Re: [PATCH 2/2] x86/retpoline,kprobes: Avoid treating rethunk as an indirect jump

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On Wed,  5 Jul 2023 10:15:47 +0200
Petr Pavlu <petr.pavlu@xxxxxxxx> wrote:

> Functions can_optimize() and insn_is_indirect_jump() consider jumps to
> the range [__indirect_thunk_start, __indirect_thunk_end] as indirect
> jumps and prevent use of optprobes in functions containing them.
> 
> Linker script arch/x86/kernel/vmlinux.lds.S places into this range also
> the special section .text.__x86.return_thunk which contains the return
> thunk. It causes that machines which use the return thunk as
> a mitigation and don't have it patched by any alternative then end up
> not being able to use optprobes in any regular function.

Ah, I got it. So with retpoline, the 'ret' instruction is replaced by
'jmp __x86_return_thunk' and the "__x86_return_thunk" is also listed in
the __indirect_thunk_start/end.
Good catch!

And I think Peter's suggestion is simpler and easier to understand.
Can you update this?

Thank you,

> 
> The return thunk doesn't need to be treated as an indirect jump from the
> perspective of insn_is_indirect_jump(). It returns to a caller and
> cannot land into an optprobe jump operand which is the purpose of the
> insn_is_indirect_jump() check.
> 
> Fix the problem by defining the symbols __indirect_thunk_start and
> __indirect_thunk_end directly in arch/x86/lib/retpoline.S. This is
> possible because commit 9bc0bb50727c ("objtool/x86: Rewrite retpoline
> thunk calls") made all indirect thunks present in a single section.
>
> Fixes: 0b53c374b9ef ("x86/retpoline: Use -mfunction-return")
> Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
> ---
>  arch/x86/kernel/vmlinux.lds.S | 2 --
>  arch/x86/lib/retpoline.S      | 4 ++++
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index a4cd04c458df..dd5b0a68cf84 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -133,9 +133,7 @@ SECTIONS
>  		KPROBES_TEXT
>  		SOFTIRQENTRY_TEXT
>  #ifdef CONFIG_RETPOLINE
> -		__indirect_thunk_start = .;
>  		*(.text..__x86.*)
> -		__indirect_thunk_end = .;
>  #endif
>  		STATIC_CALL_TEXT
>  
> diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
> index 3bea96341d00..f45a3e7f776f 100644
> --- a/arch/x86/lib/retpoline.S
> +++ b/arch/x86/lib/retpoline.S
> @@ -14,6 +14,7 @@
>  
>  	.section .text..__x86.indirect_thunk
>  
> +SYM_ENTRY(__indirect_thunk_start, SYM_L_GLOBAL, SYM_A_NONE)
>  
>  .macro POLINE reg
>  	ANNOTATE_INTRA_FUNCTION_CALL
> @@ -125,6 +126,9 @@ SYM_CODE_END(__x86_indirect_jump_thunk_array)
>  #include <asm/GEN-for-each-reg.h>
>  #undef GEN
>  #endif
> +
> +SYM_ENTRY(__indirect_thunk_end, SYM_L_GLOBAL, SYM_A_NONE)
> +
>  /*
>   * This function name is magical and is used by -mfunction-return=thunk-extern
>   * for the compiler to generate JMPs to it.
> -- 
> 2.35.3
> 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>