[PATCH v3 18/29] riscv: Implements arch agnostic shadow stack prctls

Deepak Gupta
2024-04-04T00:49:39Z

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 | 107 +++++++++++++++++++++++++++++++
3 files changed, 132 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
index b47574a7a8c9..a168ae0fa5d8 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 */
@@ -26,6 +28,10 @@ void shstk_release(struct task_struct *tsk);
void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, 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);
+void set_shstk_status(struct task_struct *task, bool enable);
+
+#define PR_SHADOW_STACK_SUPPORTED_STATUS_MASK (PR_SHADOW_STACK_ENABLE)

#else

@@ -56,6 +62,16 @@ static inline bool is_shstk_enabled(struct task_struct *task)
return false;
}

+static inline bool is_shstk_locked(struct task_struct *task)
+{
+ return false;
+}
+
+static inline void set_shstk_status(struct task_struct *task, bool enable)
+{
+
+}
+
#endif /* CONFIG_RISCV_USER_CFI */

#endif /* __ASSEMBLY__ */
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index ef48a25b0eff..3fb8b23f629b 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -145,6 +145,14 @@ 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 11ef7ab925c9..cdedf1f78b3e 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 +52,23 @@ 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)
+{
+ task->thread_info.user_cfi_state.ubcfi_en = enable ? 1 : 0;
+
+ if (enable)
+ task->thread_info.envcfg |= ENVCFG_SSE;
+ else
+ task->thread_info.envcfg &= ~ENVCFG_SSE;
+
+ csr_write(CSR_ENVCFG, task->thread_info.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
@@ -268,3 +295,83 @@ 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))
+ return -EINVAL;
+
+ set_shstk_lock(task);
+
+ return 0;
+}
--
2.43.2