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SubjectRe: [PATCH] x86/retpoline: Fix a missing return thunk warning (was: Re: [linus:master] [x86/bugs] 4535e1a417: WARNING:at_arch/x86/kernel/alternative.c:#apply_returns)
On Wed, Apr 03, 2024 at 09:45:25AM -0700, Linus Torvalds wrote:
> On Wed, 3 Apr 2024 at 05:24, Borislav Petkov <bp@alien8.de> wrote:
> >
> > Subject: [PATCH] x86/retpoline: Fix a missing return thunk warning
>
> Thanks, applied directly,

Can you pls replace it with the below one?

There's more breakage:

https://bugzilla.kernel.org/show_bug.cgi?id=218679

and I think it needs an __EXPORT_THUNK too.

Yeah, I know. Damn ifdeffery. :-\

I've changed it:

https://bugzilla.kernel.org/attachment.cgi?id=306084&action=diff

and this should be the correct fix(tm). Famous last words.

Thx.

---
From ca5b34498f997cba9c24d450f7ccab4b37377560 Mon Sep 17 00:00:00 2001
From: "Borislav Petkov (AMD)" <bp@alien8.de>
Date: Tue, 2 Apr 2024 16:05:49 +0200
Subject: [PATCH] x86/retpoline: Do the necessary fixup to the Zen3/4 srso
return thunk for !SRSO

The srso_alias_untrain_ret() dummy thunk in the !CONFIG_MITIGATION_SRSO
case is there only for the altenative in CALL_UNTRAIN_RET to have
a symbol to resolve.

However, testing with kernels which don't have CONFIG_MITIGATION_SRSO
enabled, leads to the warning in patch_return() to fire:

missing return thunk: srso_alias_untrain_ret+0x0/0x10-0x0: eb 0e 66 66 2e
WARNING: CPU: 0 PID: 0 at arch/x86/kernel/alternative.c:826 apply_returns (arch/x86/kernel/alternative.c:826

Put in a plain "ret" there so that gcc doesn't put a return thunk in
in its place which special and gets checked.

In addition:

ERROR: modpost: "srso_alias_untrain_ret" [arch/x86/kvm/kvm-amd.ko] undefined!
make[2]: *** [scripts/Makefile.modpost:145: Module.symvers] Chyba 1
make[1]: *** [/usr/src/linux-6.8.3/Makefile:1873: modpost] Chyba 2
make: *** [Makefile:240: __sub-make] Chyba 2

since !SRSO builds would use the dummy return thunk as reported by
petr.pisar@atlas.cz, https://bugzilla.kernel.org/show_bug.cgi?id=218679.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202404020901.da75a60f-oliver.sang@intel.com
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/all/202404020901.da75a60f-oliver.sang@intel.com/
---
arch/x86/lib/retpoline.S | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 02cde194a99e..0795b3464058 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -228,8 +228,11 @@ SYM_CODE_END(srso_return_thunk)
#else /* !CONFIG_MITIGATION_SRSO */
/* Dummy for the alternative in CALL_UNTRAIN_RET. */
SYM_CODE_START(srso_alias_untrain_ret)
- RET
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
SYM_FUNC_END(srso_alias_untrain_ret)
+__EXPORT_THUNK(srso_alias_untrain_ret)
#define JMP_SRSO_UNTRAIN_RET "ud2"
#endif /* CONFIG_MITIGATION_SRSO */

--
2.43.0

--
Regards/Gruss,
Boris.

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

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