[RFC PATCH] generic_entry: Add stackleak support

From: Guo Ren <redacted>

Make generic_entry supports basic STACKLEAK, and no arch custom
code is needed.

Signed-off-by: Guo Ren <redacted>
Signed-off-by: Guo Ren <guoren@kernel.org>
---
 drivers/firmware/efi/libstub/Makefile | 4 +++-
 include/linux/stackleak.h             | 3 +++
 kernel/entry/common.c                 | 5 +++++
 security/Kconfig.hardening            | 2 +-
 4 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index d0537573501e..bb6ad37a9690 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile

@@ -19,7 +19,7 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ \
 # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
 # disable the stackleak plugin
 cflags-$(CONFIG_ARM64)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
-				   -fpie $(DISABLE_STACKLEAK_PLUGIN) \
+				   -fpie \
 				   $(call cc-option,-mbranch-protection=none)
 cflags-$(CONFIG_ARM)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
 				   -fno-builtin -fpic \

@@ -27,6 +27,8 @@ cflags-$(CONFIG_ARM)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
 cflags-$(CONFIG_RISCV)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
 				   -fpic
 
+cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK) += $(DISABLE_STACKLEAK_PLUGIN)
+
 cflags-$(CONFIG_EFI_GENERIC_STUB) += -I$(srctree)/scripts/dtc/libfdt
 
 KBUILD_CFLAGS			:= $(cflags-y) -Os -DDISABLE_BRANCH_PROFILING \

diff --git a/include/linux/stackleak.h b/include/linux/stackleak.h
index c36e7a3b45e7..9890802a5868 100644
--- a/include/linux/stackleak.h
+++ b/include/linux/stackleak.h

@@ -76,8 +76,11 @@ static inline void stackleak_task_init(struct task_struct *t)
 # endif
 }
 
+void noinstr stackleak_erase(void);
+
 #else /* !CONFIG_GCC_PLUGIN_STACKLEAK */
 static inline void stackleak_task_init(struct task_struct *t) { }
+static inline void stackleak_erase(void) {}
 #endif
 
 #endif

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 063068a9ea9b..6acb1d6a1396 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c

@@ -8,6 +8,7 @@
 #include <linux/livepatch.h>
 #include <linux/audit.h>
 #include <linux/tick.h>
+#include <linux/stackleak.h>
 
 #include "common.h"
 

@@ -194,6 +195,10 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)
 
 	lockdep_assert_irqs_disabled();
 
+#ifndef CONFIG_HAVE_ARCH_STACKLEAK
+	stackleak_erase();
+#endif
+
 	/* Flush pending rcuog wakeup before the last need_resched() check */
 	tick_nohz_user_enter_prepare();
 

diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index bd2aabb2c60f..3329482beb8d 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening

@@ -152,7 +152,7 @@ config GCC_PLUGIN_STRUCTLEAK_VERBOSE
 config GCC_PLUGIN_STACKLEAK
 	bool "Poison kernel stack before returning from syscalls"
 	depends on GCC_PLUGINS
-	depends on HAVE_ARCH_STACKLEAK
+	depends on HAVE_ARCH_STACKLEAK || GENERIC_ENTRY
 	help
 	  This option makes the kernel erase the kernel stack before
 	  returning from system calls. This has the effect of leaving

-- 
2.36.1

On Tue, Sep 06, 2022 at 09:48:09PM -0400, guoren@kernel.org wrote:

From: Guo Ren <redacted>

Make generic_entry supports basic STACKLEAK, and no arch custom
code is needed.

IIUC, this change is going to cause redundant work to be done on x86 (since it
erases the stack in its entry assembly). It also means any arch relying upon
this will not clear some stack contents that could be cleared from assembly
later in the return to userspace path, after the C entry code stack frames are
gone.

I assume you're adding this so that riscv can use stackleak? WHy can't it call
stackleak_erase*() later in the return-to-userspce path?

Huh; is there a latent bug here where x86's EFI stub is instrumented with
stackleak?

Thanks,
Mark.

On Sat, Sep 17, 2022 at 10:13 PM Mark Rutland [off-list ref] wrote:

On Tue, Sep 06, 2022 at 09:48:09PM -0400, guoren@kernel.org wrote:

quoted

From: Guo Ren <redacted>

Make generic_entry supports basic STACKLEAK, and no arch custom
code is needed.

IIUC, this change is going to cause redundant work to be done on x86 (since it
erases the stack in its entry assembly). It also means any arch relying upon
this will not clear some stack contents that could be cleared from assembly
later in the return to userspace path, after the C entry code stack frames are
gone.

Yeah, it's a point, Thx.

I assume you're adding this so that riscv can use stackleak? WHy can't it call
stackleak_erase*() later in the return-to-userspce path?

Okay, I would move stackleak_erase back to riscv code and call it in
ret_from_exception of entry.S.

diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 426529b84db0..fe5f67c3ea2c 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S

@@ -130,7 +130,6 @@ END(handle_exception)
 ENTRY(ret_from_exception)
        REG_L s0, PT_STATUS(sp)

-       csrc CSR_STATUS, SR_IE
 #ifdef CONFIG_RISCV_M_MODE
        /* the MPP value is too large to be used as an immediate arg for addi */
        li t0, SR_MPP

@@ -139,6 +138,8 @@ ENTRY(ret_from_exception)
        andi s0, s0, SR_SPP
 #endif
        bnez s0, 1f
+       call stackleak_erase
+       csrc CSR_STATUS, SR_IE

        /* Save unwound kernel stack pointer in thread_info */
        addi s0, sp, PT_SIZE_ON_STACK

@@ -150,6 +151,7 @@ ENTRY(ret_from_exception)
         */
        csrw CSR_SCRATCH, tp
 1:
+       csrc CSR_STATUS, SR_IE

quoted

Signed-off-by: Guo Ren <redacted>
Signed-off-by: Guo Ren <guoren@kernel.org>
---
 drivers/firmware/efi/libstub/Makefile | 4 +++-
 include/linux/stackleak.h             | 3 +++
 kernel/entry/common.c                 | 5 +++++
 security/Kconfig.hardening            | 2 +-
 4 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index d0537573501e..bb6ad37a9690 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile

@@ -19,7 +19,7 @@ cflags-$(CONFIG_X86)                += -m$(BITS) -D__KERNEL__ \
 # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
 # disable the stackleak plugin
 cflags-$(CONFIG_ARM64)               := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
-                                -fpie $(DISABLE_STACKLEAK_PLUGIN) \
+                                -fpie \
                                 $(call cc-option,-mbranch-protection=none)
 cflags-$(CONFIG_ARM)         := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
                                 -fno-builtin -fpic \

@@ -27,6 +27,8 @@ cflags-$(CONFIG_ARM)                := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
 cflags-$(CONFIG_RISCV)               := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
                                 -fpic

+cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK) += $(DISABLE_STACKLEAK_PLUGIN)
+
 cflags-$(CONFIG_EFI_GENERIC_STUB) += -I$(srctree)/scripts/dtc/libfdt

 KBUILD_CFLAGS                        := $(cflags-y) -Os -DDISABLE_BRANCH_PROFILING \

Huh; is there a latent bug here where x86's EFI stub is instrumented with
stackleak?

Oops, I forgot x86. Thank you for reminding.

Thanks,
Mark.

quoted

diff --git a/include/linux/stackleak.h b/include/linux/stackleak.h
index c36e7a3b45e7..9890802a5868 100644
--- a/include/linux/stackleak.h
+++ b/include/linux/stackleak.h

@@ -76,8 +76,11 @@ static inline void stackleak_task_init(struct task_struct *t)
 # endif
 }

+void noinstr stackleak_erase(void);
+
 #else /* !CONFIG_GCC_PLUGIN_STACKLEAK */
 static inline void stackleak_task_init(struct task_struct *t) { }
+static inline void stackleak_erase(void) {}
 #endif

 #endif

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 063068a9ea9b..6acb1d6a1396 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c

@@ -8,6 +8,7 @@
 #include <linux/livepatch.h>
 #include <linux/audit.h>
 #include <linux/tick.h>
+#include <linux/stackleak.h>

 #include "common.h"

@@ -194,6 +195,10 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)

      lockdep_assert_irqs_disabled();

+#ifndef CONFIG_HAVE_ARCH_STACKLEAK
+     stackleak_erase();
+#endif
+
      /* Flush pending rcuog wakeup before the last need_resched() check */
      tick_nohz_user_enter_prepare();

diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index bd2aabb2c60f..3329482beb8d 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening

@@ -152,7 +152,7 @@ config GCC_PLUGIN_STRUCTLEAK_VERBOSE
 config GCC_PLUGIN_STACKLEAK
      bool "Poison kernel stack before returning from syscalls"
      depends on GCC_PLUGINS
-     depends on HAVE_ARCH_STACKLEAK
+     depends on HAVE_ARCH_STACKLEAK || GENERIC_ENTRY
      help
        This option makes the kernel erase the kernel stack before
        returning from system calls. This has the effect of leaving
--

2.36.1

--
Best Regards
 Guo Ren