Message-ID: <55e5b3f8-3e17-4962-af2f-75c98ccd414f@intel.com>
Date: Wed, 4 Jun 2025 12:46:20 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Kai Huang <kai.huang@...el.com>, peterz@...radead.org,
 tglx@...utronix.de, bp@...en8.de, mingo@...hat.com, hpa@...or.com,
 kirill.shutemov@...ux.intel.com
Cc: rick.p.edgecombe@...el.com, x86@...nel.org, samitolvanen@...gle.com,
 linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/virt/tdx: Add ENDBR for low level SEAMCALL assembly
 functions

On 6/3/25 17:38, Kai Huang wrote:
> Build warnings about missing ENDBR around making SEAMCALLs[*] were
> observed when using some randconfig[1] to build today's Linus's tree.

What actually caused it? CONFIG_CC_OPTIMIZE_FOR_SIZE?

> In the C code, the low level SEAMCALL assembly functions (__seamcall(),
> __seamcall_ret() and __seamcall_saved_ret()) are indirectly called via
> the common sc_retry() function:
> 
>     static inline u64 sc_retry(sc_func_t func, u64 fn,
> 		    	       struct tdx_module_args *args)
>     { ... }
> 
>     #define seamcall(_fn, _args) sc_retry(__seamcall, (_fn), (_args))
> 
> It turns out compilers may not always be smart enough to figure out how
> to call those assembly functions directly.

So, reading this, it didn't quite click into my brain that you were
referring to "direct call" and "indirect call" *instructions*.

I think I probably would have talked about "function pointers" because
that's what you do in C. A function pointer mostly means an indirect
call. But not always, like was intended here.

> The disassembly of the vmlinux built from the aforementioned config
> confirms that __seamcall*() are indirectly called:
> 
>     <sc_retry>:
> 			       ......
> 
>        4c 89 ee                mov    %r13,%rsi
>        4c 89 e7                mov    %r12,%rdi
>        e8 35 8c 7d 01          call   ffffffff82b3e220 <__pi___x86_indirect_thunk_rbp>
>        4c 39 f0                cmp    %r14,%rax
> 
> In this case ENDBR is needed at the beginning of __seamcall*().
> 
> Change SYM_FUNC_START() to SYM_TYPED_FUNC_START() for __seamcall*() to
> add ENDBR to them.

... and why is this? How do we know this is the correct fix? Show your
work, please.

> When the compiler can generate direct call for __seamcall*(), the
> additional ENDBR is safe since it has no impact to directly called
> functions.

Right. Direct calls are always OK. Indirect call targets need to be
handled specially.

> When kernel IBT was added to the kernel, initially the SYM_FUNC_START()
> had the ENDBR added in commit
> 
>   c4691712b546 ("x86/linkage: Add ENDBR to SYM_FUNC_START*()")
> 
> However when the commit
> 
>   582077c94052 ("x86/cfi: Clean up linkage")
> 
> removed the ENDBR from the SYM_FUNC_START() and added it to the
> SYM_TYPED_FUNC_START(), it didn't touch the SEAMCALL assembly.

I'm not sure why we need the history lesson here.

> [*] Aforementioned build warning:
> 
> vmlinux.o: warning: objtool: try_init_module_global+0x5d: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: read_sys_metadata_field+0x4a: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: do_global_key_config+0x36: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_phymem_page_reclaim+0x71: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_phymem_cache_wb+0x41: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_phymem_page_wbinvd_tdr+0x95: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdx_cpu_enable+0x7b: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: init_tdmr+0x59: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: config_tdx_module.constprop.0+0x19d: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_vp_addcx+0x91: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_vp_init+0x76: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_vp_wr+0x87: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_vp_rd+0x6d: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_vp_flush+0x4c: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_vp_create+0x85: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_mng_create+0x73: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_mem_page_aug+0xb4: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_mem_sept_add+0xb4: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_mem_page_add+0xce: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_mng_addcx+0x91: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_mem_page_remove+0x7e: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_mem_track+0x4c: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_mng_init+0x6d: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_mng_key_freeid+0x4c: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_mng_vpflushdone+0x4c: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_mr_finalize+0x4c: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_mr_extend+0x77: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_mng_rd+0x6d: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_mng_key_config+0x4c: relocation to !ENDBR: __seamcall+0x0
> vmlinux.o: warning: objtool: tdh_mem_range_block+0x7e: relocation to !ENDBR: __seamcall_ret+0x0
> vmlinux.o: warning: objtool: tdh_phymem_page_wbinvd_hkid+0x7d: relocation to !ENDBR: __seamcall+0x0

One or two of those would have sufficed...

> diff --git a/arch/x86/virt/vmx/tdx/seamcall.S b/arch/x86/virt/vmx/tdx/seamcall.S
> index 6854c52c374b..637226ae935d 100644
> --- a/arch/x86/virt/vmx/tdx/seamcall.S
> +++ b/arch/x86/virt/vmx/tdx/seamcall.S
> @@ -1,5 +1,6 @@
>  /* SPDX-License-Identifier: GPL-2.0 */
>  #include <linux/linkage.h>
> +#include <linux/cfi_types.h>
>  #include <asm/frame.h>
>  
>  #include "tdxcall.S"
> @@ -18,7 +19,7 @@
>   * Return (via RAX) TDX_SEAMCALL_VMFAILINVALID if the SEAMCALL itself
>   * fails, or the completion status of the SEAMCALL leaf function.
>   */
> -SYM_FUNC_START(__seamcall)
> +SYM_TYPED_FUNC_START(__seamcall)
>  	TDX_MODULE_CALL host=1
>  SYM_FUNC_END(__seamcall)
Personally, I'd add a _bit_ to these comments to mention that these
functions can be called via an indirect call.

Circling back around now that I've divined out what is happening here...
Is this even really about ENDBR at all? I thought that clang has some
CFI checking that's completely independent from CET/IBT and thus ENDBR.
Wouldn't this also break with that?

At the risk of giving a man a fish... Isn't this a much more coherent
changelog at about 1/4 the size?

--

Subject: x86/virt/tdx: Annotate TDX assembly to allow indirect calls

A TDX helper function (sc_retry()) passes around function pointers to
assembly functions. Normally, the compiler realizes that the function
pointer targets are completely static, can be resolved at compile time,
and generates direct call instructions.

But, other times (like when CONFIG_CC_OPTIMIZE_FOR_SIZE=y), the compiler
will instead generate indirect call instructions.

Indirect calls to assembly functions require special annotation so that
both hardware and software implementations of Control Flow Integrity
mechanisms can work correctly.

The TDX functions are declared as if they are only called directly (via
SYM_FUNC_START). Move them over to another macro (SYM_TYPED_FUNC_START)
which will annotate them as being called indirectly (see
include/linux/cfi_types.h).

This was found through randconfig testing, presumably setting
CONFIG_CC_OPTIMIZE_FOR_SIZE=1 when objtool spewed a bunch of these:

vmlinux.o: warning: objtool: tdh_mem_range_block+0x7e: relocation to
!ENDBR: __seamcall_ret+0x0

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