Re: [PATCH -v2] x86/retpoline: Ensure default return thunk isn't used at runtime
From: Borislav Petkov
Date: Mon Feb 12 2024 - 05:44:23 EST
On Wed, Feb 07, 2024 at 11:49:19AM -0800, Josh Poimboeuf wrote:
> LGTM, thanks!
Thanks, had to hoist up both THUNK macros into the header to make that
nuisance 32-bit build too :)
---
commit 4461438a8405e800f90e0e40409e5f3d07eed381 (HEAD -> refs/heads/tip-x86-bugs)
Author: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Date: Wed Jan 3 19:36:26 2024 +0100
x86/retpoline: Ensure default return thunk isn't used at runtime
Make sure the default return thunk is not used after all return
instructions have been patched by the alternatives because the default
return thunk is insufficient when it comes to mitigating Retbleed or
SRSO.
Fix based on an earlier version by David Kaplan <david.kaplan@xxxxxxx>.
[ bp: Fix the compilation error of warn_thunk_thunk being an invisible
symbol, hoist thunk macro into calling.h ]
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Co-developed-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/r/20231010171020.462211-4-david.kaplan@xxxxxxx
Link: https://lore.kernel.org/r/20240104132446.GEZZaxnrIgIyat0pqf@fat_crate.local
diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 39e069b68c6e..bd31b2534053 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -426,3 +426,63 @@ For 32-bit we have the following conventions - kernel is built with
.endm
#endif /* CONFIG_SMP */
+
+#ifdef CONFIG_X86_64
+
+/* 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
+
+#else /* CONFIG_X86_32 */
+
+/* put return address in eax (arg1) */
+.macro THUNK name, func, put_ret_addr_in_eax=0
+SYM_CODE_START_NOALIGN(\name)
+ pushl %eax
+ pushl %ecx
+ pushl %edx
+
+ .if \put_ret_addr_in_eax
+ /* Place EIP in the arg1 */
+ movl 3*4(%esp), %eax
+ .endif
+
+ call \func
+ popl %edx
+ popl %ecx
+ popl %eax
+ RET
+ _ASM_NOKPROBE(\name)
+SYM_CODE_END(\name)
+ .endm
+
+#endif
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_32.S b/arch/x86/entry/thunk_32.S
index 0103e103a657..da37f42f4549 100644
--- a/arch/x86/entry/thunk_32.S
+++ b/arch/x86/entry/thunk_32.S
@@ -4,33 +4,15 @@
* Copyright 2008 by Steven Rostedt, Red Hat, Inc
* (inspired by Andi Kleen's thunk_64.S)
*/
- #include <linux/export.h>
- #include <linux/linkage.h>
- #include <asm/asm.h>
- /* put return address in eax (arg1) */
- .macro THUNK name, func, put_ret_addr_in_eax=0
-SYM_CODE_START_NOALIGN(\name)
- pushl %eax
- pushl %ecx
- pushl %edx
+#include <linux/export.h>
+#include <linux/linkage.h>
+#include <asm/asm.h>
- .if \put_ret_addr_in_eax
- /* Place EIP in the arg1 */
- movl 3*4(%esp), %eax
- .endif
+#include "calling.h"
- call \func
- popl %edx
- popl %ecx
- popl %eax
- RET
- _ASM_NOKPROBE(\name)
-SYM_CODE_END(\name)
- .endm
-
- THUNK preempt_schedule_thunk, preempt_schedule
- THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
- EXPORT_SYMBOL(preempt_schedule_thunk)
- EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
+THUNK preempt_schedule_thunk, preempt_schedule
+THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
+EXPORT_SYMBOL(preempt_schedule_thunk)
+EXPORT_SYMBOL(preempt_schedule_notrace_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 2c0679ebe914..55754617eaee 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -357,6 +357,8 @@ extern void entry_ibpb(void);
extern void (*x86_return_thunk)(void);
+extern void __warn_thunk(void);
+
#ifdef CONFIG_MITIGATION_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 f2775417bda2..a78892b0f823 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2850,3 +2850,8 @@ 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)
+{
+ WARN_ONCE(1, "Unpatched return thunk in use. This should not happen!\n");
+}
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 0045153ba222..721b528da9ac 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -369,19 +369,16 @@ 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 ALTERNATIVE below adds a really loud warning to catch the case
+ * where the insufficient default return thunk ends up getting used for
+ * whatever reason like miscompilation or failure of
+ * objtool/alternatives/etc to patch all the return sites.
*/
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