[pxe] Warn about PXE NBPs that may be EFI executables

A relatively common user mistake is to attempt to boot an EFI
executable (such as grub.efi) using a BIOS version of iPXE.

Unfortunately there are no signature checks that we can use to
unambiguously identify a PXE NBP, since a PXE NBP is just raw machine
code.  We therefore have to accept anything sufficiently small to fit
into base memory as a valid PXE NBP.

We can detect that a file might be an EFI executable by checking for
the initial "MZ" signature bytes.  This does not necessarily preclude
the file from also being a PXE NBP (since it would be possible to
create a hybrid binary which acts as both an EFI executable and a PXE
NBP, similar to the way in which wimboot and the Linux kernel are
hybrid binaries which act as both an EFI executable and a bzImage).

If the initial "MZ" signature bytes are present, then attempt to warn
the user by setting the image type to "PXE-NBP (may be EFI?)".  We
can't (sensibly) prevent the user from accidentally running an EFI
executable as a PXE NBP, but we can at least make it easier for the
user to identify their mistake.

Inspired-by: Robin Smidsrød <robin@smidsrod.no>
Inspired-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/image/pxe_image.c

 #include <ipxe/netdevice.h>
 #include <ipxe/features.h>
 #include <ipxe/console.h>
+#include <ipxe/efi/efi.h>
+#include <ipxe/efi/IndustryStandard/PeImage.h>
 
 FEATURE ( FEATURE_IMAGE, "PXE", DHCP_EB_FEATURE_PXE, 1 );
 
 	return 0;
 }
 
+/**
+ * Probe PXE image (with rejection of potential EFI images)
+ *
+ * @v image		PXE file
+ * @ret rc		Return status code
+ */
+int pxe_probe_no_mz ( struct image *image ) {
+	uint16_t magic;
+	int rc;
+
+	/* Probe PXE image */
+	if ( ( rc = pxe_probe ( image ) ) != 0 )
+		return rc;
+
+	/* Reject image with an "MZ" signature which may indicate an
+	 * EFI image incorrectly handed out to a BIOS system.
+	 */
+	if ( image->len >= sizeof ( magic ) ) {
+		copy_from_user ( &magic, image->data, 0, sizeof ( magic ) );
+		if ( magic == cpu_to_le16 ( EFI_IMAGE_DOS_SIGNATURE ) ) {
+			DBGC ( image, "IMAGE %p may be an EFI image\n",
+			       image );
+			return -ENOTTY;
+		}
+	}
+
+	return 0;
+}
+
 /** PXE image type */
-struct image_type pxe_image_type __image_type ( PROBE_PXE ) = {
-	.name = "PXE",
-	.probe = pxe_probe,
-	.exec = pxe_exec,
+struct image_type pxe_image_type[] __image_type ( PROBE_PXE ) = {
+	{
+		.name = "PXE-NBP",
+		.probe = pxe_probe_no_mz,
+		.exec = pxe_exec,
+	},
+	{
+		.name = "PXE-NBP (may be EFI?)",
+		.probe = pxe_probe,
+		.exec = pxe_exec,
+	},
 };