arm64: omit [_text, _stext) from permanent kernel mapping

In a previous patch, we increased the size of the EFI PE/COFF header
to 64 KB, which resulted in the _stext symbol to appear at a fixed
offset of 64 KB into the image.

Since 64 KB is also the largest page size we support, this completely
removes the need to map the first 64 KB of the kernel image, given that
it only contains the arm64 Image header and the EFI header, neither of
which we ever access again after booting the kernel. More importantly,
we should avoid an executable mapping of non-executable and not entirely
predictable data, to deal with the unlikely event that we inadvertently
emitted something that looks like an opcode that could be used as a
gadget for speculative execution.

So let's limit the kernel mapping of .text to the [_stext, _etext)
region, which matches the view of generic code (such as kallsyms) when
it reasons about the boundaries of the kernel's .text section.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201117124729.12642-2-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/efi-header.S

@@ -140,13 +140,6 @@ efi_debug_entry:
 	.set	efi_debug_entry_size, . - efi_debug_entry
 #endif
 
-	/*
-	 * EFI will load .text onwards at the 4k section alignment
-	 * described in the PE/COFF header. To ensure that instruction
-	 * sequences using an adrp and a :lo12: immediate will function
-	 * correctly at this alignment, we must ensure that .text is
-	 * placed at a 4k boundary in the Image to begin with.
-	 */
 	.balign	SEGMENT_ALIGN
 efi_header_end:
 	.endm

arch/arm64/kernel/setup.c

@@ -206,7 +206,7 @@ static void __init request_standard_resources(void)
 	unsigned long i = 0;
 	size_t res_size;
 
-	kernel_code.start   = __pa_symbol(_text);
+	kernel_code.start   = __pa_symbol(_stext);
 	kernel_code.end     = __pa_symbol(__init_begin - 1);
 	kernel_data.start   = __pa_symbol(_sdata);
 	kernel_data.end     = __pa_symbol(_end - 1);
@@ -283,7 +283,7 @@ u64 cpu_logical_map(int cpu)
 
 void __init __no_sanitize_address setup_arch(char **cmdline_p)
 {
-	init_mm.start_code = (unsigned long) _text;
+	init_mm.start_code = (unsigned long) _stext;
 	init_mm.end_code   = (unsigned long) _etext;
 	init_mm.end_data   = (unsigned long) _edata;
 	init_mm.brk	   = (unsigned long) _end;

arch/arm64/kernel/vmlinux.lds.S

@@ -121,7 +121,7 @@ SECTIONS
 		_text = .;
 		HEAD_TEXT
 	}
-	.text : {			/* Real text segment		*/
+	.text : ALIGN(SEGMENT_ALIGN) {	/* Real text segment		*/
 		_stext = .;		/* Text and read-only data	*/
 			IRQENTRY_TEXT
 			SOFTIRQENTRY_TEXT

arch/arm64/mm/init.c

@@ -367,7 +367,7 @@ void __init arm64_memblock_init(void)
 	 * Register the kernel text, kernel data, initrd, and initial
 	 * pagetables with memblock.
 	 */
-	memblock_reserve(__pa_symbol(_text), _end - _text);
+	memblock_reserve(__pa_symbol(_stext), _end - _stext);
 	if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && phys_initrd_size) {
 		/* the generic initrd code expects virtual addresses */
 		initrd_start = __phys_to_virt(phys_initrd_start);

arch/arm64/mm/mmu.c

@@ -464,14 +464,14 @@ void __init mark_linear_text_alias_ro(void)
 	/*
 	 * Remove the write permissions from the linear alias of .text/.rodata
 	 */
-	update_mapping_prot(__pa_symbol(_text), (unsigned long)lm_alias(_text),
+	update_mapping_prot(__pa_symbol(_stext), (unsigned long)lm_alias(_stext),
-			    (unsigned long)__init_begin - (unsigned long)_text,
+			    (unsigned long)__init_begin - (unsigned long)_stext,
 			    PAGE_KERNEL_RO);
 }
 
 static void __init map_mem(pgd_t *pgdp)
 {
-	phys_addr_t kernel_start = __pa_symbol(_text);
+	phys_addr_t kernel_start = __pa_symbol(_stext);
 	phys_addr_t kernel_end = __pa_symbol(__init_begin);
 	phys_addr_t start, end;
 	int flags = 0;
@@ -506,7 +506,7 @@ static void __init map_mem(pgd_t *pgdp)
 	}
 
 	/*
-	 * Map the linear alias of the [_text, __init_begin) interval
+	 * Map the linear alias of the [_stext, __init_begin) interval
 	 * as non-executable now, and remove the write permission in
 	 * mark_linear_text_alias_ro() below (which will be called after
 	 * alternative patching has completed). This makes the contents
@@ -665,7 +665,7 @@ static void __init map_kernel(pgd_t *pgdp)
 	 * Only rodata will be remapped with different permissions later on,
 	 * all other segments are allowed to use contiguous mappings.
 	 */
-	map_kernel_segment(pgdp, _text, _etext, text_prot, &vmlinux_text, 0,
+	map_kernel_segment(pgdp, _stext, _etext, text_prot, &vmlinux_text, 0,
 			   VM_NO_GUARD);
 	map_kernel_segment(pgdp, __start_rodata, __inittext_begin, PAGE_KERNEL,
 			   &vmlinux_rodata, NO_CONT_MAPPINGS, VM_NO_GUARD);