[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)

[Borislav Petkov]

On Tue, Apr 02, 2024 at 10:14:07AM +0800, kernel test robot wrote:
> 
> 
> Hello,
> 
> kernel test robot noticed "WARNING:at_arch/x86/kernel/alternative.c:#apply_returns" on:
> 
> commit: 4535e1a4174c4111d92c5a9a21e542d232e0fcaa ("x86/bugs: Fix the SRSO mitigation on Zen3/4")
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master

---
From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
Date: Tue, 2 Apr 2024 16:05:49 +0200
Subject: [PATCH] x86/retpoline: Fix a missing return thunk warning

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.

Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202404020901.da75a60f-oliver.sang@xxxxxxxxx
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/all/202404020901.da75a60f-oliver.sang@xxxxxxxxx/
---
 arch/x86/lib/retpoline.S | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 02cde194a99e..440cc58986fa 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -228,7 +228,9 @@ 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)
 #define JMP_SRSO_UNTRAIN_RET "ud2"
 #endif /* CONFIG_MITIGATION_SRSO */
-- 
2.43.0

-- 
Regards/Gruss,
    Boris.

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