arm64/gcs: Context switch GCS state for EL0

There are two registers controlling the GCS state of EL0, GCSPR_EL0 which
is the current GCS pointer and GCSCRE0_EL1 which has enable bits for the
specific GCS functionality enabled for EL0. Manage these on context switch
and process lifetime events, GCS is reset on exec().  Also ensure that
any changes to the GCS memory are visible to other PEs and that changes
from other PEs are visible on this one by issuing a GCSB DSYNC when
moving to or from a thread with GCS.

Since the current GCS configuration of a thread will be visible to
userspace we store the configuration in the format used with userspace
and provide a helper which configures the system register as needed.

On systems that support GCS we always allow access to GCSPR_EL0, this
facilitates reporting of GCS faults if userspace implements disabling of
GCS on error - the GCS can still be discovered and examined even if GCS
has been disabled.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-21-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

2 files changed, 43 insertions, 0 deletions

diff --git a/arch/arm64/mm/Makefile b/arch/arm64/mm/Makefile
index 2fc8c6dd0407..fc92170a8f37 100644
--- a/arch/arm64/mm/Makefile
+++ b/arch/arm64/mm/Makefile
@@ -11,6 +11,7 @@ obj-$(CONFIG_TRANS_TABLE)	+= trans_pgd.o
 obj-$(CONFIG_TRANS_TABLE)	+= trans_pgd-asm.o
 obj-$(CONFIG_DEBUG_VIRTUAL)	+= physaddr.o
 obj-$(CONFIG_ARM64_MTE)		+= mteswap.o
+obj-$(CONFIG_ARM64_GCS)		+= gcs.o
 KASAN_SANITIZE_physaddr.o	+= n
 
 obj-$(CONFIG_KASAN)		+= kasan_init.o
diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
new file mode 100644
index 000000000000..f8f4f984a247
--- /dev/null
+++ b/arch/arm64/mm/gcs.c
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/mm.h>
+#include <linux/mman.h>
+#include <linux/syscalls.h>
+#include <linux/types.h>
+
+#include <asm/cpufeature.h>
+#include <asm/page.h>
+
+/*
+ * Apply the GCS mode configured for the specified task to the
+ * hardware.
+ */
+void gcs_set_el0_mode(struct task_struct *task)
+{
+	u64 gcscre0_el1 = GCSCRE0_EL1_nTR;
+
+	if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_ENABLE)
+		gcscre0_el1 |= GCSCRE0_EL1_RVCHKEN | GCSCRE0_EL1_PCRSEL;
+
+	if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_WRITE)
+		gcscre0_el1 |= GCSCRE0_EL1_STREn;
+
+	if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_PUSH)
+		gcscre0_el1 |= GCSCRE0_EL1_PUSHMEn;
+
+	write_sysreg_s(gcscre0_el1, SYS_GCSCRE0_EL1);
+}
+
+void gcs_free(struct task_struct *task)
+{
+	if (!system_supports_gcs())
+		return;
+
+	if (task->thread.gcs_base)
+		vm_munmap(task->thread.gcs_base, task->thread.gcs_size);
+
+	task->thread.gcspr_el0 = 0;
+	task->thread.gcs_base = 0;
+	task->thread.gcs_size = 0;
+}