linux-kernel - Re: [PATCH -v2] x86/retpoline: Ensure default return thunk isn't used at runtime

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Date: Tue, 16 Apr 2024 11:27:20 +0200
From: Borislav Petkov <bp@...en8.de>
To: Klara Modin <klarasmodin@...il.com>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>,
	"Kaplan, David" <David.Kaplan@....com>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-tip-commits@...r.kernel.org" <linux-tip-commits@...r.kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	"x86@...nel.org" <x86@...nel.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH -v2] x86/retpoline: Ensure default return thunk isn't
 used at runtime

On Wed, Apr 03, 2024 at 07:10:17PM +0200, Klara Modin wrote:
> With this patch/commit, one of my machines (older P4 Xeon, 32-bit only)
> hangs on boot with CONFIG_RETHUNK=y / CONFIG_MITIGATION_RETHUNK=y.

Ok, this should fix it:

---
From: "Borislav Petkov (AMD)" <bp@...en8.de>
Date: Mon, 15 Apr 2024 18:15:43 +0200
Subject: [PATCH] x86/retpolines: Enable the default thunk warning only on relevant configs

The using-default-thunk warning check makes sense only with
configurations which actually enable the special return thunks.

Otherwise, it fires on unrelated 32-bit configs on which the special
return thunks won't even work (they're 64-bit only) and, what is more,
those configs even go off into the weeds when booting in the
alternatives patching code, leading to a dead machine.

Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/r/78e0d19c-b77a-4169-a80f-2eef91f4a1d6@gmail.com
Link: https://lore.kernel.org/r/20240413024956.488d474e@yea
---
 arch/x86/lib/retpoline.S | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index e674ccf720b9..391059b2c6fb 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -382,8 +382,15 @@ SYM_FUNC_END(call_depth_return_thunk)
 SYM_CODE_START(__x86_return_thunk)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR
+#if defined(CONFIG_MITIGATION_UNRET_ENTRY) || \
+    defined(CONFIG_MITIGATION_SRSO) || \
+    defined(CONFIG_MITIGATION_CALL_DEPTH_TRACKING)
 	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
 		   "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
+#else
+	ANNOTATE_UNRET_SAFE
+	ret
+#endif
 	int3
 SYM_CODE_END(__x86_return_thunk)
 EXPORT_SYMBOL(__x86_return_thunk)
-- 
2.43.0

-- 
Regards/Gruss,
    Boris.

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