kasan: arm64: support specialized outlined tag mismatch checks

By using outlined checks we can achieve a significant code size
improvement by moving the tag-based ASAN checks into separate
functions. Unlike the existing CONFIG_KASAN_OUTLINE mode these
functions have a custom calling convention that preserves most
registers and is specialized to the register containing the address
and the type of access, and as a result we can eliminate the code
size and performance overhead of a standard calling convention such
as AAPCS for these functions.

This change depends on a separate series of changes to Clang [1] to
support outlined checks in the kernel, although the change works fine
without them (we just don't get outlined checks). This is because the
flag -mllvm -hwasan-inline-all-checks=0 has no effect until the Clang
changes land. The flag was introduced in the Clang 9.0 timeframe as
part of the support for outlined checks in userspace and because our
minimum Clang version is 10.0 we can pass it unconditionally.

Outlined checks require a new runtime function with a custom calling
convention. Add this function to arch/arm64/lib.

I measured the code size of defconfig + tag-based KASAN, as well
as boot time (i.e. time to init launch) on a DragonBoard 845c with
an Android arm64 GKI kernel. The results are below:

                               code size    boot time
CONFIG_KASAN_INLINE=y before    92824064      6.18s
CONFIG_KASAN_INLINE=y after     38822400      6.65s
CONFIG_KASAN_OUTLINE=y          39215616     11.48s

We can see straight away that specialized outlined checks beat the
existing CONFIG_KASAN_OUTLINE=y on both code size and boot time
for tag-based ASAN.

As for the comparison between CONFIG_KASAN_INLINE=y before and after
we saw similar performance numbers in userspace [2] and decided
that since the performance overhead is minimal compared to the
overhead of tag-based ASAN itself as well as compared to the code
size improvements we would just replace the inlined checks with the
specialized outlined checks without the option to select between them,
and that is what I have implemented in this patch.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Link: https://linux-review.googlesource.com/id/I1a30036c70ab3c3ee78d75ed9b87ef7cdc3fdb76
Link: [1] https://reviews.llvm.org/D90426
Link: [2] https://reviews.llvm.org/D56954
Link: https://lore.kernel.org/r/20210526174927.2477847-3-pcc@google.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/asm-prototypes.h

@@ -23,4 +23,10 @@ long long __ashlti3(long long a, int b);
 long long __ashrti3(long long a, int b);
 long long __lshrti3(long long a, int b);
 
+/*
+ * This function uses a custom calling convention and cannot be called from C so
+ * this prototype is not entirely accurate.
+ */
+void __hwasan_tag_mismatch(unsigned long addr, unsigned long access_info);
+
 #endif /* __ASM_PROTOTYPES_H */

arch/arm64/include/asm/module.lds.h

@@ -1,7 +1,20 @@
-#ifdef CONFIG_ARM64_MODULE_PLTS
 SECTIONS {
+#ifdef CONFIG_ARM64_MODULE_PLTS
 	.plt 0 (NOLOAD) : { BYTE(0) }
 	.init.plt 0 (NOLOAD) : { BYTE(0) }
 	.text.ftrace_trampoline 0 (NOLOAD) : { BYTE(0) }
-}
 #endif
+
+#ifdef CONFIG_KASAN_SW_TAGS
+	/*
+	 * Outlined checks go into comdat-deduplicated sections named .text.hot.
+	 * Because they are in comdats they are not combined by the linker and
+	 * we otherwise end up with multiple sections with the same .text.hot
+	 * name in the .ko file. The kernel module loader warns if it sees
+	 * multiple sections with the same name so we use this sections
+	 * directive to force them into a single section and silence the
+	 * warning.
+	 */
+	.text.hot : { *(.text.hot) }
+#endif
+}

arch/arm64/lib/Makefile

@@ -18,3 +18,5 @@ obj-$(CONFIG_CRC32) += crc32.o
 obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
 
 obj-$(CONFIG_ARM64_MTE) += mte.o
+
+obj-$(CONFIG_KASAN_SW_TAGS) += kasan_sw_tags.o

arch/arm64/lib/kasan_sw_tags.S

@@ -0,0 +1,76 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2020 Google LLC
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+/*
+ * Report a tag mismatch detected by tag-based KASAN.
+ *
+ * A compiler-generated thunk calls this with a non-AAPCS calling
+ * convention. Upon entry to this function, registers are as follows:
+ *
+ * x0:         fault address (see below for restore)
+ * x1:         fault description (see below for restore)
+ * x2 to x15:  callee-saved
+ * x16 to x17: safe to clobber
+ * x18 to x30: callee-saved
+ * sp:         pre-decremented by 256 bytes (see below for restore)
+ *
+ * The caller has decremented the SP by 256 bytes, and created a
+ * structure on the stack as follows:
+ *
+ * sp + 0..15:    x0 and x1 to be restored
+ * sp + 16..231:  free for use
+ * sp + 232..247: x29 and x30 (same as in GPRs)
+ * sp + 248..255: free for use
+ *
+ * Note that this is not a struct pt_regs.
+ *
+ * To call a regular AAPCS function we must save x2 to x15 (which we can
+ * store in the gaps), and create a frame record (for which we can use
+ * x29 and x30 spilled by the caller as those match the GPRs).
+ *
+ * The caller expects x0 and x1 to be restored from the structure, and
+ * for the structure to be removed from the stack (i.e. the SP must be
+ * incremented by 256 prior to return).
+ */
+SYM_CODE_START(__hwasan_tag_mismatch)
+#ifdef BTI_C
+	BTI_C
+#endif
+	add	x29, sp, #232
+	stp	x2, x3, [sp, #8 * 2]
+	stp	x4, x5, [sp, #8 * 4]
+	stp	x6, x7, [sp, #8 * 6]
+	stp	x8, x9, [sp, #8 * 8]
+	stp	x10, x11, [sp, #8 * 10]
+	stp	x12, x13, [sp, #8 * 12]
+	stp	x14, x15, [sp, #8 * 14]
+#ifndef CONFIG_SHADOW_CALL_STACK
+	str	x18, [sp, #8 * 18]
+#endif
+
+	mov	x2, x30
+	bl	kasan_tag_mismatch
+
+	ldp	x0, x1, [sp]
+	ldp	x2, x3, [sp, #8 * 2]
+	ldp	x4, x5, [sp, #8 * 4]
+	ldp	x6, x7, [sp, #8 * 6]
+	ldp	x8, x9, [sp, #8 * 8]
+	ldp	x10, x11, [sp, #8 * 10]
+	ldp	x12, x13, [sp, #8 * 12]
+	ldp	x14, x15, [sp, #8 * 14]
+#ifndef CONFIG_SHADOW_CALL_STACK
+	ldr	x18, [sp, #8 * 18]
+#endif
+	ldp	x29, x30, [sp, #8 * 29]
+
+	/* remove the structure from the stack */
+	add	sp, sp, #256
+	ret
+SYM_CODE_END(__hwasan_tag_mismatch)
+EXPORT_SYMBOL(__hwasan_tag_mismatch)

mm/kasan/sw_tags.c

@@ -207,3 +207,10 @@ struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
 
 	return &alloc_meta->free_track[i];
 }
+
+void kasan_tag_mismatch(unsigned long addr, unsigned long access_info,
+			unsigned long ret_ip)
+{
+	kasan_report(addr, 1 << (access_info & 0xf), access_info & 0x10,
+		     ret_ip);
+}

scripts/Makefile.kasan

@@ -50,6 +50,7 @@ endif
 CFLAGS_KASAN := -fsanitize=kernel-hwaddress \
 		$(call cc-param,hwasan-instrument-stack=$(stack_enable)) \
 		$(call cc-param,hwasan-use-short-granules=0) \
+		$(call cc-param,hwasan-inline-all-checks=0) \
 		$(instrumentation_flags)
 
 endif # CONFIG_KASAN_SW_TAGS