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>

1 files changed, 5 insertions, 5 deletions

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 1c0f3e02f731..e6f2accaeade 100644
--- a/arch/arm64/mm/mmu.c
+++ b/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),
-			    (unsigned long)__init_begin - (unsigned long)_text,
+	update_mapping_prot(__pa_symbol(_stext), (unsigned long)lm_alias(_stext),
+			    (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);