Message-ID: <CANXhq0oNc2=dXbNmBN29JUQu1FYn3fVSJ8NWYU7jiGQ5qDRPsw@mail.gmail.com>
Date: Mon, 17 Mar 2025 09:29:00 +0800
From: Zong Li <zong.li@...ive.com>
To: Deepak Gupta <debug@...osinc.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
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Subject: Re: [PATCH v12 12/28] riscv: Implements arch agnostic shadow stack prctls

On Sat, Mar 15, 2025 at 6:51 AM Deepak Gupta <debug@...osinc.com> wrote:
>
> Implement architecture agnostic prctls() interface for setting and getting
> shadow stack status.
>
> prctls implemented are PR_GET_SHADOW_STACK_STATUS,
> PR_SET_SHADOW_STACK_STATUS and PR_LOCK_SHADOW_STACK_STATUS.
>
> As part of PR_SET_SHADOW_STACK_STATUS/PR_GET_SHADOW_STACK_STATUS, only
> PR_SHADOW_STACK_ENABLE is implemented because RISCV allows each mode to
> write to their own shadow stack using `sspush` or `ssamoswap`.
>
> PR_LOCK_SHADOW_STACK_STATUS locks current configuration of shadow stack
> enabling.
>
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>  arch/riscv/include/asm/usercfi.h |  18 ++++++-
>  arch/riscv/kernel/process.c      |   8 +++
>  arch/riscv/kernel/usercfi.c      | 110 +++++++++++++++++++++++++++++++++++++++
>  3 files changed, 135 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
> index 82d28ac98d76..c4dcd256f19a 100644
> --- a/arch/riscv/include/asm/usercfi.h
> +++ b/arch/riscv/include/asm/usercfi.h
> @@ -7,6 +7,7 @@
>
>  #ifndef __ASSEMBLY__
>  #include <linux/types.h>
> +#include <linux/prctl.h>
>
>  struct task_struct;
>  struct kernel_clone_args;
> @@ -14,7 +15,8 @@ struct kernel_clone_args;
>  #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 ubcfi_locked : 1;
> +       unsigned long rsvd : ((sizeof(unsigned long) * 8) - 2);
>         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 */
> @@ -27,6 +29,12 @@ void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, unsigned
>  unsigned long get_shstk_base(struct task_struct *task, unsigned long *size);
>  void set_active_shstk(struct task_struct *task, unsigned long shstk_addr);
>  bool is_shstk_enabled(struct task_struct *task);
> +bool is_shstk_locked(struct task_struct *task);
> +bool is_shstk_allocated(struct task_struct *task);
> +void set_shstk_lock(struct task_struct *task);
> +void set_shstk_status(struct task_struct *task, bool enable);
> +
> +#define PR_SHADOW_STACK_SUPPORTED_STATUS_MASK (PR_SHADOW_STACK_ENABLE)
>
>  #else
>
> @@ -42,6 +50,14 @@ bool is_shstk_enabled(struct task_struct *task);
>
>  #define is_shstk_enabled(task) false
>
> +#define is_shstk_locked(task) false
> +
> +#define is_shstk_allocated(task) false
> +
> +#define set_shstk_lock(task)
> +
> +#define set_shstk_status(task, enable)
> +
>  #endif /* CONFIG_RISCV_USER_CFI */
>
>  #endif /* __ASSEMBLY__ */
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index 99acb6342a37..cd11667593fe 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -153,6 +153,14 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
>         regs->epc = pc;
>         regs->sp = sp;
>
> +       /*
> +        * clear shadow stack state on exec.
> +        * libc will set it later via prctl.
> +        */
> +       set_shstk_status(current, false);
> +       set_shstk_base(current, 0, 0);
> +       set_active_shstk(current, 0);
> +
>  #ifdef CONFIG_64BIT
>         regs->status &= ~SR_UXL;
>
> diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
> index 73cf87dab186..b93b324eed26 100644
> --- a/arch/riscv/kernel/usercfi.c
> +++ b/arch/riscv/kernel/usercfi.c
> @@ -24,6 +24,16 @@ bool is_shstk_enabled(struct task_struct *task)
>         return task->thread_info.user_cfi_state.ubcfi_en ? true : false;
>  }
>
> +bool is_shstk_allocated(struct task_struct *task)
> +{
> +       return task->thread_info.user_cfi_state.shdw_stk_base ? true : false;
> +}
> +
> +bool is_shstk_locked(struct task_struct *task)
> +{
> +       return task->thread_info.user_cfi_state.ubcfi_locked ? true : false;
> +}
> +
>  void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, unsigned long size)
>  {
>         task->thread_info.user_cfi_state.shdw_stk_base = shstk_addr;
> @@ -42,6 +52,26 @@ void set_active_shstk(struct task_struct *task, unsigned long shstk_addr)
>         task->thread_info.user_cfi_state.user_shdw_stk = shstk_addr;
>  }
>
> +void set_shstk_status(struct task_struct *task, bool enable)
> +{
> +       if (!cpu_supports_shadow_stack())
> +               return;
> +
> +       task->thread_info.user_cfi_state.ubcfi_en = enable ? 1 : 0;
> +
> +       if (enable)
> +               task->thread.envcfg |= ENVCFG_SSE;
> +       else
> +               task->thread.envcfg &= ~ENVCFG_SSE;
> +
> +       csr_write(CSR_ENVCFG, task->thread.envcfg);
> +}
> +
> +void set_shstk_lock(struct task_struct *task)
> +{
> +       task->thread_info.user_cfi_state.ubcfi_locked = 1;
> +}
> +
>  /*
>   * If size is 0, then to be compatible with regular stack we want it to be as big as
>   * regular stack. Else PAGE_ALIGN it and return back
> @@ -262,3 +292,83 @@ void shstk_release(struct task_struct *tsk)
>         vm_munmap(base, size);
>         set_shstk_base(tsk, 0, 0);
>  }
> +
> +int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *status)
> +{
> +       unsigned long bcfi_status = 0;
> +
> +       if (!cpu_supports_shadow_stack())
> +               return -EINVAL;
> +
> +       /* this means shadow stack is enabled on the task */
> +       bcfi_status |= (is_shstk_enabled(t) ? PR_SHADOW_STACK_ENABLE : 0);
> +
> +       return copy_to_user(status, &bcfi_status, sizeof(bcfi_status)) ? -EFAULT : 0;
> +}
> +
> +int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status)
> +{
> +       unsigned long size = 0, addr = 0;
> +       bool enable_shstk = false;
> +
> +       if (!cpu_supports_shadow_stack())
> +               return -EINVAL;
> +
> +       /* Reject unknown flags */
> +       if (status & ~PR_SHADOW_STACK_SUPPORTED_STATUS_MASK)
> +               return -EINVAL;
> +
> +       /* bcfi status is locked and further can't be modified by user */
> +       if (is_shstk_locked(t))
> +               return -EINVAL;
> +
> +       enable_shstk = status & PR_SHADOW_STACK_ENABLE;
> +       /* Request is to enable shadow stack and shadow stack is not enabled already */
> +       if (enable_shstk && !is_shstk_enabled(t)) {
> +               /* shadow stack was allocated and enable request again
> +                * no need to support such usecase and return EINVAL.
> +                */
> +               if (is_shstk_allocated(t))
> +                       return -EINVAL;
> +
> +               size = calc_shstk_size(0);
> +               addr = allocate_shadow_stack(0, size, 0, false);
> +               if (IS_ERR_VALUE(addr))
> +                       return -ENOMEM;
> +               set_shstk_base(t, addr, size);
> +               set_active_shstk(t, addr + size);
> +       }
> +
> +       /*
> +        * If a request to disable shadow stack happens, let's go ahead and release it
> +        * Although, if CLONE_VFORKed child did this, then in that case we will end up
> +        * not releasing the shadow stack (because it might be needed in parent). Although
> +        * we will disable it for VFORKed child. And if VFORKed child tries to enable again
> +        * then in that case, it'll get entirely new shadow stack because following condition
> +        * are true
> +        *  - shadow stack was not enabled for vforked child
> +        *  - shadow stack base was anyways pointing to 0
> +        * This shouldn't be a big issue because we want parent to have availability of shadow
> +        * stack whenever VFORKed child releases resources via exit or exec but at the same
> +        * time we want VFORKed child to break away and establish new shadow stack if it desires
> +        *
> +        */
> +       if (!enable_shstk)
> +               shstk_release(t);
> +
> +       set_shstk_status(t, enable_shstk);
> +       return 0;
> +}
> +
> +int arch_lock_shadow_stack_status(struct task_struct *task,
> +                                 unsigned long arg)
> +{
> +       /* If shtstk not supported or not enabled on task, nothing to lock here */
> +       if (!cpu_supports_shadow_stack() ||
> +           !is_shstk_enabled(task) || arg != 0)
> +               return -EINVAL;
> +
> +       set_shstk_lock(task);
> +
> +       return 0;
> +}
>

LGTM

Reviewed-by: Zong Li <zong.li@...ive.com>

> --
> 2.34.1
>
>
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