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: <stable@vger.kernel.org> # v5.10+
Link: https://lore.kernel.org/all/20220904165321.1140894-1-Jason@zx2c4.com/
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>

diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index 43ca665af610c4ff4dc91d8f954976e2ce2445be..7a7abc8959d2b0e3e44dcb06f15fa12432b93c18 100644 (file)
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/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 */