linux-kernel - [PATCH v2 1/6] x86/cfi: Extend {JMP,CAKK}

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Date:   Thu, 22 Jun 2023 16:42:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, alyssa.milburn@...ux.intel.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        samitolvanen@...gle.com, keescook@...omium.org,
        jpoimboe@...nel.org, joao@...rdrivepizza.com, brgerst@...il.com
Subject: [PATCH v2 1/6] x86/cfi: Extend {JMP,CAKK}_NOSPEC comment

With the introduction of kCFI these helpers are no longer equivalent
to C indirect calls and should be used with care.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/nospec-branch.h |    4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -234,6 +234,10 @@
  * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
  * indirect jmp/call which may be susceptible to the Spectre variant 2
  * attack.
+ *
+ * NOTE: these do not take kCFI into account and are thus not comparable to C
+ * indirect calls, take care when using. The target of these should be an ENDBR
+ * instruction irrespective of kCFI.
  */
 .macro JMP_NOSPEC reg:req
 #ifdef CONFIG_RETPOLINE