arm64/mm: Allocate PIE slots for EL0 guarded control stack

Pages used for guarded control stacks need to be described to the hardware
using the Permission Indirection Extension, GCS is not supported without
PIE. In order to support copy on write for guarded stacks we allocate two
values, one for active GCSs and one for GCS pages marked as read only prior
to copy.

Since the actual effect is defined using PIE the specific bit pattern used
does not matter to the hardware but we choose two values which differ only
in PTE_WRITE in order to help share code with non-PIE cases.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-13-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/pgtable-prot.h

@@ -144,22 +144,32 @@ static inline bool __pure lpa2_is_enabled(void)
 /* 6:                                PTE_PXN | PTE_WRITE            */
 /* 7: PAGE_SHARED_EXEC               PTE_PXN | PTE_WRITE | PTE_USER */
 /* 8: PAGE_KERNEL_ROX      PTE_UXN                                  */
-/* 9:                      PTE_UXN |                       PTE_USER */
+/* 9: PAGE_GCS_RO          PTE_UXN |                       PTE_USER */
 /* a: PAGE_KERNEL_EXEC     PTE_UXN |           PTE_WRITE            */
-/* b:                      PTE_UXN |           PTE_WRITE | PTE_USER */
+/* b: PAGE_GCS             PTE_UXN |           PTE_WRITE | PTE_USER */
 /* c: PAGE_KERNEL_RO       PTE_UXN | PTE_PXN                        */
 /* d: PAGE_READONLY        PTE_UXN | PTE_PXN |             PTE_USER */
 /* e: PAGE_KERNEL          PTE_UXN | PTE_PXN | PTE_WRITE            */
 /* f: PAGE_SHARED          PTE_UXN | PTE_PXN | PTE_WRITE | PTE_USER */
 
+#define _PAGE_GCS	(_PAGE_DEFAULT | PTE_NG | PTE_UXN | PTE_WRITE | PTE_USER)
+#define _PAGE_GCS_RO	(_PAGE_DEFAULT | PTE_NG | PTE_UXN | PTE_USER)
+
+#define PAGE_GCS	__pgprot(_PAGE_GCS)
+#define PAGE_GCS_RO	__pgprot(_PAGE_GCS_RO)
+
 #define PIE_E0	( \
+	PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS),           PIE_GCS)  | \
+	PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS_RO),        PIE_R)   | \
 	PIRx_ELx_PERM(pte_pi_index(_PAGE_EXECONLY),      PIE_X_O) | \
 	PIRx_ELx_PERM(pte_pi_index(_PAGE_READONLY_EXEC), PIE_RX_O)  | \
 	PIRx_ELx_PERM(pte_pi_index(_PAGE_SHARED_EXEC),   PIE_RWX_O) | \
 	PIRx_ELx_PERM(pte_pi_index(_PAGE_READONLY),      PIE_R_O)   | \
 	PIRx_ELx_PERM(pte_pi_index(_PAGE_SHARED),        PIE_RW_O))
 
 #define PIE_E1	( \
+	PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS),           PIE_NONE_O) | \
+	PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS_RO),        PIE_NONE_O) | \
 	PIRx_ELx_PERM(pte_pi_index(_PAGE_EXECONLY),      PIE_NONE_O) | \
 	PIRx_ELx_PERM(pte_pi_index(_PAGE_READONLY_EXEC), PIE_R)      | \
 	PIRx_ELx_PERM(pte_pi_index(_PAGE_SHARED_EXEC),   PIE_RW)     | \