RISC-V: Protect all kernel sections including init early

Currently, .init.text & .init.data are intermixed which makes it impossible
apply different permissions to them. .init.data shouldn't need exec
permissions while .init.text shouldn't have write permission. Moreover,
the strict permission are only enforced /init starts. This leaves the
kernel vulnerable from possible buggy built-in modules.

Keep .init.text & .data in separate sections so that different permissions
are applied to each section. Apply permissions to individual sections as
early as possible. This improves the kernel protection under
CONFIG_STRICT_KERNEL_RWX. We also need to restore the permissions for the
entire _init section after it is freed so that those pages can be used
for other purpose.
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Tested-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/sections.h

@@ -9,5 +9,7 @@
 
 extern char _start[];
 extern char _start_kernel[];
+extern char __init_data_begin[], __init_data_end[];
+extern char __init_text_begin[], __init_text_end[];
 
 #endif /* __ASM_SECTIONS_H */

arch/riscv/include/asm/set_memory.h

@@ -15,11 +15,15 @@ int set_memory_ro(unsigned long addr, int numpages);
 int set_memory_rw(unsigned long addr, int numpages);
 int set_memory_x(unsigned long addr, int numpages);
 int set_memory_nx(unsigned long addr, int numpages);
+int set_memory_rw_nx(unsigned long addr, int numpages);
+void protect_kernel_text_data(void);
 #else
 static inline int set_memory_ro(unsigned long addr, int numpages) { return 0; }
 static inline int set_memory_rw(unsigned long addr, int numpages) { return 0; }
 static inline int set_memory_x(unsigned long addr, int numpages) { return 0; }
 static inline int set_memory_nx(unsigned long addr, int numpages) { return 0; }
+static inline void protect_kernel_text_data(void) {};
+static inline int set_memory_rw_nx(unsigned long addr, int numpages) { return 0; }
 #endif
 
 int set_direct_map_invalid_noflush(struct page *page);

arch/riscv/kernel/setup.c

@@ -24,6 +24,7 @@
 #include <asm/cpu_ops.h>
 #include <asm/early_ioremap.h>
 #include <asm/setup.h>
+#include <asm/set_memory.h>
 #include <asm/sections.h>
 #include <asm/sbi.h>
 #include <asm/tlbflush.h>
@@ -252,6 +253,8 @@ void __init setup_arch(char **cmdline_p)
 	if (IS_ENABLED(CONFIG_RISCV_SBI))
 		sbi_init();
 
+	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
+		protect_kernel_text_data();
 #ifdef CONFIG_SWIOTLB
 	swiotlb_init(1);
 #endif
@@ -281,3 +284,12 @@ static int __init topology_init(void)
 	return 0;
 }
 subsys_initcall(topology_init);
+
+void free_initmem(void)
+{
+	unsigned long init_begin = (unsigned long)__init_begin;
+	unsigned long init_end = (unsigned long)__init_end;
+
+	set_memory_rw_nx(init_begin, (init_end - init_begin) >> PAGE_SHIFT);
+	free_initmem_default(POISON_FREE_INITMEM);
+}

arch/riscv/kernel/vmlinux.lds.S

@@ -29,6 +29,22 @@ SECTIONS
 	HEAD_TEXT_SECTION
 	. = ALIGN(PAGE_SIZE);
 
+	.text : {
+		_text = .;
+		_stext = .;
+		TEXT_TEXT
+		SCHED_TEXT
+		CPUIDLE_TEXT
+		LOCK_TEXT
+		KPROBES_TEXT
+		ENTRY_TEXT
+		IRQENTRY_TEXT
+		SOFTIRQENTRY_TEXT
+		*(.fixup)
+		_etext = .;
+	}
+
+	. = ALIGN(SECTION_ALIGN);
 	__init_begin = .;
 	__init_text_begin = .;
 	.init.text : AT(ADDR(.init.text) - LOAD_OFFSET) ALIGN(SECTION_ALIGN) { \
@@ -53,35 +69,24 @@ SECTIONS
 	{
 		EXIT_TEXT
 	}
-	.exit.data :
-	{
-		EXIT_DATA
-	}
-	PERCPU_SECTION(L1_CACHE_BYTES)
-	__init_end = .;
 
+	__init_text_end = .;
 	. = ALIGN(SECTION_ALIGN);
-	.text : {
-		_text = .;
-		_stext = .;
-		TEXT_TEXT
-		SCHED_TEXT
-		CPUIDLE_TEXT
-		LOCK_TEXT
-		KPROBES_TEXT
-		ENTRY_TEXT
-		IRQENTRY_TEXT
-		SOFTIRQENTRY_TEXT
-		*(.fixup)
-		_etext = .;
-	}
-
 #ifdef CONFIG_EFI
 	. = ALIGN(PECOFF_SECTION_ALIGNMENT);
 	__pecoff_text_end = .;
 #endif
-
+	/* Start of init data section */
+	__init_data_begin = .;
 	INIT_DATA_SECTION(16)
+	.exit.data :
+	{
+		EXIT_DATA
+	}
+	PERCPU_SECTION(L1_CACHE_BYTES)
+
+	__init_data_end = .;
+	__init_end = .;
 
 	/* Start of data section */
 	_sdata = .;

arch/riscv/mm/init.c

@@ -612,18 +612,29 @@ static inline void setup_vm_final(void)
 #endif /* CONFIG_MMU */
 
 #ifdef CONFIG_STRICT_KERNEL_RWX
-void mark_rodata_ro(void)
+void protect_kernel_text_data(void)
 {
-	unsigned long text_start = (unsigned long)_text;
-	unsigned long text_end = (unsigned long)_etext;
+	unsigned long text_start = (unsigned long)_start;
+	unsigned long init_text_start = (unsigned long)__init_text_begin;
+	unsigned long init_data_start = (unsigned long)__init_data_begin;
 	unsigned long rodata_start = (unsigned long)__start_rodata;
 	unsigned long data_start = (unsigned long)_data;
 	unsigned long max_low = (unsigned long)(__va(PFN_PHYS(max_low_pfn)));
 
-	set_memory_ro(text_start, (text_end - text_start) >> PAGE_SHIFT);
-	set_memory_ro(rodata_start, (data_start - rodata_start) >> PAGE_SHIFT);
+	set_memory_ro(text_start, (init_text_start - text_start) >> PAGE_SHIFT);
+	set_memory_ro(init_text_start, (init_data_start - init_text_start) >> PAGE_SHIFT);
+	set_memory_nx(init_data_start, (rodata_start - init_data_start) >> PAGE_SHIFT);
+	/* rodata section is marked readonly in mark_rodata_ro */
 	set_memory_nx(rodata_start, (data_start - rodata_start) >> PAGE_SHIFT);
 	set_memory_nx(data_start, (max_low - data_start) >> PAGE_SHIFT);
+}
+
+void mark_rodata_ro(void)
+{
+	unsigned long rodata_start = (unsigned long)__start_rodata;
+	unsigned long data_start = (unsigned long)_data;
+
+	set_memory_ro(rodata_start, (data_start - rodata_start) >> PAGE_SHIFT);
 
 	debug_checkwx();
 }

arch/riscv/mm/pageattr.c

@@ -128,6 +128,12 @@ static int __set_memory(unsigned long addr, int numpages, pgprot_t set_mask,
 	return ret;
 }
 
+int set_memory_rw_nx(unsigned long addr, int numpages)
+{
+	return __set_memory(addr, numpages, __pgprot(_PAGE_READ | _PAGE_WRITE),
+			    __pgprot(_PAGE_EXEC));
+}
+
 int set_memory_ro(unsigned long addr, int numpages)
 {
 	return __set_memory(addr, numpages, __pgprot(_PAGE_READ),