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Date: Wed, 3 Jan 2024 19:46:56 +0100
From: Borislav Petkov <bp@alien8.de>
To: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: "Kaplan, David" <David.Kaplan@amd.com>, Ingo Molnar <mingo@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-tip-commits@vger.kernel.org" <linux-tip-commits@vger.kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	"x86@kernel.org" <x86@kernel.org>,
	David Howells <dhowells@redhat.com>
Subject: Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't
 used at runtime
Message-ID: <20240103184656.GEZZWroPmHLJuP6y5H@fat_crate.local>
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On Tue, Oct 24, 2023 at 10:19:13PM +0200, Borislav Petkov wrote:
> On Thu, Oct 19, 2023 at 08:20:51AM -0700, Josh Poimboeuf wrote:
> > GCC doesn't read asm.  Even if it did that wouldn't fix things for
> > callers of custom-ABI return-thunk-using functions.
> > 
> > The below seems to work.
> 
> Right, I guess we can do something like that. Linker is not happy here
> about that symbol, tho:
> 
> ld: arch/x86/lib/retpoline.o:(.altinstr_replacement+0x95): undefined reference to `warn_thunk_thunk'
> make[2]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 1
> make[1]: *** [/mnt/kernel/kernel/5th/linux/Makefile:1165: vmlinux] Error 2
> make: *** [Makefile:234: __sub-make] Error 2

Ok, back to playing with this. A fix below along with a beefed up
warning message.

If only I can remember now how we did trigger the warning in the first
place in order to test it...

---

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index e59d3073e7cf..a4679e8f30ad 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -426,3 +426,36 @@ For 32-bit we have the following conventions - kernel is built with
 .endm
 
 #endif /* CONFIG_SMP */
+
+/* rdi:	arg1 ... normal C conventions. rax is saved/restored. */
+.macro THUNK name, func
+SYM_FUNC_START(\name)
+	pushq %rbp
+	movq %rsp, %rbp
+
+	pushq %rdi
+	pushq %rsi
+	pushq %rdx
+	pushq %rcx
+	pushq %rax
+	pushq %r8
+	pushq %r9
+	pushq %r10
+	pushq %r11
+
+	call \func
+
+	popq %r11
+	popq %r10
+	popq %r9
+	popq %r8
+	popq %rax
+	popq %rcx
+	popq %rdx
+	popq %rsi
+	popq %rdi
+	popq %rbp
+	RET
+SYM_FUNC_END(\name)
+	_ASM_NOKPROBE(\name)
+.endm
diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
index 8c8d38f0cb1d..582731f74dc8 100644
--- a/arch/x86/entry/entry.S
+++ b/arch/x86/entry/entry.S
@@ -7,6 +7,8 @@
 #include <linux/linkage.h>
 #include <asm/msr-index.h>
 
+#include "calling.h"
+
 .pushsection .noinstr.text, "ax"
 
 SYM_FUNC_START(entry_ibpb)
@@ -20,3 +22,5 @@ SYM_FUNC_END(entry_ibpb)
 EXPORT_SYMBOL_GPL(entry_ibpb);
 
 .popsection
+
+THUNK warn_thunk_thunk, __warn_thunk
diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S
index 416b400f39db..119ebdc3d362 100644
--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -9,39 +9,6 @@
 #include "calling.h"
 #include <asm/asm.h>
 
-	/* rdi:	arg1 ... normal C conventions. rax is saved/restored. */
-	.macro THUNK name, func
-SYM_FUNC_START(\name)
-	pushq %rbp
-	movq %rsp, %rbp
-
-	pushq %rdi
-	pushq %rsi
-	pushq %rdx
-	pushq %rcx
-	pushq %rax
-	pushq %r8
-	pushq %r9
-	pushq %r10
-	pushq %r11
-
-	call \func
-
-	popq %r11
-	popq %r10
-	popq %r9
-	popq %r8
-	popq %rax
-	popq %rcx
-	popq %rdx
-	popq %rsi
-	popq %rdi
-	popq %rbp
-	RET
-SYM_FUNC_END(\name)
-	_ASM_NOKPROBE(\name)
-	.endm
-
 THUNK preempt_schedule_thunk, preempt_schedule
 THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
 EXPORT_SYMBOL(preempt_schedule_thunk)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 691ff1ef701b..64b175f03cdb 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -348,6 +348,8 @@ extern void entry_ibpb(void);
 
 extern void (*x86_return_thunk)(void);
 
+extern void __warn_thunk(void);
+
 #ifdef CONFIG_CALL_DEPTH_TRACKING
 extern void call_depth_return_thunk(void);
 
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index bb0ab8466b91..00a6d024ddf7 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2849,3 +2849,19 @@ ssize_t cpu_show_gds(struct device *dev, struct device_attribute *attr, char *bu
 	return cpu_show_common(dev, attr, buf, X86_BUG_GDS);
 }
 #endif
+
+void __warn_thunk(void)
+{
+	pr_warn_once("\n");
+	pr_warn_once("**********************************************************\n");
+	pr_warn_once("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
+	pr_warn_once("**                                                      **\n");
+	pr_warn_once("**   unpatched return thunk in use. This should not     **\n");
+	pr_warn_once("**   on a production kernel. Please report this to      **\n");
+	pr_warn_once("**   x86@kernel.org.                                    **\n");
+	pr_warn_once("**                                                      **\n");
+	pr_warn_once("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
+	pr_warn_once("**********************************************************\n");
+
+	dump_stack();
+}
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 7b2589877d06..dfcb7d64d05a 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -369,19 +369,14 @@ SYM_FUNC_END(call_depth_return_thunk)
  * 'JMP __x86_return_thunk' sites are changed to something else by
  * apply_returns().
  *
- * This should be converted eventually to call a warning function which
- * should scream loudly when the default return thunk is called after
- * alternatives have been applied.
- *
- * That warning function cannot BUG() because the bug splat cannot be
- * displayed in all possible configurations, leading to users not really
- * knowing why the machine froze.
+ * The RET is replaced with a WARN_ONCE() to ensure it is never used at
+ * runtime.  Alternative instructions are applied after apply_returns().
  */
 SYM_CODE_START(__x86_return_thunk)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR
-	ANNOTATE_UNRET_SAFE
-	ret
+	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
+		   "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
 	int3
 SYM_CODE_END(__x86_return_thunk)
 EXPORT_SYMBOL(__x86_return_thunk)

-- 
Regards/Gruss,
    Boris.

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