On Thu, Jan 25, 2024 at 01:24:16PM -0800, Charlie Jenkins wrote:
On Wed, Jan 24, 2024 at 10:21:40PM -0800, debug@rivosinc.com wrote:
From: Deepak Gupta debug@rivosinc.com
As discussed extensively in the changelog for the addition of this syscall on x86 ("x86/shstk: Introduce map_shadow_stack syscall") the existing mmap() and madvise() syscalls do not map entirely well onto the security requirements for guarded control stacks since they lead to windows where memory is allocated but not yet protected or stacks which are not properly and safely initialised. Instead a new syscall map_shadow_stack() has been defined which allocates and initialises a shadow stack page.
This patch implements this syscall for riscv. riscv doesn't require token to be setup by kernel because user mode can do that by itself. However to provide compatiblity and portability with other architectues, user mode can specify token set flag.
Signed-off-by: Deepak Gupta debug@rivosinc.com
arch/riscv/kernel/Makefile | 2 + arch/riscv/kernel/usercfi.c | 150 ++++++++++++++++++++++++++++++++ include/uapi/asm-generic/mman.h | 1 + 3 files changed, 153 insertions(+) create mode 100644 arch/riscv/kernel/usercfi.c
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile index fee22a3d1b53..8c668269e886 100644 --- a/arch/riscv/kernel/Makefile +++ b/arch/riscv/kernel/Makefile @@ -102,3 +102,5 @@ obj-$(CONFIG_COMPAT) += compat_vdso/
obj-$(CONFIG_64BIT) += pi/ obj-$(CONFIG_ACPI) += acpi.o
+obj-$(CONFIG_RISCV_USER_CFI) += usercfi.o diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c new file mode 100644 index 000000000000..35ede2cbc05b --- /dev/null +++ b/arch/riscv/kernel/usercfi.c @@ -0,0 +1,150 @@ +// SPDX-License-Identifier: GPL-2.0 +/*
- Copyright (C) 2023 Rivos, Inc.
Nit: Should be updated to 2024
noted
- Deepak Gupta debug@rivosinc.com
- */
+#include <linux/sched.h> +#include <linux/bitops.h> +#include <linux/types.h> +#include <linux/mm.h> +#include <linux/mman.h> +#include <linux/uaccess.h> +#include <linux/sizes.h> +#include <linux/user.h> +#include <linux/syscalls.h> +#include <linux/prctl.h> +#include <asm/csr.h> +#include <asm/usercfi.h>
+#define SHSTK_ENTRY_SIZE sizeof(void *)
+/*
- Writes on shadow stack can either be `sspush` or `ssamoswap`. `sspush` can happen
- implicitly on current shadow stack pointed to by CSR_SSP. `ssamoswap` takes pointer to
- shadow stack. To keep it simple, we plan to use `ssamoswap` to perform writes on shadow
- stack.
- */
+static noinline unsigned long amo_user_shstk(unsigned long *addr, unsigned long val) +{
- /*
* In case ssamoswap faults, return -1.* Never expect -1 on shadow stack. Expect return addresses and zero*/- unsigned long swap = -1;
- __enable_user_access();
- asm_volatile_goto(
".option push\n"".option arch, +zicfiss\n"+#ifdef CONFIG_64BIT
"1: ssamoswap.d %0, %2, %1\n"+#else
"1: ssamoswap.w %0, %2, %1\n"A SSAMOSWAP macro that conditionally defines this would be cleaner
Yes I need to do that. Infact I need to gate CONFIG_RISCV_USER_CFI behind some riscv-gnu toolchain version as well. Becuase not all toolchain versions will recognize this.
+#endif
_ASM_EXTABLE(1b, %l[fault])RISCV_ACQUIRE_BARRIER".option pop\n": "=r" (swap), "+A" (*addr)I just ran into this on one of my patches that not every compiler supports output args in asm goto blocks. You need to guard this with the kconfig option CC_HAS_ASM_GOTO_TIED_OUTPUT. Unfortunately, that means that this code needs two versions, or you can choose to gate CFI behind this option, it's supported by recent versions of GCC/CLANG.
Thanks. I'll gate behind CC_HAS_ASM_GOTO_TIED_OUTPUT. Earlier versions of GCC/CLANG won't have CFI support in them anyways.
For readability it is also nice to use labels for the asm variables such as `"=r" (swap)` can be `[swap] "=r" (swap)` and then replace %0 with %[swap].
noted, will do that. I copied it from gcc asm snippet `amoswap` somewhere in kernel. Goes without saying, I am terrible with gcc asm syntax.
- Charlie
: "r" (val): "memory": fault