[PATCH v3 2/4] riscv/shstk: release shadow stack on failed fork

[Iván Ezequiel Rodriguez]

Same hole as arm64: copy_thread() maps a CLONE_VM shadow stack,
copy_process() fails later, exit_thread() ran but was never
implemented. Select HAVE_EXIT_THREAD and call shstk_release().

shstk_release() already documented this path; connect it. Clear
inherited tracking on the args->fn path for the same CLONE_VM
reason as arm64.

Fixes: fd44a4a85516 ("riscv/shstk: If needed allocate a new shadow stack on clone")
Cc: stable@xxxxxxxxxxxxxxx
Cc: Deepak Gupta <debug@xxxxxxxxxxxx>
Assisted-by: LLM
Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@xxxxxxxxx>
---
 arch/riscv/Kconfig          | 1 +
 arch/riscv/kernel/process.c | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 6c9d61d83d71..ba1ee563aa54 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -168,6 +168,7 @@ config RISCV
 	select HAVE_FUNCTION_GRAPH_FREGS
 	select HAVE_FUNCTION_TRACER if HAVE_DYNAMIC_FTRACE
 	select HAVE_EBPF_JIT if MMU
+	select HAVE_EXIT_THREAD
 	select HAVE_GENERIC_TIF_BITS
 	select HAVE_GUP_FAST if MMU
 	select HAVE_FUNCTION_ARG_ACCESS_API
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 7cc5a6a5c020..5bb93377d67f 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -12,6 +12,7 @@
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/sched/debug.h>
+#include <linux/sched/task.h>
 #include <linux/sched/task_stack.h>
 #include <linux/tick.h>
 #include <linux/ptrace.h>
@@ -213,6 +214,12 @@ void arch_release_task_struct(struct task_struct *tsk)
 		riscv_v_thread_free(tsk);
 }
 
+void exit_thread(struct task_struct *tsk)
+{
+	/* Failed fork after copy_thread(); deactivate_mm() does not run. */
+	shstk_release(tsk);
+}
+
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 {
 	fstate_save(src, task_pt_regs(src));
@@ -261,6 +268,8 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		p->thread.s[0] = (unsigned long)args->fn;
 		p->thread.s[1] = (unsigned long)args->fn_arg;
 		p->thread.ra = (unsigned long)ret_from_fork_kernel_asm;
+		/* No user shstk alloc here; drop inherited tracking for CLONE_VM. */
+		set_shstk_base(p, 0, 0);
 	} else {
 		/* allocate new shadow stack if needed. In case of CLONE_VM we have to */
 		ssp = shstk_alloc_thread_stack(p, args);
-- 
2.43.0