x86/boot/compressed/64: Do not read legacy ROM on EFI system

commit 6f913de3 upstream.

EFI systems do not necessarily provide a legacy ROM. If the ROM is missing
the memory is not mapped at all.

Trying to dereference values in the legacy ROM area leads to a crash on
Macbook Pro.

Only look for values in the legacy ROM area for non-EFI system.

Fixes: 3548e131 ("x86/boot/compressed/64: Find a place for 32-bit trampoline")
Reported-by: Pitam Mitra <pitamm@gmail.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Bockjoo Kim <bockjoo@phys.ufl.edu>
Cc: bp@alien8.de
Cc: hpa@zytor.com
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20190219075224.35058-1-kirill.shutemov@linux.intel.com
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=202351Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/boot/compressed/pgtable_64.c

+#include <linux/efi.h>
 #include <asm/e820/types.h>
 #include <asm/processor.h>
+#include <asm/efi.h>
 #include "pgtable.h"
 #include "../string.h"
 
@@ -37,9 +39,10 @@ int cmdline_find_option_bool(const char *option);
 
 static unsigned long find_trampoline_placement(void)
 {
-	unsigned long bios_start, ebda_start;
+	unsigned long bios_start = 0, ebda_start = 0;
 	unsigned long trampoline_start;
 	struct boot_e820_entry *entry;
+	char *signature;
 	int i;
 
 	/*
@@ -47,8 +50,18 @@ static unsigned long find_trampoline_placement(void)
 	 * This code is based on reserve_bios_regions().
 	 */
 
+	/*
+	 * EFI systems may not provide legacy ROM. The memory may not be mapped
+	 * at all.
+	 *
+	 * Only look for values in the legacy ROM for non-EFI system.
+	 */
+	signature = (char *)&boot_params->efi_info.efi_loader_signature;
+	if (strncmp(signature, EFI32_LOADER_SIGNATURE, 4) &&
+	    strncmp(signature, EFI64_LOADER_SIGNATURE, 4)) {
 		ebda_start = *(unsigned short *)0x40e << 4;
 		bios_start = *(unsigned short *)0x413 << 10;
+	}
 
 	if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
 		bios_start = BIOS_START_MAX;