Re: [RFC PATCH v1 07/28] riscv: kernel handling on trap entry/exit for user cfi

Stefan O'Rear
2024-01-25T07:29:24Z

On Thu, Jan 25, 2024, at 1:21 AM, debug@rivosinc.com wrote:
> From: Deepak Gupta <debug@rivosinc.com>
>
> Carves out space in arch specific thread struct for cfi status and shadow stack
> in usermode on riscv.
>
> This patch does following
> - defines a new structure cfi_status with status bit for cfi feature
> - defines shadow stack pointer, base and size in cfi_status structure
> - defines offsets to new member fields in thread in asm-offsets.c
> - Saves and restore shadow stack pointer on trap entry (U --> S) and exit
> (S --> U)
>
> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> ---
> arch/riscv/include/asm/processor.h | 1 +
> arch/riscv/include/asm/thread_info.h | 3 +++
> arch/riscv/include/asm/usercfi.h | 24 ++++++++++++++++++++++++
> arch/riscv/kernel/asm-offsets.c | 5 ++++-
> arch/riscv/kernel/entry.S | 25 +++++++++++++++++++++++++
> 5 files changed, 57 insertions(+), 1 deletion(-)
> create mode 100644 arch/riscv/include/asm/usercfi.h
>
> diff --git a/arch/riscv/include/asm/processor.h
> b/arch/riscv/include/asm/processor.h
> index ee2f51787ff8..d4dc298880fc 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -14,6 +14,7 @@
>
> #include <asm/ptrace.h>
> #include <asm/hwcap.h>
> +#include <asm/usercfi.h>
>
> #ifdef CONFIG_64BIT
> #define DEFAULT_MAP_WINDOW (UL(1) << (MMAP_VA_BITS - 1))
> diff --git a/arch/riscv/include/asm/thread_info.h
> b/arch/riscv/include/asm/thread_info.h
> index 320bc899a63b..6a2acecec546 100644
> --- a/arch/riscv/include/asm/thread_info.h
> +++ b/arch/riscv/include/asm/thread_info.h
> @@ -58,6 +58,9 @@ struct thread_info {
> int cpu;
> unsigned long syscall_work; /* SYSCALL_WORK_ flags */
> unsigned long envcfg;
> +#ifdef CONFIG_RISCV_USER_CFI
> + struct cfi_status user_cfi_state;
> +#endif
> #ifdef CONFIG_SHADOW_CALL_STACK
> void *scs_base;
> void *scs_sp;
> diff --git a/arch/riscv/include/asm/usercfi.h
> b/arch/riscv/include/asm/usercfi.h
> new file mode 100644
> index 000000000000..080d7077d12c
> --- /dev/null
> +++ b/arch/riscv/include/asm/usercfi.h
> @@ -0,0 +1,24 @@
> +/* SPDX-License-Identifier: GPL-2.0
> + * Copyright (C) 2023 Rivos, Inc.
> + * Deepak Gupta <debug@rivosinc.com>
> + */
> +#ifndef _ASM_RISCV_USERCFI_H
> +#define _ASM_RISCV_USERCFI_H
> +
> +#ifndef __ASSEMBLY__
> +#include <linux/types.h>
> +
> +#ifdef CONFIG_RISCV_USER_CFI
> +struct cfi_status {
> + unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
> + unsigned long rsvd : ((sizeof(unsigned long)*8) - 1);
> + unsigned long user_shdw_stk; /* Current user shadow stack pointer */
> + unsigned long shdw_stk_base; /* Base address of shadow stack */
> + unsigned long shdw_stk_size; /* size of shadow stack */
> +};
> +
> +#endif /* CONFIG_RISCV_USER_CFI */
> +
> +#endif /* __ASSEMBLY__ */
> +
> +#endif /* _ASM_RISCV_USERCFI_H */
> diff --git a/arch/riscv/kernel/asm-offsets.c
> b/arch/riscv/kernel/asm-offsets.c
> index cdd8f095c30c..5e1f412e96ba 100644
> --- a/arch/riscv/kernel/asm-offsets.c
> +++ b/arch/riscv/kernel/asm-offsets.c
> @@ -43,8 +43,11 @@ void asm_offsets(void)
> #ifdef CONFIG_SHADOW_CALL_STACK
> OFFSET(TASK_TI_SCS_SP, task_struct, thread_info.scs_sp);
> #endif
> -
> OFFSET(TASK_TI_CPU_NUM, task_struct, thread_info.cpu);
> +#ifdef CONFIG_RISCV_USER_CFI
> + OFFSET(TASK_TI_CFI_STATUS, task_struct, thread_info.user_cfi_state);
> + OFFSET(TASK_TI_USER_SSP, task_struct,
> thread_info.user_cfi_state.user_shdw_stk);
> +#endif
> OFFSET(TASK_THREAD_F0, task_struct, thread.fstate.f[0]);
> OFFSET(TASK_THREAD_F1, task_struct, thread.fstate.f[1]);
> OFFSET(TASK_THREAD_F2, task_struct, thread.fstate.f[2]);
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 63c3855ba80d..410659e2eadb 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -49,6 +49,21 @@ SYM_CODE_START(handle_exception)
> REG_S x5, PT_T0(sp)
> save_from_x6_to_x31
>
> +#ifdef CONFIG_RISCV_USER_CFI
> + /*
> + * we need to save cfi status only when previous mode was U
> + */
> + csrr s2, CSR_STATUS
> + andi s2, s2, SR_SPP
> + bnez s2, skip_bcfi_save
> + /* load cfi status word */
> + lw s3, TASK_TI_CFI_STATUS(tp)
> + andi s3, s3, 1
> + beqz s3, skip_bcfi_save
> + csrr s3, CSR_SSP
> + REG_S s3, TASK_TI_USER_SSP(tp) /* save user ssp in thread_info */
> +skip_bcfi_save:
> +#endif
> /*
> * Disable user-mode memory access as it should only be set in the
> * actual user copy routines.
> @@ -141,6 +156,16 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
> * structures again.
> */
> csrw CSR_SCRATCH, tp
> +
> +#ifdef CONFIG_RISCV_USER_CFI
> + lw s3, TASK_TI_CFI_STATUS(tp)
> + andi s3, s3, 1
> + beqz s3, skip_bcfi_resume
> + REG_L s3, TASK_TI_USER_SSP(tp) /* restore user ssp from thread struct */
> + csrw CSR_SSP, s3
> +skip_bcfi_resume:
> +#endif
> +

We shouldn't need any of this in the entry/exit code, at least as long as
the kernel itself is not using Zicfiss. ssp can keep its value in the
kernel and swap it on task switches. Our entry/exit code is rather short
and I'd like to keep it that way.

-s

> 1:
> REG_L a0, PT_STATUS(sp)
> /*
> --
> 2.43.0
>
>
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