efi: x86: Wipe setup_data on pure EFI boot

When booting the x86 kernel via EFI using the LoadImage/StartImage boot
services [as opposed to the deprecated EFI handover protocol], the setup
header is taken from the image directly, and given that EFI's LoadImage
has no Linux/x86 specific knowledge regarding struct bootparams or
struct setup_header, any absolute addresses in the setup header must
originate from the file and not from a prior loading stage.

Since we cannot generally predict where LoadImage() decides to load an
image (*), such absolute addresses must be treated as suspect: even if a
prior boot stage intended to make them point somewhere inside the
[signed] image, there is no way to validate that, and if they point at
an arbitrary location in memory, the setup_data nodes will not be
covered by any signatures or TPM measurements either, and could be made
to contain an arbitrary sequence of SETUP_xxx nodes, which could
interfere quite badly with the early x86 boot sequence.

(*) Note that, while LoadImage() does take a buffer/size tuple in
addition to a device path, which can be used to provide the image
contents directly, it will re-allocate such images, as the memory
footprint of an image is generally larger than the PE/COFF file
representation.

Cc: <[email protected]> # v5.10+
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Ard Biesheuvel <[email protected]>
Acked-by: Jason A. Donenfeld <[email protected]>

Author: ardbiesheuvel

drivers/firmware/efi/libstub/x86-stub.c

@@ -516,6 +516,13 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
 	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);
 	/* not reached */