[PATCH 02/17] x86/efi: Disregard setup header of loaded image

Ard Biesheuvel
2023-08-18T13:45:53Z

The native EFI entrypoint does not take a struct boot_params from the
loader, but instead, it constructs one from scratch, using the setup
header data placed at the start of the image.

This setup header is placed in a way that permits legacy loaders to
manipulate the contents (i.e., to pass the kernel command line or the
address and size of an initial ramdisk), but EFI boot does not use it in
that way - it only copies the contents that were placed there at build
time, but EFI loaders will not (and should not) manipulate the setup
header to configure the boot. (Commit 63bf28ceb3ebbe76 "efi: x86: Wipe
setup_data on pure EFI boot" deals with some of the fallout of using
setup_data in a way that breaks EFI boot.)

As it turns out, there is another reason why copying this header is
slightly problematic: it is placed at a fixed offset of 0x1f1 bytes into
the image, and this means the PE/COFF view of the image payload has to
start there as the setup header is consumed by the program and therefore
part of the payload, and this leaves little space to describe all the
sections of the image. The only reason to describe the setup header is
that the EFI stub may copy this setup header at boot time, but beyond
that, none of the contents of this header section are needed by the
running code. If the setup header does not need to be accessed at
runtime, there is no need to describe it in the first place, freeing up
a slot in the section header array.

And actually, none of the non-zero values that are copied from the
setup header into the EFI stub's struct boot_params are relevant to the
boot now that the EFI stub no longer enters via the legacy decompressor.
So omit the copy altogether.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
drivers/firmware/efi/libstub/x86-stub.c | 46 +++-----------------
1 file changed, 6 insertions(+), 40 deletions(-)

diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index e976288728e975f1..d35cc47d195debcf 100644
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -449,9 +449,8 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
efi_system_table_t *sys_table_arg)
{
- struct boot_params *boot_params;
- struct setup_header *hdr;
- void *image_base;
+ static struct boot_params boot_params __page_aligned_bss;
+ struct setup_header *hdr = &boot_params.hdr;
efi_guid_t proto = LOADED_IMAGE_PROTOCOL_GUID;
int options_size = 0;
efi_status_t status;
@@ -469,30 +468,9 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
efi_exit(handle, status);
}

- image_base = efi_table_attr(image, image_base);
-
- status = efi_allocate_pages(sizeof(struct boot_params),
- (unsigned long *)&boot_params, ULONG_MAX);
- if (status != EFI_SUCCESS) {
- efi_err("Failed to allocate lowmem for boot params\n");
- efi_exit(handle, status);
- }
-
- memset(boot_params, 0x0, sizeof(struct boot_params));
-
- hdr = &boot_params->hdr;
-
- /* Copy the setup header from the second sector to boot_params */
- memcpy(&hdr->jump, image_base + 512,
- sizeof(struct setup_header) - offsetof(struct setup_header, jump));
-
- /*
- * Fill out some of the header fields ourselves because the
- * EFI firmware loader doesn't load the first sector.
- */
+ /* assign the setup_header fields that the kernel actually cares about */
hdr->root_flags = 1;
hdr->vid_mode = 0xffff;
- hdr->boot_flag = 0xAA55;

hdr->type_of_loader = 0x21;

@@ -501,25 +479,13 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
if (!cmdline_ptr)
goto fail;

- efi_set_u64_split((unsigned long)cmdline_ptr,
- &hdr->cmd_line_ptr, &boot_params->ext_cmd_line_ptr);
+ efi_set_u64_split((unsigned long)cmdline_ptr, &hdr->cmd_line_ptr,
+ &boot_params.ext_cmd_line_ptr);

- hdr->ramdisk_image = 0;
- hdr->ramdisk_size = 0;
-
- /*
- * Disregard any setup data that was provided by the bootloader:
- * setup_data could be pointing anywhere, and we have no way of
- * authenticating or validating the payload.
- */
- hdr->setup_data = 0;
-
- efi_stub_entry(handle, sys_table_arg, boot_params);
+ efi_stub_entry(handle, sys_table_arg, &boot_params);
/* not reached */

fail:
- efi_free(sizeof(struct boot_params), (unsigned long)boot_params);
-
efi_exit(handle, status);
}

--
2.39.2