[RFC PATCH 07/12] riscv/mm: prepare shadow stack for init task for kernel cfi

[Deepak Gupta <debug@rivosinc.com>]

Under CONFIG_SHADOW_CALL_STACK, shadow call stack goes into data section.
Although with CONFIG_DYNAMIC_SCS on riscv, hardware assisted shadow stack
are used. Hardware assisted shadow stack on riscv uses PTE.R=0, PTE.W=1 &
PTE.X=0 encodings. Without CONFIG_DYNAMIC_SCS, shadow stack for init is
placed in data section and thus regular read/write encodings are applied
to it. Although with with CONFIG_DYNAMIC_SCS, they need to go into
different section. This change places it into `.shadowstack` section.
As part of this change early boot code (`setup_vm`), applies appropriate
PTE encodings to shadow call stack for init placed in `.shadowstack`
section.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 arch/riscv/include/asm/pgtable.h     |  4 ++++
 arch/riscv/include/asm/sections.h    | 22 +++++++++++++++++++++
 arch/riscv/include/asm/thread_info.h | 10 ++++++++--
 arch/riscv/kernel/vmlinux.lds.S      | 12 ++++++++++++
 arch/riscv/mm/init.c                 | 29 +++++++++++++++++++++-------
 5 files changed, 68 insertions(+), 9 deletions(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 9f8ea0e33eb1..3409b250390d 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -197,6 +197,10 @@ extern struct pt_alloc_ops pt_ops __initdata;
 #define PAGE_KERNEL_READ_EXEC	__pgprot((_PAGE_KERNEL & ~_PAGE_WRITE) \
 					 | _PAGE_EXEC)
 
+#ifdef CONFIG_DYNAMIC_SCS
+#define PAGE_KERNEL_SHADOWSTACK __pgprot(_PAGE_KERNEL & ~(_PAGE_READ | _PAGE_EXEC))
+#endif
+
 #define PAGE_TABLE		__pgprot(_PAGE_TABLE)
 
 #define _PAGE_IOREMAP	((_PAGE_KERNEL & ~_PAGE_MTMASK) | _PAGE_IO)
diff --git a/arch/riscv/include/asm/sections.h b/arch/riscv/include/asm/sections.h
index a393d5035c54..4c4154d0021e 100644
--- a/arch/riscv/include/asm/sections.h
+++ b/arch/riscv/include/asm/sections.h
@@ -14,6 +14,10 @@ extern char __init_data_begin[], __init_data_end[];
 extern char __init_text_begin[], __init_text_end[];
 extern char __alt_start[], __alt_end[];
 extern char __exittext_begin[], __exittext_end[];
+#ifdef CONFIG_DYNAMIC_SCS
+extern char __init_shstk_start[], __init_shstk_end[];
+#endif
+extern char __end_srodata[];
 
 static inline bool is_va_kernel_text(uintptr_t va)
 {
@@ -31,4 +35,22 @@ static inline bool is_va_kernel_lm_alias_text(uintptr_t va)
 	return va >= start && va < end;
 }
 
+#ifdef CONFIG_DYNAMIC_SCS
+static inline bool is_va_init_shadow_stack_early(uintptr_t va)
+{
+	uintptr_t start = (uintptr_t)(kernel_mapping_pa_to_va(__init_shstk_start));
+	uintptr_t end = (uintptr_t)(kernel_mapping_pa_to_va(__init_shstk_end));
+
+	return va >= start && va < end;
+}
+
+static inline bool is_va_init_shadow_stack(uintptr_t va)
+{
+	uintptr_t start = (uintptr_t)(__init_shstk_start);
+	uintptr_t end = (uintptr_t)(__init_shstk_end);
+
+	return va >= start && va < end;
+}
+#endif
+
 #endif /* __ASM_SECTIONS_H */
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 5d473343634b..7ae28d627f84 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -63,12 +63,18 @@ struct thread_info {
 };
 
 #ifdef CONFIG_SHADOW_CALL_STACK
+#ifdef CONFIG_DYNAMIC_SCS
 #define INIT_SCS							\
-	.scs_base	= init_shadow_call_stack,			\
+	.scs_base	= init_shadow_call_stack,	\
+	.scs_sp		= &init_shadow_call_stack[SCS_SIZE / sizeof(long)],
+#else
+#define INIT_SCS							\
+	.scs_base	= init_shadow_call_stack,	\
 	.scs_sp		= init_shadow_call_stack,
+#endif /* CONFIG_DYNAMIC_SCS */
 #else
 #define INIT_SCS
-#endif
+#endif /* CONFIG_SHADOW_CALL_STACK */
 
 /*
  * macros/functions for gaining access to the thread information structure
diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
index 002ca58dd998..cccc51f845ab 100644
--- a/arch/riscv/kernel/vmlinux.lds.S
+++ b/arch/riscv/kernel/vmlinux.lds.S
@@ -126,6 +126,18 @@ SECTIONS
 		*(.srodata*)
 	}
 
+	. = ALIGN(SECTION_ALIGN);
+	__end_srodata = .;
+
+#ifdef CONFIG_DYNAMIC_SCS
+	.shadowstack : AT(ADDR(.shadowstack) - LOAD_OFFSET){
+		__init_shstk_start = .;
+		KEEP(*(.shadowstack..init))
+		. = __init_shstk_start + PAGE_SIZE;
+		__init_shstk_end = .;
+	}
+#endif
+
 	. = ALIGN(SECTION_ALIGN);
 	_data = .;
 
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index fe8e159394d8..5b6f0cfa5719 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -713,14 +713,22 @@ static __init pgprot_t pgprot_from_va(uintptr_t va)
 	if (IS_ENABLED(CONFIG_64BIT) && is_va_kernel_lm_alias_text(va))
 		return PAGE_KERNEL_READ;
 
+#ifdef CONFIG_DYNAMIC_SCS
+	/* If init task's shadow stack va, return write only page protections */
+	if (IS_ENABLED(CONFIG_64BIT) && is_va_init_shadow_stack(va)) {
+		pr_info("Shadow stack protections are being applied to for init\n");
+		return PAGE_KERNEL_SHADOWSTACK;
+	}
+#endif
+
 	return PAGE_KERNEL;
 }
 
 void mark_rodata_ro(void)
 {
-	set_kernel_memory(__start_rodata, _data, set_memory_ro);
+	set_kernel_memory(__start_rodata, __end_srodata, set_memory_ro);
 	if (IS_ENABLED(CONFIG_64BIT))
-		set_kernel_memory(lm_alias(__start_rodata), lm_alias(_data),
+		set_kernel_memory(lm_alias(__start_rodata), lm_alias(__end_srodata),
 				  set_memory_ro);
 }
 #else
@@ -913,14 +921,21 @@ static void __init create_kernel_page_table(pgd_t *pgdir,
 static void __init create_kernel_page_table(pgd_t *pgdir, bool early)
 {
 	uintptr_t va, end_va;
+	pgprot_t prot;
 
 	end_va = kernel_map.virt_addr + kernel_map.size;
-	for (va = kernel_map.virt_addr; va < end_va; va += PMD_SIZE)
+	for (va = kernel_map.virt_addr; va < end_va; va += PMD_SIZE) {
+		prot = PAGE_KERNEL_EXEC;
+#ifdef CONFIG_DYNAMIC_SCS
+		if (early && is_va_init_shadow_stack_early(va))
+			prot = PAGE_KERNEL_SHADOWSTACK;
+#endif
 		create_pgd_mapping(pgdir, va,
-				   kernel_map.phys_addr + (va - kernel_map.virt_addr),
-				   PMD_SIZE,
-				   early ?
-					PAGE_KERNEL_EXEC : pgprot_from_va(va));
+					kernel_map.phys_addr + (va - kernel_map.virt_addr),
+					PMD_SIZE,
+					early ?
+					prot : pgprot_from_va(va));
+	}
 }
 #endif
 
-- 
2.43.2