x86/boot/compressed/64: Fix boot on machines with broken E820 table

BIOS on Samsung 500C Chromebook reports very rudimentary E820 table that
consists of 2 entries:

  BIOS-e820: [mem 0x0000000000000000-0x0000000000000fff] usable
  BIOS-e820: [mem 0x00000000fffff000-0x00000000ffffffff] reserved

It breaks logic in find_trampoline_placement(): bios_start lands on the
end of the first 4k page and trampoline start gets placed below 0.

Detect underflow and don't touch bios_start for such cases. It makes
kernel ignore E820 table on machines that doesn't have two usable pages
below BIOS_START_MAX.

Fixes: 1b3a626 ("x86/boot/compressed/64: Validate trampoline placement against E820")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=203463
Link: https://lkml.kernel.org/r/20190813131654.24378-1-kirill.shutemov@linux.intel.com

arch/x86/boot/compressed/pgtable_64.c

@@ -72,6 +72,8 @@ static unsigned long find_trampoline_placement(void)
 
 	/* Find the first usable memory region under bios_start. */
 	for (i = boot_params->e820_entries - 1; i >= 0; i--) {
+		unsigned long new;
+
 		entry = &boot_params->e820_table[i];
 
 		/* Skip all entries above bios_start. */
@@ -84,15 +86,20 @@ static unsigned long find_trampoline_placement(void)
 
 		/* Adjust bios_start to the end of the entry if needed. */
 		if (bios_start > entry->addr + entry->size)
-			bios_start = entry->addr + entry->size;
+			new = entry->addr + entry->size;
 
 		/* Keep bios_start page-aligned. */
-		bios_start = round_down(bios_start, PAGE_SIZE);
+		new = round_down(new, PAGE_SIZE);
 
 		/* Skip the entry if it's too small. */
-		if (bios_start - TRAMPOLINE_32BIT_SIZE < entry->addr)
+		if (new - TRAMPOLINE_32BIT_SIZE < entry->addr)
 			continue;
 
+		/* Protect against underflow. */
+		if (new - TRAMPOLINE_32BIT_SIZE > bios_start)
+			break;
+
+		bios_start = new;
 		break;
 	}