[PATCH v3 1/4] arm64/gcs: release GCS on failed fork

["Iván Ezequiel Rodriguez" <ivanrwcm25@gmail.com>]

copy_thread() can map a CLONE_VM GCS and still return 0. A later
copy_process() failure calls exit_thread(), but arm64 did not
implement it, so the VMA stayed in the parent's mm. Wire
HAVE_EXIT_THREAD to gcs_free().

x86 already does this via exit_thread() -> shstk_free(). The separate
x86 TLS leak is different: copy_thread() itself fails after the
alloc, so copy_process() skips exit_thread().

args->fn tasks inherit GCS tracking without allocating; clear it so
a failed CLONE_VM copy_process() cannot gcs_free() the caller's
stack. Keep that helper in process.c with the other fork helpers.

Fixes: 506496bcbb42 ("arm64/gcs: Ensure that new threads have a GCS")
Cc: stable@vger.kernel.org
Cc: Mark Brown <broonie@kernel.org>
Assisted-by: LLM
Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>
---
 arch/arm64/Kconfig          |  1 +
 arch/arm64/kernel/process.c | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index cfef22aee709..3010ced4b4f2 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -177,6 +177,7 @@ config ARM64
 	select HAVE_ARM_SMCCC
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_EBPF_JIT
+	select HAVE_EXIT_THREAD
 	select HAVE_C_RECORDMCOUNT
 	select HAVE_CMPXCHG_DOUBLE
 	select HAVE_CMPXCHG_LOCAL
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 581f80e9b9b7..d17172e60acb 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -320,6 +320,13 @@ static int copy_thread_gcs(struct task_struct *p,
 	return 0;
 }
 
+static void gcs_reset_thread(struct task_struct *task)
+{
+	task->thread.gcs_base = 0;
+	task->thread.gcs_size = 0;
+	task->thread.gcspr_el0 = 0;
+}
+
 #else
 
 static void flush_gcs(void) { }
@@ -328,6 +335,7 @@ static int copy_thread_gcs(struct task_struct *p,
 {
 	return 0;
 }
+static void gcs_reset_thread(struct task_struct *task) { }
 
 #endif
 
@@ -341,6 +349,12 @@ void flush_thread(void)
 	flush_gcs();
 }
 
+void exit_thread(struct task_struct *tsk)
+{
+	/* Failed fork after copy_thread(); deactivate_mm() does not run. */
+	gcs_free(tsk);
+}
+
 void arch_release_task_struct(struct task_struct *tsk)
 {
 	fpsimd_release_task(tsk);
@@ -503,6 +517,8 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 
 		p->thread.cpu_context.x19 = (unsigned long)args->fn;
 		p->thread.cpu_context.x20 = (unsigned long)args->fn_arg;
+		/* No user GCS alloc here; drop inherited tracking for CLONE_VM. */
+		gcs_reset_thread(p);
 
 		if (system_supports_poe())
 			p->thread.por_el0 = POR_EL0_INIT;
-- 
2.43.0