Re: [PATCH v12 12/28] riscv: Implements arch agnostic shadow stack prctls

On Sat, Mar 15, 2025 at 6:51 AM Deepak Gupta debug@rivosinc.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@rivosinc.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@sifive.com

-- 2.34.1

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