[PATCH v3 3/3] kasan: arm64: support specialized outlined tag mismatch checks
Peter Collingbourne
pcc at google.com
Thu Dec 3 00:12:24 EST 2020
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. But we may make a
different decision for the kernel such as having CONFIG_KASAN_OUTLINE=y
turn on specialized outlined checks if Clang is new enough.
Signed-off-by: Peter Collingbourne <pcc at google.com>
Link: https://linux-review.googlesource.com/id/I1a30036c70ab3c3ee78d75ed9b87ef7cdc3fdb76
Link: [1] https://reviews.llvm.org/D90426
Link: [2] https://reviews.llvm.org/D56954
---
v3:
- adopt Mark Rutland's suggested changes
- move frame record alignment patches behind this one
v2:
- use calculations in the stack spills and restores
- improve the comment at the top of the function
- add a BTI instruction
arch/arm64/include/asm/asm-prototypes.h | 6 ++
arch/arm64/include/asm/module.lds.h | 17 +++++-
arch/arm64/lib/Makefile | 2 +
arch/arm64/lib/kasan_sw_tags.S | 76 +++++++++++++++++++++++++
mm/kasan/tags.c | 7 +++
scripts/Makefile.kasan | 1 +
6 files changed, 107 insertions(+), 2 deletions(-)
create mode 100644 arch/arm64/lib/kasan_sw_tags.S
diff --git a/arch/arm64/include/asm/asm-prototypes.h b/arch/arm64/include/asm/asm-prototypes.h
index 1c9a3a0c5fa5..ec1d9655f885 100644
--- a/arch/arm64/include/asm/asm-prototypes.h
+++ b/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 */
diff --git a/arch/arm64/include/asm/module.lds.h b/arch/arm64/include/asm/module.lds.h
index 691f15af788e..4a6d717f75f3 100644
--- a/arch/arm64/include/asm/module.lds.h
+++ b/arch/arm64/include/asm/module.lds.h
@@ -1,7 +1,20 @@
-#ifdef CONFIG_ARM64_MODULE_PLTS
SECTIONS {
+#ifdef CONFIG_ARM64_MODULE_PLTS
.plt (NOLOAD) : { BYTE(0) }
.init.plt (NOLOAD) : { BYTE(0) }
.text.ftrace_trampoline (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
+}
diff --git a/arch/arm64/lib/Makefile b/arch/arm64/lib/Makefile
index d31e1169d9b8..8e60d76a1b47 100644
--- a/arch/arm64/lib/Makefile
+++ b/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
diff --git a/arch/arm64/lib/kasan_sw_tags.S b/arch/arm64/lib/kasan_sw_tags.S
new file mode 100644
index 000000000000..5b04464c045e
--- /dev/null
+++ b/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)
diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
index e02a36a51f42..d00613956c79 100644
--- a/mm/kasan/tags.c
+++ b/mm/kasan/tags.c
@@ -198,3 +198,10 @@ 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);
+}
diff --git a/scripts/Makefile.kasan b/scripts/Makefile.kasan
index 1e000cc2e7b4..1f2cccc264d8 100644
--- a/scripts/Makefile.kasan
+++ b/scripts/Makefile.kasan
@@ -44,6 +44,7 @@ endif
CFLAGS_KASAN := -fsanitize=kernel-hwaddress \
-mllvm -hwasan-instrument-stack=$(CONFIG_KASAN_STACK) \
-mllvm -hwasan-use-short-granules=0 \
+ -mllvm -hwasan-inline-all-checks=0 \
$(instrumentation_flags)
endif # CONFIG_KASAN_SW_TAGS
--
2.29.2.454.gaff20da3a2-goog
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