Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't used at runtime

From: Borislav Petkov
Date: Wed Jan 03 2024 - 13:47:31 EST

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@xxxxxxxxxx. **\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.

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