2023-08-09 08:54:41

by Peter Zijlstra

Subject: [RFC][PATCH 01/17] x86/alternative: Unconditional custom return thunk

There is infrastructure to rewrite return thunks to point to any
random thunk one desires, unwrap that from CALL_THUNKS, which up to
now was the sole user of that.

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
---
arch/x86/include/asm/nospec-branch.h | 4 ----
arch/x86/kernel/alternative.c | 2 --
2 files changed, 6 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -347,11 +347,7 @@ extern void srso_untrain_ret(void);
extern void srso_untrain_ret_alias(void);
extern void entry_ibpb(void);

-#ifdef CONFIG_CALL_THUNKS
extern void (*x86_return_thunk)(void);
-#else
-#define x86_return_thunk (&__x86_return_thunk)
-#endif

#ifdef CONFIG_CALL_DEPTH_TRACKING
extern void __x86_return_skl(void);
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -698,9 +698,7 @@ void __init_or_module noinline apply_ret

#ifdef CONFIG_RETHUNK

-#ifdef CONFIG_CALL_THUNKS
void (*x86_return_thunk)(void) __ro_after_init = &__x86_return_thunk;
-#endif

/*
* Rewrite the compiler generated return thunk tail-calls.

2023-08-09 09:59:23

by Nikolay Borisov

Subject: Re: [RFC][PATCH 01/17] x86/alternative: Unconditional custom return thunk

On 9.08.23 г. 10:12 ч., Peter Zijlstra wrote:
> There is infrastructure to rewrite return thunks to point to any
> random thunk one desires, unwrap that from CALL_THUNKS, which up to
> now was the sole user of that.
>
> Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
> ---
> arch/x86/include/asm/nospec-branch.h | 4 ----
> arch/x86/kernel/alternative.c | 2 --
> 2 files changed, 6 deletions(-)
>
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -347,11 +347,7 @@ extern void srso_untrain_ret(void);
> extern void srso_untrain_ret_alias(void);
> extern void entry_ibpb(void);
>
> -#ifdef CONFIG_CALL_THUNKS
> extern void (*x86_return_thunk)(void);
> -#else
> -#define x86_return_thunk (&__x86_return_thunk)
> -#endif
>
> #ifdef CONFIG_CALL_DEPTH_TRACKING
> extern void __x86_return_skl(void);
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -698,9 +698,7 @@ void __init_or_module noinline apply_ret
>
> #ifdef CONFIG_RETHUNK
>
> -#ifdef CONFIG_CALL_THUNKS
> void (*x86_return_thunk)(void) __ro_after_init = &__x86_return_thunk;
> -#endif
>
> /*
> * Rewrite the compiler generated return thunk tail-calls.
>
>


Reviewed-by: Nikolay Borisov <[email protected]>

2023-08-10 13:05:55

by Borislav Petkov

Subject: Re: [RFC][PATCH 01/17] x86/alternative: Unconditional custom return thunk

On Wed, Aug 09, 2023 at 09:12:19AM +0200, Peter Zijlstra wrote:

Please add a verb to the subject:

"Make the custom return thunk unconditional"

or so.

> There is infrastructure to rewrite return thunks to point to any
> random thunk one desires, unwrap that from CALL_THUNKS, which up to
> now was the sole user of that.

And yes, provided we can do the thing in the next patch which makes
sense to me, ack.

--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette