x86/speculation: Add a conditional CS prefix to CALL_NOSPEC

commit 052040e34c08428a5a388b85787e8531970c0c67 upstream.

Retpoline mitigation for spectre-v2 uses thunks for indirect branches. To
support this mitigation compilers add a CS prefix with
-mindirect-branch-cs-prefix. For an indirect branch in asm, this needs to
be added manually.

CS prefix is already being added to indirect branches in asm files, but not
in inline asm. Add CS prefix to CALL_NOSPEC for inline asm as well. There
is no JMP_NOSPEC for inline asm.

Reported-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250228-call-nospec-v3-2-96599fed0f33@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 7b5b93da50f59b48c6a0d257fd2db858d0bd8b2d..dab6e9ef01b647c1acf420f8848527925f658135 100644 (file)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -119,9 +119,8 @@
 .endm
 
 /*
- * Equivalent to -mindirect-branch-cs-prefix; emit the 5 byte jmp/call
- * to the retpoline thunk with a CS prefix when the register requires
- * a RAX prefix byte to encode. Also see apply_retpolines().
+ * Emits a conditional CS prefix that is compatible with
+ * -mindirect-branch-cs-prefix.
  */
 .macro __CS_PREFIX reg:req
        .irp rs,r8,r9,r10,r11,r12,r13,r14,r15
@@ -280,12 +279,24 @@ extern void (*x86_return_thunk)(void);
 
 #ifdef CONFIG_X86_64
 
+/*
+ * Emits a conditional CS prefix that is compatible with
+ * -mindirect-branch-cs-prefix.
+ */
+#define __CS_PREFIX(reg)                               \
+       ".irp rs,r8,r9,r10,r11,r12,r13,r14,r15\n"       \
+       ".ifc \\rs," reg "\n"                           \
+       ".byte 0x2e\n"                                  \
+       ".endif\n"                                      \
+       ".endr\n"
+
 /*
  * Inline asm uses the %V modifier which is only in newer GCC
  * which is ensured when CONFIG_RETPOLINE is defined.
  */
 #ifdef CONFIG_RETPOLINE
-#define CALL_NOSPEC    "call __x86_indirect_thunk_%V[thunk_target]\n"
+#define CALL_NOSPEC    __CS_PREFIX("%V[thunk_target]") \
+                       "call __x86_indirect_thunk_%V[thunk_target]\n"
 #else
 #define CALL_NOSPEC    "call *%[thunk_target]\n"
 #endif