[PATCH] riscv: ptrace: reject CFI regset access when extensions are absent

[Chen Pei <cp0613@linux.alibaba.com>]

riscv_cfi_get() and riscv_cfi_set() do not check whether the Zicfilp
or Zicfiss extensions are present. On systems without them,
PTRACE_GETREGSET on REGSET_CFI still succeeds and returns a zeroed
user_cfi_state, misleading debuggers into believing the register set
is available, and PTRACE_SETREGSET silently accepts writes that have
no effect (e.g. clearing SR_ELP).

Reject the access with -EINVAL when neither branch landing pads nor
shadow stack is available to userspace, using the same availability
helpers as the prctl path.

Fixes: 2af7c9cf021c ("riscv/ptrace: expose riscv CFI status and state via ptrace and in core files")
Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
---
This follows the behaviour of the arm64 GCS regset. Unlike vector
registers, no ENODATA case applies here: the CFI status regset has a
valid all-zero representation even when CFI has not been enabled for
the traced task.

is_user_lpad_enabled()/is_user_shstk_enabled() are used instead of a
plain ISA check so that the regset visibility matches what userspace
can actually use via prctl: both helpers also honor the
riscv_nousercfi kernel command line switch.

 arch/riscv/kernel/ptrace.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c
index f336a183667e..566856e42f65 100644
--- a/arch/riscv/kernel/ptrace.c
+++ b/arch/riscv/kernel/ptrace.c
@@ -299,6 +299,9 @@ static int riscv_cfi_get(struct task_struct *target,
 	struct user_cfi_state user_cfi;
 	struct pt_regs *regs;
 
+	if (!is_user_lpad_enabled() && !is_user_shstk_enabled())
+		return -EINVAL;
+
 	memset(&user_cfi, 0, sizeof(user_cfi));
 	regs = task_pt_regs(target);
 
@@ -337,6 +340,9 @@ static int riscv_cfi_set(struct task_struct *target,
 	struct user_cfi_state user_cfi;
 	struct pt_regs *regs;
 
+	if (!is_user_lpad_enabled() && !is_user_shstk_enabled())
+		return -EINVAL;
+
 	regs = task_pt_regs(target);
 
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &user_cfi, 0, -1);
-- 
2.50.1