Re: [RFC/RFT] CFI: Add support for gcc CFI in aarch64

Mark Rutland
2023-01-03T08:53:33Z

On Sun, Dec 18, 2022 at 10:17:58PM -0800, Dan Li wrote:
> Based on Sami's patch[1], this patch makes the corresponding kernel
> configuration of CFI available when compiling the kernel with the gcc[2].
>
> The code after enabling cfi is as follows:
>
> int (*p)(void);
> int func (int)
> {
> p();
> }
>
> __cfi_func:
> .4byte 0x439d3502
> func:
> ......
> adrp x0, p
> add x0, x0, :lo12:p
> mov w1, 23592
> movk w1, 0x4601, lsl 16
> cmp w0, w1
> beq .L2
> ......
> bl cfi_check_failed
> .L2:
> blr x19
> ret
>
> In the compiler part[4], there are some differences from Sami's
> implementation[3], mainly including:
>
> 1. When a typeid mismatch is detected, the cfi_check_failed function
> will be called instead of the brk instruction. This function needs
> to be implemented by the compiler user.

For arm64, we specifically wanted the BRK approach (with the information endoed
into an immediate) because it gave us all the information we needed to diagnose
the failed check, while giving the compiler freedom to perform the check
however it wanted, and without needing to shuffle a bunch of registers (or
having a weird calling convention for the cfi_check_failed handler).

> If there are user mode programs or other systems that want to use
> this feature, it may be more convenient to use a callback (so this
> compilation option is set to -fsanitize=cfi instead of kcfi).

I appreciate that may be nicer for userspace, but it would be far nicer for the
kernel if we could have a kcfi mode that behaves the same as LLVM, using a BRK.
That's going to be simpler for the kernel to deal with, and should result in
nicer code / smaller binary size (for the reasons given above).

Can we please have an LLVM-compatible KCFI mode, and have the -fsanitize=cfi be
a separate option from -fsanitize=kcfi?

> 2. A reserved typeid (such as 0x0U on the aarch64 platform) is always
> inserted in front of functions that should not be called indirectly.
> Functions that can be called indirectly will not use this hash value,
> which prevents instructions/data before the function from being used
> as a typeid by an attacker.

That sounds sensible, though it meanse we'll need to go audit all the assembly
without type annotations.

I presume that "functions that should not be called indirectly" only includes
those which are not directly visible outside the compilation unit AND whose
address is never taken / escaped from the compilation unit. Is that the case?

> 3. Some bits are ignored in the typeid to avoid conflicts between the
> typeid and the instruction set of a specific platform, thereby
> preventing an attacker from bypassing the CFI check by using the
> instruction as a typeid, such as on the aarch64 platform:
> * If the following instruction sequence exists:
> 400620: a9be7bfd stp x29, x30, [sp, #-32]!
> 400624: 910003fd mov x29, sp
> 400628: f9000bf3 str x19, [sp, #16]
> * If the expected typeid of the indirect call is exactly 0x910003fd,
> the attacker can jump to the next instruction position of any
> "mov x29,sp" instruction (such as 0x400628 here).

Which bits exactly are ignored on arm64?

e.g. are these encoded into UDF immediates?

> 4. Insert a symbol __cfi_<function> before each function's typeid,
> which may be helpful for fine-grained KASLR implementations (or not?).

I can imagine this is useful, but I am not immediately sure.

> 5. The current implementation of gcc only supports the aarch64 platform.
>
> This produces the following oops on CFI failure (generated using lkdtm):
>
> /kselftest_install/lkdtm # ./CFI_FORWARD_PROTO.sh
> [ 74.856516] lkdtm: Performing direct entry CFI_FORWARD_PROTO
> [ 74.856878] lkdtm: Calling matched prototype ...
> [ 74.857011] lkdtm: Calling mismatched prototype ...
> [ 74.857133] CFI failure at lkdtm_indirect_call+0x30/0x50 (target: lkdtm_increment_int+0x0/0x1c; expected type: 0xc59c68f1)
> [ 74.858185] Kernel panic - not syncing: Oops - CFI
> [ 74.859240] CPU: 0 PID: 129 Comm: cat Not tainted 6.0.0-rc4-00024-g32bf7f14f497-dirty #150
> [ 74.859481] Hardware name: linux,dummy-virt (DT)
> [ 74.859795] Call trace:
> [ 74.859959] dump_backtrace.part.0+0xcc/0xe0
> [ 74.860212] show_stack+0x18/0x5c
> [ 74.860327] dump_stack_lvl+0x64/0x84
> [ 74.860398] dump_stack+0x18/0x38
> [ 74.860443] panic+0x170/0x36c
> [ 74.860496] cfi_check_failed+0x38/0x44
> [ 74.860564] lkdtm_indirect_call+0x30/0x50
> [ 74.860614] lkdtm_CFI_FORWARD_PROTO+0x3c/0x6c
> [ 74.860701] lkdtm_do_action+0x44/0x58
> [ 74.860764] direct_entry+0x148/0x160
> [ 74.860814] full_proxy_write+0x74/0xe0
> [ 74.860874] vfs_write+0xd8/0x2d0
> [ 74.860942] ksys_write+0x70/0x110
> [ 74.861000] __arm64_sys_write+0x1c/0x30
> [ 74.861067] invoke_syscall+0x5c/0x140
> [ 74.861117] el0_svc_common.constprop.0+0x44/0xf0
> [ 74.861190] do_el0_svc+0x2c/0xc0
> [ 74.861233] el0_svc+0x20/0x60
> [ 74.861287] el0t_64_sync_handler+0xf4/0x124
> [ 74.861340] el0t_64_sync+0x160/0x164
> [ 74.861782] SMP: stopping secondary CPUs
> [ 74.862336] Kernel Offset: disabled
> [ 74.862439] CPU features: 0x0000,00075024,699418af
> [ 74.862799] Memory Limit: none
> [ 74.863373] ---[ end Kernel panic - not syncing: Oops - CFI ]---
>
> The gcc-related patches[4] are based on tag: releases/gcc-12.2.0.
>
> Any suggestion please let me know :).

As a general thing, how does this work with -fpatchable-function-entry=M,N,
where N is non-zero?

We still need to fix that for LLVM, and it would be good to align on the same behaviour.

Thanks,
Mark.

>
> Thanks, Dan.
>
> [1] https://lore.kernel.org/all/20220908215504.3686827-1-samitolvanen@google.com/
> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107048
> [3] https://reviews.llvm.org/D119296
> [4] https://lore.kernel.org/linux-hardening/20221219055431.22596-1-ashimida.1990@gmail.com/
>
> Signed-off-by: Dan Li <ashimida.1990@gmail.com>
> ---
> Makefile | 6 ++++++
> arch/Kconfig | 24 +++++++++++++++++++++++-
> arch/arm64/Kconfig | 1 +
> include/linux/cfi_types.h | 15 +++++++++++----
> include/linux/compiler-gcc.h | 4 ++++
> kernel/Makefile | 1 +
> kernel/cfi.c | 23 +++++++++++++++++++++++
> scripts/kallsyms.c | 4 +++-
> 8 files changed, 72 insertions(+), 6 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 43e08c9f95e9..7c74dac57aa4 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -926,6 +926,12 @@ KBUILD_CFLAGS += $(CC_FLAGS_CFI)
> export CC_FLAGS_CFI
> endif
>
> +ifdef CONFIG_CFI_GCC
> +CC_FLAGS_CFI := -fsanitize=cfi
> +KBUILD_CFLAGS += $(CC_FLAGS_CFI)
> +export CC_FLAGS_CFI
> +endif
> +
> ifdef CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B
> KBUILD_CFLAGS += -falign-functions=64
> endif
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 1c1eca0c0019..8b43a9fd3b54 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -756,9 +756,31 @@ config CFI_CLANG
>
> https://clang.llvm.org/docs/ControlFlowIntegrity.html
>
> +config ARCH_SUPPORTS_CFI_GCC
> + bool
> + help
> + An architecture should select this option if it can support GCC's
> + Control-Flow Integrity (CFI) checking.
> +
> +config CFI_GCC
> + bool "Use GCC's Control Flow Integrity (CFI)"
> + depends on ARCH_SUPPORTS_CFI_GCC
> + depends on $(cc-option,-fsanitize=cfi)
> + help
> + This option enables GCC’s forward-edge Control Flow Integrity
> + (CFI) checking, where the compiler injects a runtime check to each
> + indirect function call to ensure the target is a valid function with
> + the correct static type. This restricts possible call targets and
> + makes it more difficult for an attacker to exploit bugs that allow
> + the modification of stored function pointers. More information can be
> + found from the compiler's documentation:
> +
> + - Clang: https://clang.llvm.org/docs/ControlFlowIntegrity.html
> + - GCC: https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#Instrumentation-Options
> +
> config CFI_PERMISSIVE
> bool "Use CFI in permissive mode"
> - depends on CFI_CLANG
> + depends on CFI_CLANG || CFI_GCC
> help
> When selected, Control Flow Integrity (CFI) violations result in a
> warning instead of a kernel panic. This option should only be used
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 9fb9fff08c94..60fdfb01cecb 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -89,6 +89,7 @@ config ARM64
> select ARCH_SUPPORTS_LTO_CLANG if CPU_LITTLE_ENDIAN
> select ARCH_SUPPORTS_LTO_CLANG_THIN
> select ARCH_SUPPORTS_CFI_CLANG
> + select ARCH_SUPPORTS_CFI_GCC
> select ARCH_SUPPORTS_ATOMIC_RMW
> select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
> select ARCH_SUPPORTS_NUMA_BALANCING
> diff --git a/include/linux/cfi_types.h b/include/linux/cfi_types.h
> index 6b8713675765..1c3b7ea6a79f 100644
> --- a/include/linux/cfi_types.h
> +++ b/include/linux/cfi_types.h
> @@ -8,18 +8,25 @@
> #ifdef __ASSEMBLY__
> #include <linux/linkage.h>
>
> -#ifdef CONFIG_CFI_CLANG
> +#if defined(CONFIG_CFI_CLANG) || defined(CONFIG_CFI_GCC)
> /*
> - * Use the __kcfi_typeid_<function> type identifier symbol to
> + * Use the __[k]cfi_typeid_<function> type identifier symbol to
> * annotate indirectly called assembly functions. The compiler emits
> * these symbols for all address-taken function declarations in C
> * code.
> */
> #ifndef __CFI_TYPE
> +
> +#ifdef CONFIG_CFI_GCC
> +#define __CFI_TYPE(name) \
> + .4byte __cfi_typeid_##name
> +#else
> #define __CFI_TYPE(name) \
> .4byte __kcfi_typeid_##name
> #endif
>
> +#endif
> +
> #define SYM_TYPED_ENTRY(name, linkage, align...) \
> linkage(name) ASM_NL \
> align ASM_NL \
> @@ -29,12 +36,12 @@
> #define SYM_TYPED_START(name, linkage, align...) \
> SYM_TYPED_ENTRY(name, linkage, align)
>
> -#else /* CONFIG_CFI_CLANG */
> +#else /* defined(CONFIG_CFI_CLANG) || defined(CONFIG_CFI_GCC) */
>
> #define SYM_TYPED_START(name, linkage, align...) \
> SYM_START(name, linkage, align)
>
> -#endif /* CONFIG_CFI_CLANG */
> +#endif /* defined(CONFIG_CFI_CLANG) || defined(CONFIG_CFI_GCC) */
>
> #ifndef SYM_TYPED_FUNC_START
> #define SYM_TYPED_FUNC_START(name) \
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index 9b157b71036f..aec1ce327b1a 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -82,6 +82,10 @@
> #define __noscs __attribute__((__no_sanitize__("shadow-call-stack")))
> #endif
>
> +#ifdef CONFIG_CFI_GCC
> +#define __nocfi __attribute__((no_sanitize("cfi")))
> +#endif
> +
> #if __has_attribute(__no_sanitize_address__)
> #define __no_sanitize_address __attribute__((no_sanitize_address))
> #else
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 318789c728d3..923d3e060852 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -114,6 +114,7 @@ obj-$(CONFIG_SHADOW_CALL_STACK) += scs.o
> obj-$(CONFIG_HAVE_STATIC_CALL) += static_call.o
> obj-$(CONFIG_HAVE_STATIC_CALL_INLINE) += static_call_inline.o
> obj-$(CONFIG_CFI_CLANG) += cfi.o
> +obj-$(CONFIG_CFI_GCC) += cfi.o
>
> obj-$(CONFIG_PERF_EVENTS) += events/
>
> diff --git a/kernel/cfi.c b/kernel/cfi.c
> index 08caad776717..9bff35736756 100644
> --- a/kernel/cfi.c
> +++ b/kernel/cfi.c
> @@ -25,6 +25,7 @@ enum bug_trap_type report_cfi_failure(struct pt_regs *regs, unsigned long addr,
> return BUG_TRAP_TYPE_BUG;
> }
>
> +#ifdef CONFIG_CFI_CLANG
> #ifdef CONFIG_ARCH_USES_CFI_TRAPS
> static inline unsigned long trap_address(s32 *p)
> {
> @@ -99,3 +100,25 @@ bool is_cfi_trap(unsigned long addr)
> return is_module_cfi_trap(addr);
> }
> #endif /* CONFIG_ARCH_USES_CFI_TRAPS */
> +#endif /* CONFIG_CFI_CLANG */
> +
> +
> +#ifdef CONFIG_CFI_GCC
> +void cfi_check_failed(u32 caller_hash, u32 callee_hash, void *callee_addr)
> +{
> + unsigned long pc, target;
> +
> + pc = (unsigned long)__builtin_return_address(0);
> + target = (unsigned long)callee_addr;
> +
> + switch (report_cfi_failure(NULL, pc, &target, caller_hash)) {
> + case BUG_TRAP_TYPE_WARN:
> + break;
> +
> + default:
> + panic("Oops - CFI");
> + }
> +}
> +EXPORT_SYMBOL(cfi_check_failed);
> +
> +#endif /* CONFIG_CFI_GCC */
> diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
> index ccdf0c897f31..ed8db513b918 100644
> --- a/scripts/kallsyms.c
> +++ b/scripts/kallsyms.c
> @@ -119,7 +119,9 @@ static bool is_ignored_symbol(const char *name, char type)
> "__ThumbV7PILongThunk_",
> "__LA25Thunk_", /* mips lld */
> "__microLA25Thunk_",
> - "__kcfi_typeid_", /* CFI type identifiers */
> + "__kcfi_typeid_", /* CFI type identifiers in Clang */
> + "__cfi_", /* CFI type identifiers in GCC */
> + "__pi___cfi", /* CFI type identifiers in GCC */
> NULL
> };
>
> --
> 2.17.1
>