x86/efistub: Remap kernel text read-only before dropping NX attribute

commit 9c55461040a9264b7e44444c53d26480b438eda6 upstream.

Currently, the EFI stub invokes the EFI memory attributes protocol to
strip any NX restrictions from the entire loaded kernel, resulting in
all code and data being mapped read-write-execute.

The point of the EFI memory attributes protocol is to remove the need
for all memory allocations to be mapped with both write and execute
permissions by default, and make it the OS loader's responsibility to
transition data mappings to code mappings where appropriate.

Even though the UEFI specification does not appear to leave room for
denying memory attribute changes based on security policy, let's be
cautious and avoid relying on the ability to create read-write-execute
mappings. This is trivially achievable, given that the amount of kernel
code executing via the firmware's 1:1 mapping is rather small and
limited to the .head.text region. So let's drop the NX restrictions only
on that subregion, but not before remapping it as read-only first.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 13 insertions, 2 deletions

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 71fc531b95b4..583c11664c63 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -84,7 +84,7 @@ LDFLAGS_vmlinux += -T
 hostprogs	:= mkpiggy
 HOST_EXTRACFLAGS += -I$(srctree)/tools/include
 
-sed-voffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] \(_text\|__bss_start\|_end\)$$/\#define VO_\2 _AC(0x\1,UL)/p'
+sed-voffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] \(_text\|__start_rodata\|__bss_start\|_end\)$$/\#define VO_\2 _AC(0x\1,UL)/p'
 
 quiet_cmd_voffset = VOFFSET $@
       cmd_voffset = $(NM) $< | sed -n $(sed-voffset) > $@
diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index f711f2a85862..9fe4e6ff693f 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -330,6 +330,7 @@ static size_t parse_elf(void *output)
 	return ehdr.e_entry - LOAD_PHYSICAL_ADDR;
 }
 
+const unsigned long kernel_text_size = VO___start_rodata - VO__text;
 const unsigned long kernel_total_size = VO__end - VO__text;
 
 static u8 boot_heap[BOOT_HEAP_SIZE] __aligned(4);
diff --git a/arch/x86/include/asm/boot.h b/arch/x86/include/asm/boot.h
index b3a7cfb0d99e..c945c893c52e 100644
--- a/arch/x86/include/asm/boot.h
+++ b/arch/x86/include/asm/boot.h
@@ -81,6 +81,7 @@
 
 #ifndef __ASSEMBLY__
 extern unsigned int output_len;
+extern const unsigned long kernel_text_size;
 extern const unsigned long kernel_total_size;
 
 unsigned long decompress_kernel(unsigned char *outbuf, unsigned long virt_addr,
diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index 8307950fe3ce..1c9e9909dcd7 100644
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -238,6 +238,15 @@ efi_status_t efi_adjust_memory_range_protection(unsigned long start,
 	rounded_end = roundup(start + size, EFI_PAGE_SIZE);
 
 	if (memattr != NULL) {
+		status = efi_call_proto(memattr, set_memory_attributes,
+					rounded_start,
+					rounded_end - rounded_start,
+					EFI_MEMORY_RO);
+		if (status != EFI_SUCCESS) {
+			efi_warn("Failed to set EFI_MEMORY_RO attribute\n");
+			return status;
+		}
+
 		status = efi_call_proto(memattr, clear_memory_attributes,
 					rounded_start,
 					rounded_end - rounded_start,
@@ -816,7 +825,7 @@ static efi_status_t efi_decompress_kernel(unsigned long *kernel_entry)
 
 	*kernel_entry = addr + entry;
 
-	return efi_adjust_memory_range_protection(addr, kernel_total_size);
+	return efi_adjust_memory_range_protection(addr, kernel_text_size);
 }
 
 static void __noreturn enter_kernel(unsigned long kernel_addr,