[PATCH v2] kasan: arm64: support specialized outlined tag mismatch checks
Andrey Konovalov
andreyknvl at google.com
Mon Nov 23 13:21:02 EST 2020
On Sat, Nov 21, 2020 at 12:02 AM Peter Collingbourne <pcc at google.com> wrote:
>
> 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
> ---
> 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 | 60 +++++++++++++++++++++++++
> mm/kasan/tags.c | 7 +++
> scripts/Makefile.kasan | 1 +
> 6 files changed, 91 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..5c179b32af71
> --- /dev/null
> +++ b/arch/arm64/lib/kasan_sw_tags.S
> @@ -0,0 +1,60 @@
> +/* 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.
> + *
> + * This function has a custom calling convention in order to minimize the sizes
> + * of the compiler-generated thunks that call it. All registers except for x16
> + * and x17 must be preserved. This includes x0 and x1 which are used by the
> + * caller to pass arguments. In order to allow these registers to be restored
> + * the caller spills x0 and x1 to sp+0 and sp+8. The registers x29 and x30 are
> + * spilled to sp+232 and sp+240, and although it is not strictly necessary for
> + * the caller to spill them, that is how the ABI for these functions has been
> + * defined. The 256 bytes of stack space allocated by the caller must be
> + * deallocated on return.
> + *
> + * This function takes care of transitioning to the standard AAPCS calling
> + * convention and calls the C function kasan_tag_mismatch to report the error.
> + *
> + * Parameters:
> + * x0 - the fault address
> + * x1 - an encoded description of the faulting access
> + */
> +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 x29, x30, [sp, #8 * 29]
> +#ifndef CONFIG_SHADOW_CALL_STACK
> + ldr x18, [sp, #8 * 18]
> +#endif
> + ldp x14, x15, [sp, #8 * 14]
> + ldp x12, x13, [sp, #8 * 12]
> + ldp x10, x11, [sp, #8 * 10]
> + ldp x8, x9, [sp, #8 * 8]
> + ldp x6, x7, [sp, #8 * 6]
> + ldp x4, x5, [sp, #8 * 4]
> + ldp x2, x3, [sp, #8 * 2]
> + ldp x0, x1, [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
>
Acked-by: Andrey Konovalov <andreyknvl at google.com>
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