[PATCH] x86/retbleed: Add __x86_return_thunk alignment checks

[Borislav Petkov]

From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>

Add a linker assertion and compute the 0xcc padding dynamically so that
__x86_return_thunk is always cacheline-aligned. Leave the SYM_START()
macro in as the untraining doesn't need ENDBR annotations anyway.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
---
 arch/x86/kernel/vmlinux.lds.S | 4 ++++
 arch/x86/lib/retpoline.S      | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 25f155205770..03c885d3640f 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -508,4 +508,8 @@ INIT_PER_CPU(irq_stack_backing_store);
            "fixed_percpu_data is not at start of per-cpu area");
 #endif
 
+#ifdef CONFIG_RETHUNK
+. = ASSERT((__x86_return_thunk & 0x3f) == 0, "__x86_return_thunk not cacheline-aligned");
+#endif
+
 #endif /* CONFIG_X86_64 */
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index b3b1e376dce8..3fd066d42ec0 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -143,7 +143,7 @@ SYM_CODE_END(__x86_indirect_jump_thunk_array)
  *    from re-poisioning the BTB prediction.
  */
 	.align 64
-	.skip 63, 0xcc
+	.skip 64 - (__x86_return_thunk - zen_untrain_ret), 0xcc
 SYM_START(zen_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
 	ANNOTATE_NOENDBR
 	/*
-- 
2.35.1