[PATCH RFC v10 21/22] arm64: mm: Map kernel image alias of init_pg_dir read-only

[Kevin Brodsky]

Page tables in init_pg_dir are always modified via the linear map.
kpkeys_hardened_pgtables now prevents arbitrary code from writing
page tables via the linear map; map the kernel image alias of
init_pg_dir read-only to ensure that the kpkeys protection cannot be
bypassed.

Reviewed-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
Signed-off-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
---
 arch/arm64/include/asm/sections.h | 1 +
 arch/arm64/kernel/pi/map_kernel.c | 9 +++++++--
 arch/arm64/mm/mmu.c               | 8 ++++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/sections.h b/arch/arm64/include/asm/sections.h
index 51b0d594239e..dc961cf67168 100644
--- a/arch/arm64/include/asm/sections.h
+++ b/arch/arm64/include/asm/sections.h
@@ -23,6 +23,7 @@ extern char __irqentry_text_start[], __irqentry_text_end[];
 extern char __mmuoff_data_start[], __mmuoff_data_end[];
 extern char __entry_tramp_text_start[], __entry_tramp_text_end[];
 extern char __relocate_new_kernel_start[], __relocate_new_kernel_end[];
+extern char __pi_init_pg_dir[], __pi_init_pg_end[];
 
 static inline size_t entry_tramp_text_size(void)
 {
diff --git a/arch/arm64/kernel/pi/map_kernel.c b/arch/arm64/kernel/pi/map_kernel.c
index fb44cbdd2f29..81ca3e98f0f7 100644
--- a/arch/arm64/kernel/pi/map_kernel.c
+++ b/arch/arm64/kernel/pi/map_kernel.c
@@ -92,8 +92,13 @@ static void __init map_kernel(u64 kaslr_offset, u64 va_offset, int root_level)
 		    __inittext_end, prot, false, root_level);
 	map_segment(init_pg_dir, &pgdp, va_offset, __initdata_begin,
 		    __initdata_end, data_prot, false, root_level);
-	map_segment(init_pg_dir, &pgdp, va_offset, _data, _end, data_prot,
-		    true, root_level);
+	map_segment(init_pg_dir, &pgdp, va_offset, _data, init_pg_dir,
+		    data_prot, true, root_level);
+	/* Map init_pg_dir separately as we will remap it read-only later on */
+	map_segment(init_pg_dir, &pgdp, va_offset, init_pg_dir, init_pg_end,
+		    data_prot, true, root_level);
+	map_segment(init_pg_dir, &pgdp, va_offset, init_pg_end, _end,
+		    data_prot, true, root_level);
 	dsb(ishst);
 
 	idmap_cpu_replace_ttbr1((phys_addr_t)init_pg_dir);
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index c1e3260bd0db..b6a4371dd03f 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1267,6 +1267,14 @@ void mark_rodata_ro(void)
 	update_mapping_prot(__pa_symbol(_text), (unsigned long)_text,
 			    (unsigned long)_stext - (unsigned long)_text,
 			    PAGE_KERNEL_RO);
+	/*
+	 * Map the kernel image mapping of init_pg_dir read-only; it should
+	 * only be written via the linear map.
+	 */
+	section_size = (unsigned long)__pi_init_pg_end - (unsigned long)__pi_init_pg_dir;
+	update_mapping_prot(__pa_symbol(__pi_init_pg_dir),
+			    (unsigned long)__pi_init_pg_dir,
+			    section_size, PAGE_KERNEL_RO);
 
 	/* Map the kernel data/bss as invalid in the linear map */
 	mark_linear_data_alias_valid(false);

-- 
2.56.0