
+         Contemporary processors may use speculative execution as a
+         performance optimisation, but this can potentially be abused by an
+         attacker to leak data via speculative sidechannels.
+
+         One source of misbehaviour is by executing the wrong basic block
+         following a conditional jump.
+
+         When enabled, specific conditions which have been deemed liable to
+         be speculatively abused will be hardened to avoid entering the wrong
+         basic block.
+
+         This is a best-effort mitigation.  There are no guarantees that all
+         areas of code open to abuse have been hardened, nor that
+         optimisations in the compiler haven't subverted the attempts to
+         harden.
+
+         If unsure, say Y.
+
 endmenu
 
 config KEXEC
diff --git a/xen/include/asm-x86/nospec.h b/xen/include/asm-x86/nospec.h
index 427b5ff9df..154e92aed8 100644
--- a/xen/include/asm-x86/nospec.h
+++ b/xen/include/asm-x86/nospec.h
@@ -9,7 +9,7 @@
 /* Allow to insert a read memory barrier into conditionals */
 static always_inline bool barrier_nospec_true(void)
 {
-#ifdef CONFIG_HVM
+#ifdef CONFIG_SPECULATIVE_HARDEN_BRANCH
     alternative("", "lfence", X86_FEATURE_SC_L1TF_VULN);
 #endif
     return true;
-- 
2.11.0


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