[PATCH] x86/kvm/tdx: Save %rbp in TDX_MODULE_CALL

[Juergen Gross <jgross@suse.com>]

While testing TDX host support patches, a crash of the host has been
observed a few instructions after doing a seamcall. Reason was a
clobbered %rbp (set to 0), which occurred in spite of the TDX module
offering the feature NOT to modify %rbp across TDX module calls.

In order not having to build the host kernel with CONFIG_FRAME_POINTER,
save %rbp across a seamcall/tdcall.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/virt/vmx/tdx/tdxcall.S | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
index 016a2a1ec1d6..68728acf0d3a 100644
--- a/arch/x86/virt/vmx/tdx/tdxcall.S
+++ b/arch/x86/virt/vmx/tdx/tdxcall.S
@@ -44,6 +44,10 @@
  */
 .macro TDX_MODULE_CALL host:req ret=0 saved=0
 	FRAME_BEGIN
+#ifndef CONFIG_FRAME_POINTER
+	/* Buggy firmware sometimes clobbers %rbp, so save it. */
+	pushq	%rbp
+#endif
 
 	/* Move Leaf ID to RAX */
 	mov %rdi, %rax
@@ -187,6 +191,9 @@
 	popq	%rbx
 .endif	/* \saved */
 
+#ifndef CONFIG_FRAME_POINTER
+	popq	%rbp
+#endif
 	FRAME_END
 	RET
 
-- 
2.35.3