riscv: Implement arch-agnostic shadow stack prctls

Implement an architecture-agnostic prctl() interface for setting and
getting shadow stack status.  The 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 the current shadow stack enablement
configuration.

Reviewed-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de> # QEMU, custom CVA6
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-12-b55691eacf4f@rivosinc.com
[pjw@kernel.org: cleaned up patch description]
Signed-off-by: Paul Walmsley <pjw@kernel.org>

diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
index a16a5dff8b0ec44b9866676a534e5ee68f6e6cb1..d71093f414df1eff932eeb4fcee21bf967027f28 100644 (file)
--- a/arch/riscv/include/asm/usercfi.h
+++ b/arch/riscv/include/asm/usercfi.h
@@ -7,6 +7,7 @@
 
 #ifndef __ASSEMBLER__
 #include <linux/types.h>
+#include <linux/prctl.h>
 
 struct task_struct;
 struct kernel_clone_args;
@@ -14,6 +15,7 @@ struct kernel_clone_args;
 #ifdef CONFIG_RISCV_USER_CFI
 struct cfi_state {
        unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
+       unsigned long ubcfi_locked : 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 */
@@ -26,6 +28,12 @@ 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
 
@@ -41,6 +49,14 @@ 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) do {} while (0)
+
+#define set_shstk_status(task, enable) do {} while (0)
+
 #endif /* CONFIG_RISCV_USER_CFI */
 
 #endif /* __ASSEMBLER__ */

diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 72d35adc6e0efb70f3ee8a4b10c998dad45283db..a137d3483646219afeab164da0eb34db518273fe 100644 (file)
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -156,6 +156,14 @@ 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 59cdd6f69d3f5429f5b430173ca92875ebaff158..778930f79cfa302179fb78455e8cfdaa087ecedd 100644 (file)
--- 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;
 }
 
+bool is_shstk_allocated(struct task_struct *task)
+{
+       return task->thread_info.user_cfi_state.shdw_stk_base;
+}
+
+bool is_shstk_locked(struct task_struct *task)
+{
+       return task->thread_info.user_cfi_state.ubcfi_locked;
+}
+
 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
@@ -258,3 +288,83 @@ 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;
+}