Re: [PATCH v3 07/29] riscv: usercfi state for task and save/restore of CSR_SSP on trap entry/exit

[Charlie Jenkins <charlie@rivosinc.com>]

On Wed, Apr 03, 2024 at 04:34:55PM -0700, Deepak Gupta wrote:
> 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)
> 
> Shadow stack save/restore is gated on feature availiblity and implemented
> using alternative. CSR can be context switched in `switch_to` as well but
> soon as kernel shadow stack support gets rolled in, shadow stack pointer
> will need to be switched at trap entry/exit point (much like `sp`). It can
> be argued that kernel using shadow stack deployment scenario may not be as
> prevalant as user mode using this feature. But even if there is some
> minimal deployment of kernel shadow stack, that means that it needs to be
> supported. And thus save/restore of shadow stack pointer in entry.S instead
> of in `switch_to.h`.
> 
> 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      |  4 ++++
>  arch/riscv/kernel/entry.S            | 26 ++++++++++++++++++++++++++
>  5 files changed, 58 insertions(+)
>  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 6c5b3d928b12..f8decf357804 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 a503bdc2f6dd..f1dee307806e 100644
> --- a/arch/riscv/include/asm/thread_info.h
> +++ b/arch/riscv/include/asm/thread_info.h
> @@ -57,6 +57,9 @@ 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..4fa201b4fc4e
> --- /dev/null
> +++ b/arch/riscv/include/asm/usercfi.h
> @@ -0,0 +1,24 @@
> +/* SPDX-License-Identifier: GPL-2.0
> + * Copyright (C) 2024 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 a03129f40c46..5c5ea015c776 100644
> --- a/arch/riscv/kernel/asm-offsets.c
> +++ b/arch/riscv/kernel/asm-offsets.c
> @@ -44,6 +44,10 @@ void asm_offsets(void)
>  #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 9d1a305d5508..7245a0ea25c1 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -60,6 +60,20 @@ SYM_CODE_START(handle_exception)
>  
>  	REG_L s0, TASK_TI_USER_SP(tp)
>  	csrrc s1, CSR_STATUS, t0
> +	/*
> +	 * If previous mode was U, capture shadow stack pointer and save it away
> +	 * Zero CSR_SSP at the same time for sanitization.
> +	 */
> +	ALTERNATIVE("nop; nop; nop; nop",
> +				__stringify(			\
> +				andi s2, s1, SR_SPP;	\
> +				bnez s2, skip_ssp_save;	\
> +				csrrw s2, CSR_SSP, x0;	\
> +				REG_S s2, TASK_TI_USER_SSP(tp); \
> +				skip_ssp_save:),
> +				0,
> +				RISCV_ISA_EXT_ZICFISS,
> +				CONFIG_RISCV_USER_CFI)
>  	csrr s2, CSR_EPC
>  	csrr s3, CSR_TVAL
>  	csrr s4, CSR_CAUSE
> @@ -141,6 +155,18 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
>  	 * structures again.
>  	 */
>  	csrw CSR_SCRATCH, tp
> +
> +	/*
> +	 * Going back to U mode, restore shadow stack pointer
> +	 */
> +	ALTERNATIVE("nop; nop",
> +				__stringify(					\
> +				REG_L s3, TASK_TI_USER_SSP(tp); \
> +				csrw CSR_SSP, s3),
> +				0,
> +				RISCV_ISA_EXT_ZICFISS,
> +				CONFIG_RISCV_USER_CFI)
> +
>  1:
>  #ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
>  	move a0, sp
> -- 
> 2.43.2
> 

Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>