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

BugLink: https://bugs.launchpad.net/bugs/1819515
commit 6f913de3231e1d70a871135b38219da7810df218 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: 3548e131ec6a ("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=202351
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index 9e215737149103dcb627ebc9138a9b7126b24f99..f8debf7aeb4c144b286c8a12570cad23d46decc8 100644 (file)
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -1,5 +1,7 @@
+#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().
         */
 
-       ebda_start = *(unsigned short *)0x40e << 4;
-       bios_start = *(unsigned short *)0x413 << 10;
+       /*
+        * 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;