FAILED: patch "[PATCH] arm64/gcs: release GCS on failed fork" failed to apply to 6.18-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 078e122f3bf000dd45965997b66deac10ee580fa
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107051555-failed-6.18-078e122f3bf0@kernel.org' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 078e122f3bf000dd45965997b66deac10ee580fa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Iv=C3=A1n=20Ezequiel=20Rodriguez?= <ivanrwcm25@gmail.com>
Date: Wed, 28 Oct 2026 12:43:12 -0300
Subject: [PATCH] arm64/gcs: release GCS on failed fork
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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().

Kernel threads created with CLONE_VM (io_uring and vhost workers)
inherit the parent's gcs_base/gcs_size from arch_dup_task_struct()
without allocating their own GCS. When such a thread exits,
exit_mm() -> mm_release() -> deactivate_mm() -> gcs_free() unmaps the
parent's GCS. With the new exit_thread(), a failed copy_process() for
one of these threads would do the same. Clear these fields in
copy_thread().

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>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
---
 arch/arm64/Kconfig          |  1 +
 arch/arm64/kernel/process.c | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 8593ad10a9c48..7587cf86d9411 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -199,6 +199,7 @@ config ARM64
 	select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
 	select HAVE_BUILDTIME_MCOUNT_SORT
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS
+	select HAVE_EXIT_THREAD
 	select HAVE_GUP_FAST
 	select HAVE_FTRACE_GRAPH_FUNC
 	select HAVE_FUNCTION_TRACER
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 581f80e9b9b73..42b15b0c069ff 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -320,9 +320,18 @@ 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) { }
+static void gcs_reset_thread(struct task_struct *task) { }
+
 static int copy_thread_gcs(struct task_struct *p,
 			   const struct kernel_clone_args *args)
 {
@@ -341,6 +350,12 @@ 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 +518,8 @@ 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;
+		/* Kernel threads sharing the parent's mm must not free its GCS. */
+		gcs_reset_thread(p);
 
 		if (system_supports_poe())
 			p->thread.por_el0 = POR_EL0_INIT;
-- 
2.53.0