riscv/signal: save and restore the shadow stack on a signal

Save the shadow stack pointer in the sigcontext structure when
delivering a signal.  Restore the shadow stack pointer from sigcontext
on sigreturn.

As part of the save operation, the kernel uses the 'ssamoswap'
instruction to save a snapshot of the current shadow stack on the
shadow stack itself (this can be called a "save token"). During
restore on sigreturn, the kernel retrieves the save token from the top
of the shadow stack and validates it. This ensures that user mode
can't arbitrarily pivot to any shadow stack address without having a
token and thus provides a strong security assurance during the window
between signal delivery and sigreturn.

Use an ABI-compatible way of saving/restoring the shadow stack pointer
into the signal stack. This follows the vector extension, where extra
registers are placed in a form of extension header + extension body in
the stack.  The extension header indicates the size of the extra
architectural states plus the size of header itself, and a magic
identifier for the extension. Then, the extension body contains the
new architectural states in the form defined by uapi.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de>
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-17-b55691eacf4f@rivosinc.com
[pjw@kernel.org: cleaned patch description, code comments; resolved checkpatch warning]
Signed-off-by: Paul Walmsley <pjw@kernel.org>

3 files changed, 15 insertions, 0 deletions

diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
index 4501d741a609..ec4b8a53eb74 100644
--- a/arch/riscv/include/asm/usercfi.h
+++ b/arch/riscv/include/asm/usercfi.h
@@ -8,6 +8,7 @@
 #ifndef __ASSEMBLER__
 #include <linux/types.h>
 #include <linux/prctl.h>
+#include <linux/errno.h>
 
 struct task_struct;
 struct kernel_clone_args;
@@ -34,6 +35,9 @@ 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);
+unsigned long get_active_shstk(struct task_struct *task);
+int restore_user_shstk(struct task_struct *tsk, unsigned long shstk_ptr);
+int save_user_shstk(struct task_struct *tsk, unsigned long *saved_shstk_ptr);
 bool is_indir_lp_enabled(struct task_struct *task);
 bool is_indir_lp_locked(struct task_struct *task);
 void set_indir_lp_status(struct task_struct *task, bool enable);
@@ -71,6 +75,12 @@ void set_indir_lp_lock(struct task_struct *task);
 
 #define set_indir_lp_lock(task) do {} while (0)
 
+#define restore_user_shstk(tsk, shstk_ptr) -EINVAL
+
+#define save_user_shstk(tsk, saved_shstk_ptr) -EINVAL
+
+#define get_active_shstk(task) 0UL
+
 #endif /* CONFIG_RISCV_USER_CFI */
 
 #endif /* __ASSEMBLER__ */
diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h
index beff8df80ac9..261bfe70f60a 100644
--- a/arch/riscv/include/uapi/asm/ptrace.h
+++ b/arch/riscv/include/uapi/asm/ptrace.h
@@ -127,6 +127,10 @@ struct __riscv_v_regset_state {
  */
 #define RISCV_MAX_VLENB (8192)
 
+struct __sc_riscv_cfi_state {
+	unsigned long ss_ptr;   /* shadow stack pointer */
+};
+
 #endif /* __ASSEMBLER__ */
 
 #endif /* _UAPI_ASM_RISCV_PTRACE_H */
diff --git a/arch/riscv/include/uapi/asm/sigcontext.h b/arch/riscv/include/uapi/asm/sigcontext.h
index 748dffc9ae19..d22d0815d605 100644
--- a/arch/riscv/include/uapi/asm/sigcontext.h
+++ b/arch/riscv/include/uapi/asm/sigcontext.h
@@ -10,6 +10,7 @@
 
 /* The Magic number for signal context frame header. */
 #define RISCV_V_MAGIC	0x53465457
+#define RISCV_ZICFISS_MAGIC		0x9487
 #define END_MAGIC	0x0
 
 /* The size of END signal context header. */