[PATCH v4 1/4] compiler_types: Introduce the Clang __preserve_most function attribute

[PATCH v4 1/4] compiler_types: Introduce the Clang __preserve_most function attribute

[Marco Elver]

[1]: "On X86-64 and AArch64 targets, this attribute changes the calling
convention of a function. The preserve_most calling convention attempts
to make the code in the caller as unintrusive as possible. This
convention behaves identically to the C calling convention on how
arguments and return values are passed, but it uses a different set of
caller/callee-saved registers. This alleviates the burden of saving and
recovering a large register set before and after the call in the caller.
If the arguments are passed in callee-saved registers, then they will be
preserved by the callee across the call. This doesn't apply for values
returned in callee-saved registers.

 * On X86-64 the callee preserves all general purpose registers, except
   for R11. R11 can be used as a scratch register. Floating-point
   registers (XMMs/YMMs) are not preserved and need to be saved by the
   caller.

 * On AArch64 the callee preserve all general purpose registers, except
   x0-X8 and X16-X18."

[1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most

Introduce the attribute to compiler_types.h as __preserve_most.

Use of this attribute results in better code generation for calls to
very rarely called functions, such as error-reporting functions, or
rarely executed slow paths.

Beware that the attribute conflicts with instrumentation calls inserted
on function entry which do not use __preserve_most themselves. Notably,
function tracing which assumes the normal C calling convention for the
given architecture.  Where the attribute is supported, __preserve_most
will imply notrace. It is recommended to restrict use of the attribute
to functions that should or already disable tracing.

Note: The additional preprocessor check against architecture should not
be necessary if __has_attribute() only returns true where supported;
also see https://github.com/ClangBuiltLinux/linux/issues/1908. But until
__has_attribute() does the right thing, we also guard by known-supported
architectures to avoid build warnings on other architectures.

The attribute may be supported by a future GCC version (see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899).

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
---
v4:
* Guard attribute based on known-supported architectures to avoid
  compiler warnings about the attribute being ignored.

v3:
* Quote more from LLVM documentation about which registers are
  callee/caller with preserve_most.
* Code comment to restrict use where tracing is meant to be disabled.

v2:
* Imply notrace, to avoid any conflicts with tracing which is inserted
  on function entry. See added comments.
---
 include/linux/compiler_types.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 547ea1ff806e..c523c6683789 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -106,6 +106,34 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 #define __cold
 #endif
 
+/*
+ * On x86-64 and arm64 targets, __preserve_most changes the calling convention
+ * of a function to make the code in the caller as unintrusive as possible. This
+ * convention behaves identically to the C calling convention on how arguments
+ * and return values are passed, but uses a different set of caller- and callee-
+ * saved registers.
+ *
+ * The purpose is to alleviates the burden of saving and recovering a large
+ * register set before and after the call in the caller.  This is beneficial for
+ * rarely taken slow paths, such as error-reporting functions that may be called
+ * from hot paths.
+ *
+ * Note: This may conflict with instrumentation inserted on function entry which
+ * does not use __preserve_most or equivalent convention (if in assembly). Since
+ * function tracing assumes the normal C calling convention, where the attribute
+ * is supported, __preserve_most implies notrace.  It is recommended to restrict
+ * use of the attribute to functions that should or already disable tracing.
+ *
+ * Optional: not supported by gcc.
+ *
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#preserve-most
+ */
+#if __has_attribute(__preserve_most__) && (defined(CONFIG_X86_64) || defined(CONFIG_ARM64))
+# define __preserve_most notrace __attribute__((__preserve_most__))
+#else
+# define __preserve_most
+#endif
+
 /* Builtins */
 
 /*
-- 
2.41.0.694.ge786442a9b-goog

[PATCH v4 2/4] list_debug: Introduce inline wrappers for debug checks

[Marco Elver]

Turn the list debug checking functions __list_*_valid() into inline
functions that wrap the out-of-line functions. Care is taken to ensure
the inline wrappers are always inlined, so that additional compiler
instrumentation (such as sanitizers) does not result in redundant
outlining.

This change is preparation for performing checks in the inline wrappers.

No functional change intended.

Signed-off-by: Marco Elver <elver@google.com>
---
v3:
* Rename ___list_*_valid() to __list_*_valid_or_report().
* Some documentation.
---
 arch/arm64/kvm/hyp/nvhe/list_debug.c |  6 ++---
 include/linux/list.h                 | 37 +++++++++++++++++++++++++---
 lib/list_debug.c                     | 11 ++++-----
 3 files changed, 41 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/list_debug.c b/arch/arm64/kvm/hyp/nvhe/list_debug.c
index d68abd7ea124..16266a939a4c 100644
--- a/arch/arm64/kvm/hyp/nvhe/list_debug.c
+++ b/arch/arm64/kvm/hyp/nvhe/list_debug.c
@@ -26,8 +26,8 @@ static inline __must_check bool nvhe_check_data_corruption(bool v)
 
 /* The predicates checked here are taken from lib/list_debug.c. */
 
-bool __list_add_valid(struct list_head *new, struct list_head *prev,
-		      struct list_head *next)
+bool __list_add_valid_or_report(struct list_head *new, struct list_head *prev,
+				struct list_head *next)
 {
 	if (NVHE_CHECK_DATA_CORRUPTION(next->prev != prev) ||
 	    NVHE_CHECK_DATA_CORRUPTION(prev->next != next) ||
@@ -37,7 +37,7 @@ bool __list_add_valid(struct list_head *new, struct list_head *prev,
 	return true;
 }
 
-bool __list_del_entry_valid(struct list_head *entry)
+bool __list_del_entry_valid_or_report(struct list_head *entry)
 {
 	struct list_head *prev, *next;
 
diff --git a/include/linux/list.h b/include/linux/list.h
index f10344dbad4d..130c6a1bb45c 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -39,10 +39,39 @@ static inline void INIT_LIST_HEAD(struct list_head *list)
 }
 
 #ifdef CONFIG_DEBUG_LIST
-extern bool __list_add_valid(struct list_head *new,
-			      struct list_head *prev,
-			      struct list_head *next);
-extern bool __list_del_entry_valid(struct list_head *entry);
+/*
+ * Performs the full set of list corruption checks before __list_add().
+ * On list corruption reports a warning, and returns false.
+ */
+extern bool __list_add_valid_or_report(struct list_head *new,
+				       struct list_head *prev,
+				       struct list_head *next);
+
+/*
+ * Performs list corruption checks before __list_add(). Returns false if a
+ * corruption is detected, true otherwise.
+ */
+static __always_inline bool __list_add_valid(struct list_head *new,
+					     struct list_head *prev,
+					     struct list_head *next)
+{
+	return __list_add_valid_or_report(new, prev, next);
+}
+
+/*
+ * Performs the full set of list corruption checks before __list_del_entry().
+ * On list corruption reports a warning, and returns false.
+ */
+extern bool __list_del_entry_valid_or_report(struct list_head *entry);
+
+/*
+ * Performs list corruption checks before __list_del_entry(). Returns false if a
+ * corruption is detected, true otherwise.
+ */
+static __always_inline bool __list_del_entry_valid(struct list_head *entry)
+{
+	return __list_del_entry_valid_or_report(entry);
+}
 #else
 static inline bool __list_add_valid(struct list_head *new,
 				struct list_head *prev,
diff --git a/lib/list_debug.c b/lib/list_debug.c
index d98d43f80958..2def33b1491f 100644
--- a/lib/list_debug.c
+++ b/lib/list_debug.c
@@ -17,8 +17,8 @@
  * attempt).
  */
 
-bool __list_add_valid(struct list_head *new, struct list_head *prev,
-		      struct list_head *next)
+bool __list_add_valid_or_report(struct list_head *new, struct list_head *prev,
+				struct list_head *next)
 {
 	if (CHECK_DATA_CORRUPTION(prev == NULL,
 			"list_add corruption. prev is NULL.\n") ||
@@ -37,9 +37,9 @@ bool __list_add_valid(struct list_head *new, struct list_head *prev,
 
 	return true;
 }
-EXPORT_SYMBOL(__list_add_valid);
+EXPORT_SYMBOL(__list_add_valid_or_report);
 
-bool __list_del_entry_valid(struct list_head *entry)
+bool __list_del_entry_valid_or_report(struct list_head *entry)
 {
 	struct list_head *prev, *next;
 
@@ -65,6 +65,5 @@ bool __list_del_entry_valid(struct list_head *entry)
 		return false;
 
 	return true;
-
 }
-EXPORT_SYMBOL(__list_del_entry_valid);
+EXPORT_SYMBOL(__list_del_entry_valid_or_report);
-- 
2.41.0.694.ge786442a9b-goog

[PATCH v4 3/4] list: Introduce CONFIG_LIST_HARDENED

[Marco Elver]

Numerous production kernel configs (see [1, 2]) are choosing to enable
CONFIG_DEBUG_LIST, which is also being recommended by KSPP for hardened
configs [3]. The motivation behind this is that the option can be used
as a security hardening feature (e.g. CVE-2019-2215 and CVE-2019-2025
are mitigated by the option [4]).

The feature has never been designed with performance in mind, yet common
list manipulation is happening across hot paths all over the kernel.

Introduce CONFIG_LIST_HARDENED, which performs list pointer checking
inline, and only upon list corruption calls the reporting slow path.

To generate optimal machine code with CONFIG_LIST_HARDENED:

  1. Elide checking for pointer values which upon dereference would
     result in an immediate access fault (i.e. minimal hardening
     checks).  The trade-off is lower-quality error reports.

  2. Use the __preserve_most function attribute (available with Clang,
     but not yet with GCC) to minimize the code footprint for calling
     the reporting slow path. As a result, function size of callers is
     reduced by avoiding saving registers before calling the rarely
     called reporting slow path.

     Note that all TUs in lib/Makefile already disable function tracing,
     including list_debug.c, and __preserve_most's implied notrace has
     no effect in this case.

  3. Because the inline checks are a subset of the full set of checks in
     __list_*_valid_or_report(), always return false if the inline
     checks failed.  This avoids redundant compare and conditional
     branch right after return from the slow path.

As a side-effect of the checks being inline, if the compiler can prove
some condition to always be true, it can completely elide some checks.

Since DEBUG_LIST is functionally a superset of LIST_HARDENED, the
Kconfig variables are changed to reflect that: DEBUG_LIST selects
LIST_HARDENED, whereas LIST_HARDENED itself has no dependency on
DEBUG_LIST.

Running netperf with CONFIG_LIST_HARDENED (using a Clang compiler with
"preserve_most") shows throughput improvements, in my case of ~7% on
average (up to 20-30% on some test cases).

Link: https://r.android.com/1266735 [1]
Link: https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/blob/main/config [2]
Link: https://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project/Recommended_Settings [3]
Link: https://googleprojectzero.blogspot.com/2019/11/bad-binder-android-in-wild-exploit.html [4]
Signed-off-by: Marco Elver <elver@google.com>
---
v4:
* Rename to CONFIG_LIST_HARDENED, which can independently be selected
  from CONFIG_DEBUG_LIST.
* LKDTM test should just check CONFIG_LIST_HARDENED (which is also
  implied by DEBUG_LIST).
* Comment word smithing.

v3:
* Rename ___list_*_valid() to __list_*_valid_or_report().
* More comments.

v2:
* Note that lib/Makefile disables function tracing for everything and
  __preserve_most's implied notrace is a noop here.
---
 arch/arm64/kvm/hyp/nvhe/Makefile     |  2 +-
 arch/arm64/kvm/hyp/nvhe/list_debug.c |  2 +
 drivers/misc/lkdtm/bugs.c            |  4 +-
 include/linux/list.h                 | 64 +++++++++++++++++++++++++---
 lib/Kconfig.debug                    |  9 +++-
 lib/Makefile                         |  2 +-
 lib/list_debug.c                     |  5 ++-
 security/Kconfig.hardening           | 13 ++++++
 8 files changed, 88 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
index 9ddc025e4b86..2250253a6429 100644
--- a/arch/arm64/kvm/hyp/nvhe/Makefile
+++ b/arch/arm64/kvm/hyp/nvhe/Makefile
@@ -25,7 +25,7 @@ hyp-obj-y := timer-sr.o sysreg-sr.o debug-sr.o switch.o tlb.o hyp-init.o host.o
 	 cache.o setup.o mm.o mem_protect.o sys_regs.o pkvm.o stacktrace.o ffa.o
 hyp-obj-y += ../vgic-v3-sr.o ../aarch32.o ../vgic-v2-cpuif-proxy.o ../entry.o \
 	 ../fpsimd.o ../hyp-entry.o ../exception.o ../pgtable.o
-hyp-obj-$(CONFIG_DEBUG_LIST) += list_debug.o
+hyp-obj-$(CONFIG_LIST_HARDENED) += list_debug.o
 hyp-obj-y += $(lib-objs)
 
 ##
diff --git a/arch/arm64/kvm/hyp/nvhe/list_debug.c b/arch/arm64/kvm/hyp/nvhe/list_debug.c
index 16266a939a4c..46a2d4f2b3c6 100644
--- a/arch/arm64/kvm/hyp/nvhe/list_debug.c
+++ b/arch/arm64/kvm/hyp/nvhe/list_debug.c
@@ -26,6 +26,7 @@ static inline __must_check bool nvhe_check_data_corruption(bool v)
 
 /* The predicates checked here are taken from lib/list_debug.c. */
 
+__list_valid_slowpath
 bool __list_add_valid_or_report(struct list_head *new, struct list_head *prev,
 				struct list_head *next)
 {
@@ -37,6 +38,7 @@ bool __list_add_valid_or_report(struct list_head *new, struct list_head *prev,
 	return true;
 }
 
+__list_valid_slowpath
 bool __list_del_entry_valid_or_report(struct list_head *entry)
 {
 	struct list_head *prev, *next;
diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index 3c95600ab2f7..963b4dee6a7d 100644
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -393,7 +393,7 @@ static void lkdtm_CORRUPT_LIST_ADD(void)
 		pr_err("Overwrite did not happen, but no BUG?!\n");
 	else {
 		pr_err("list_add() corruption not detected!\n");
-		pr_expected_config(CONFIG_DEBUG_LIST);
+		pr_expected_config(CONFIG_LIST_HARDENED);
 	}
 }
 
@@ -420,7 +420,7 @@ static void lkdtm_CORRUPT_LIST_DEL(void)
 		pr_err("Overwrite did not happen, but no BUG?!\n");
 	else {
 		pr_err("list_del() corruption not detected!\n");
-		pr_expected_config(CONFIG_DEBUG_LIST);
+		pr_expected_config(CONFIG_LIST_HARDENED);
 	}
 }
 
diff --git a/include/linux/list.h b/include/linux/list.h
index 130c6a1bb45c..164b4d0e9d2a 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -38,39 +38,91 @@ static inline void INIT_LIST_HEAD(struct list_head *list)
 	WRITE_ONCE(list->prev, list);
 }
 
+#ifdef CONFIG_LIST_HARDENED
+
 #ifdef CONFIG_DEBUG_LIST
+# define __list_valid_slowpath
+#else
+# define __list_valid_slowpath __cold __preserve_most
+#endif
+
 /*
  * Performs the full set of list corruption checks before __list_add().
  * On list corruption reports a warning, and returns false.
  */
-extern bool __list_add_valid_or_report(struct list_head *new,
-				       struct list_head *prev,
-				       struct list_head *next);
+extern bool __list_valid_slowpath __list_add_valid_or_report(struct list_head *new,
+							     struct list_head *prev,
+							     struct list_head *next);
 
 /*
  * Performs list corruption checks before __list_add(). Returns false if a
  * corruption is detected, true otherwise.
+ *
+ * With CONFIG_LIST_HARDENED only, performs minimal list integrity checking
+ * inline to catch non-faulting corruptions, and only if a corruption is
+ * detected calls the reporting function __list_add_valid_or_report().
  */
 static __always_inline bool __list_add_valid(struct list_head *new,
 					     struct list_head *prev,
 					     struct list_head *next)
 {
-	return __list_add_valid_or_report(new, prev, next);
+	bool ret = true;
+
+	if (!IS_ENABLED(CONFIG_DEBUG_LIST)) {
+		/*
+		 * With the hardening version, elide checking if next and prev
+		 * are NULL, since the immediate dereference of them below would
+		 * result in a fault if NULL.
+		 *
+		 * With the reduced set of checks, we can afford to inline the
+		 * checks, which also gives the compiler a chance to elide some
+		 * of them completely if they can be proven at compile-time. If
+		 * one of the pre-conditions does not hold, the slow-path will
+		 * show a report which pre-condition failed.
+		 */
+		if (likely(next->prev == prev && prev->next == next && new != prev && new != next))
+			return true;
+		ret = false;
+	}
+
+	ret &= __list_add_valid_or_report(new, prev, next);
+	return ret;
 }
 
 /*
  * Performs the full set of list corruption checks before __list_del_entry().
  * On list corruption reports a warning, and returns false.
  */
-extern bool __list_del_entry_valid_or_report(struct list_head *entry);
+extern bool __list_valid_slowpath __list_del_entry_valid_or_report(struct list_head *entry);
 
 /*
  * Performs list corruption checks before __list_del_entry(). Returns false if a
  * corruption is detected, true otherwise.
+ *
+ * With CONFIG_LIST_HARDENED only, performs minimal list integrity checking
+ * inline to catch non-faulting corruptions, and only if a corruption is
+ * detected calls the reporting function __list_del_entry_valid_or_report().
  */
 static __always_inline bool __list_del_entry_valid(struct list_head *entry)
 {
-	return __list_del_entry_valid_or_report(entry);
+	bool ret = true;
+
+	if (!IS_ENABLED(CONFIG_DEBUG_LIST)) {
+		struct list_head *prev = entry->prev;
+		struct list_head *next = entry->next;
+
+		/*
+		 * With the hardening version, elide checking if next and prev
+		 * are NULL, LIST_POISON1 or LIST_POISON2, since the immediate
+		 * dereference of them below would result in a fault.
+		 */
+		if (likely(prev->next == entry && next->prev == entry))
+			return true;
+		ret = false;
+	}
+
+	ret &= __list_del_entry_valid_or_report(entry);
+	return ret;
 }
 #else
 static inline bool __list_add_valid(struct list_head *new,
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index fbc89baf7de6..c38745ad46eb 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1674,9 +1674,14 @@ menu "Debug kernel data structures"
 config DEBUG_LIST
 	bool "Debug linked list manipulation"
 	depends on DEBUG_KERNEL || BUG_ON_DATA_CORRUPTION
+	select LIST_HARDENED
 	help
-	  Enable this to turn on extended checks in the linked-list
-	  walking routines.
+	  Enable this to turn on extended checks in the linked-list walking
+	  routines.
+
+	  This option trades better quality error reports for performance, and
+	  is more suitable for kernel debugging. If you care about performance,
+	  you should only enable CONFIG_LIST_HARDENED instead.
 
 	  If unsure, say N.
 
diff --git a/lib/Makefile b/lib/Makefile
index 1ffae65bb7ee..d1397785ec16 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -167,7 +167,7 @@ obj-$(CONFIG_BTREE) += btree.o
 obj-$(CONFIG_INTERVAL_TREE) += interval_tree.o
 obj-$(CONFIG_ASSOCIATIVE_ARRAY) += assoc_array.o
 obj-$(CONFIG_DEBUG_PREEMPT) += smp_processor_id.o
-obj-$(CONFIG_DEBUG_LIST) += list_debug.o
+obj-$(CONFIG_LIST_HARDENED) += list_debug.o
 obj-$(CONFIG_DEBUG_OBJECTS) += debugobjects.o
 
 obj-$(CONFIG_BITREVERSE) += bitrev.o
diff --git a/lib/list_debug.c b/lib/list_debug.c
index 2def33b1491f..db602417febf 100644
--- a/lib/list_debug.c
+++ b/lib/list_debug.c
@@ -2,7 +2,8 @@
  * Copyright 2006, Red Hat, Inc., Dave Jones
  * Released under the General Public License (GPL).
  *
- * This file contains the linked list validation for DEBUG_LIST.
+ * This file contains the linked list validation and error reporting for
+ * LIST_HARDENED and DEBUG_LIST.
  */
 
 #include <linux/export.h>
@@ -17,6 +18,7 @@
  * attempt).
  */
 
+__list_valid_slowpath
 bool __list_add_valid_or_report(struct list_head *new, struct list_head *prev,
 				struct list_head *next)
 {
@@ -39,6 +41,7 @@ bool __list_add_valid_or_report(struct list_head *new, struct list_head *prev,
 }
 EXPORT_SYMBOL(__list_add_valid_or_report);
 
+__list_valid_slowpath
 bool __list_del_entry_valid_or_report(struct list_head *entry)
 {
 	struct list_head *prev, *next;
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 0f295961e773..ffc3c702b461 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -279,6 +279,19 @@ config ZERO_CALL_USED_REGS
 
 endmenu
 
+menu "Hardening of kernel data structures"
+
+config LIST_HARDENED
+	bool "Check integrity of linked list manipulation"
+	help
+	  Minimal integrity checking in the linked-list manipulation routines
+	  to catch memory corruptions that are not guaranteed to result in an
+	  immediate access fault.
+
+	  If unsure, say N.
+
+endmenu
+
 config CC_HAS_RANDSTRUCT
 	def_bool $(cc-option,-frandomize-layout-seed-file=/dev/null)
 	# Randstruct was first added in Clang 15, but it isn't safe to use until
-- 
2.41.0.694.ge786442a9b-goog

[PATCH v4 4/4] hardening: Move BUG_ON_DATA_CORRUPTION to hardening options

[Marco Elver]

BUG_ON_DATA_CORRUPTION is turning detected corruptions of list data
structures from WARNings into BUGs. This can be useful to stop further
corruptions or even exploitation attempts.

However, the option has less to do with debugging than with hardening.
With the introduction of LIST_HARDENED, it makes more sense to move it
to the hardening options, where it selects LIST_HARDENED instead.

Without this change, combining BUG_ON_DATA_CORRUPTION with LIST_HARDENED
alone wouldn't be possible, because DEBUG_LIST would always be selected
by BUG_ON_DATA_CORRUPTION.

Signed-off-by: Marco Elver <elver@google.com>
---
v4:
* New patch, after LIST_HARDENED was made independent of DEBUG_LIST, and
  now DEBUG_LIST depends on LIST_HARDENED.
---
 lib/Kconfig.debug          | 12 +-----------
 security/Kconfig.hardening | 10 ++++++++++
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index c38745ad46eb..c7348d1fabe5 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1673,7 +1673,7 @@ menu "Debug kernel data structures"
 
 config DEBUG_LIST
 	bool "Debug linked list manipulation"
-	depends on DEBUG_KERNEL || BUG_ON_DATA_CORRUPTION
+	depends on DEBUG_KERNEL
 	select LIST_HARDENED
 	help
 	  Enable this to turn on extended checks in the linked-list walking
@@ -1715,16 +1715,6 @@ config DEBUG_NOTIFIERS
 	  This is a relatively cheap check but if you care about maximum
 	  performance, say N.
 
-config BUG_ON_DATA_CORRUPTION
-	bool "Trigger a BUG when data corruption is detected"
-	select DEBUG_LIST
-	help
-	  Select this option if the kernel should BUG when it encounters
-	  data corruption in kernel memory structures when they get checked
-	  for validity.
-
-	  If unsure, say N.
-
 config DEBUG_MAPLE_TREE
 	bool "Debug maple trees"
 	depends on DEBUG_KERNEL
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index ffc3c702b461..2cff851ebfd7 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -290,6 +290,16 @@ config LIST_HARDENED
 
 	  If unsure, say N.
 
+config BUG_ON_DATA_CORRUPTION
+	bool "Trigger a BUG when data corruption is detected"
+	select LIST_HARDENED
+	help
+	  Select this option if the kernel should BUG when it encounters
+	  data corruption in kernel memory structures when they get checked
+	  for validity.
+
+	  If unsure, say N.
+
 endmenu
 
 config CC_HAS_RANDSTRUCT
-- 
2.41.0.694.ge786442a9b-goog

Re: [PATCH v4 1/4] compiler_types: Introduce the Clang __preserve_most function attribute

[Kees Cook]

On Fri, Aug 11, 2023 at 05:18:38PM +0200, Marco Elver wrote:
> [1]: "On X86-64 and AArch64 targets, this attribute changes the calling
> convention of a function. The preserve_most calling convention attempts
> to make the code in the caller as unintrusive as possible. This
> convention behaves identically to the C calling convention on how
> arguments and return values are passed, but it uses a different set of
> caller/callee-saved registers. This alleviates the burden of saving and
> recovering a large register set before and after the call in the caller.
> If the arguments are passed in callee-saved registers, then they will be
> preserved by the callee across the call. This doesn't apply for values
> returned in callee-saved registers.
> 
>  * On X86-64 the callee preserves all general purpose registers, except
>    for R11. R11 can be used as a scratch register. Floating-point
>    registers (XMMs/YMMs) are not preserved and need to be saved by the
>    caller.
> 
>  * On AArch64 the callee preserve all general purpose registers, except
>    x0-X8 and X16-X18."
> 
> [1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most
> 
> Introduce the attribute to compiler_types.h as __preserve_most.
> 
> Use of this attribute results in better code generation for calls to
> very rarely called functions, such as error-reporting functions, or
> rarely executed slow paths.
> 
> Beware that the attribute conflicts with instrumentation calls inserted
> on function entry which do not use __preserve_most themselves. Notably,
> function tracing which assumes the normal C calling convention for the
> given architecture.  Where the attribute is supported, __preserve_most
> will imply notrace. It is recommended to restrict use of the attribute
> to functions that should or already disable tracing.
> 
> Note: The additional preprocessor check against architecture should not
> be necessary if __has_attribute() only returns true where supported;
> also see https://github.com/ClangBuiltLinux/linux/issues/1908. But until
> __has_attribute() does the right thing, we also guard by known-supported
> architectures to avoid build warnings on other architectures.
> 
> The attribute may be supported by a future GCC version (see
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899).
> 
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Acked-by: Mark Rutland <mark.rutland@arm.com>

Should this go via -mm, the hardening tree, or something else? I'm happy
to carry it if no one else wants it?

-Kees

-- 
Kees Cook

Re: [PATCH v4 1/4] compiler_types: Introduce the Clang __preserve_most function attribute

[Marco Elver]

On Tue, 15 Aug 2023 at 01:21, Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Aug 11, 2023 at 05:18:38PM +0200, Marco Elver wrote:
> > [1]: "On X86-64 and AArch64 targets, this attribute changes the calling
> > convention of a function. The preserve_most calling convention attempts
> > to make the code in the caller as unintrusive as possible. This
> > convention behaves identically to the C calling convention on how
> > arguments and return values are passed, but it uses a different set of
> > caller/callee-saved registers. This alleviates the burden of saving and
> > recovering a large register set before and after the call in the caller.
> > If the arguments are passed in callee-saved registers, then they will be
> > preserved by the callee across the call. This doesn't apply for values
> > returned in callee-saved registers.
> >
> >  * On X86-64 the callee preserves all general purpose registers, except
> >    for R11. R11 can be used as a scratch register. Floating-point
> >    registers (XMMs/YMMs) are not preserved and need to be saved by the
> >    caller.
> >
> >  * On AArch64 the callee preserve all general purpose registers, except
> >    x0-X8 and X16-X18."
> >
> > [1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most
> >
> > Introduce the attribute to compiler_types.h as __preserve_most.
> >
> > Use of this attribute results in better code generation for calls to
> > very rarely called functions, such as error-reporting functions, or
> > rarely executed slow paths.
> >
> > Beware that the attribute conflicts with instrumentation calls inserted
> > on function entry which do not use __preserve_most themselves. Notably,
> > function tracing which assumes the normal C calling convention for the
> > given architecture.  Where the attribute is supported, __preserve_most
> > will imply notrace. It is recommended to restrict use of the attribute
> > to functions that should or already disable tracing.
> >
> > Note: The additional preprocessor check against architecture should not
> > be necessary if __has_attribute() only returns true where supported;
> > also see https://github.com/ClangBuiltLinux/linux/issues/1908. But until
> > __has_attribute() does the right thing, we also guard by known-supported
> > architectures to avoid build warnings on other architectures.
> >
> > The attribute may be supported by a future GCC version (see
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899).
> >
> > Signed-off-by: Marco Elver <elver@google.com>
> > Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
> > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> > Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> > Acked-by: Mark Rutland <mark.rutland@arm.com>
>
> Should this go via -mm, the hardening tree, or something else? I'm happy
> to carry it if no one else wants it?

v3 of this series is already in mm-unstable, and has had some -next
exposure (which was helpful in uncovering some additional issues).
Therefore, I think it's appropriate that it continues in mm and Andrew
picks up the latest v4 here.

Your official Ack would nevertheless be much appreciated!

Thanks,
-- Marco

Re: [PATCH v4 1/4] compiler_types: Introduce the Clang __preserve_most function attribute

[Andrew Morton]

On Mon, 14 Aug 2023 16:21:43 -0700 Kees Cook <keescook@chromium.org> wrote:

> Should this go via -mm, the hardening tree, or something else? I'm happy
> to carry it if no one else wants it?

Please do so.

Re: [PATCH v4 1/4] compiler_types: Introduce the Clang __preserve_most function attribute

[Kees Cook]

On Fri, 11 Aug 2023 17:18:38 +0200, Marco Elver wrote:
> [1]: "On X86-64 and AArch64 targets, this attribute changes the calling
> convention of a function. The preserve_most calling convention attempts
> to make the code in the caller as unintrusive as possible. This
> convention behaves identically to the C calling convention on how
> arguments and return values are passed, but it uses a different set of
> caller/callee-saved registers. This alleviates the burden of saving and
> recovering a large register set before and after the call in the caller.
> If the arguments are passed in callee-saved registers, then they will be
> preserved by the callee across the call. This doesn't apply for values
> returned in callee-saved registers.
> 
> [...]

Applied to for-next/hardening, thanks!

[1/4] compiler_types: Introduce the Clang __preserve_most function attribute
      https://git.kernel.org/kees/c/7a0fd5e16785
[2/4] list_debug: Introduce inline wrappers for debug checks
      https://git.kernel.org/kees/c/b16c42c8fde8
[3/4] list: Introduce CONFIG_LIST_HARDENED
      https://git.kernel.org/kees/c/aebc7b0d8d91
[4/4] hardening: Move BUG_ON_DATA_CORRUPTION to hardening options
      https://git.kernel.org/kees/c/aa9f10d57056

Take care,

-- 
Kees Cook