[tip: x86/urgent] x86/cfi: Extend {JMP,CAKK}_NOSPEC comment

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     0479a42d4c15bd554f54d89d12bf68218e3e70da
Gitweb:        https://git.kernel.org/tip/0479a42d4c15bd554f54d89d12bf68218e3e70da
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 22 Jun 2023 16:27:13 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 10 Jul 2023 09:52:23 +02:00

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@xxxxxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20230622144321.360957723%40infradead.org
---
 arch/x86/include/asm/nospec-branch.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 55388c9..1a65cf4 100644
--- 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