arm64/efi: map the entire UEFI vendor string before reading it

commit f91b1fea upstream.

At boot, the UTF-16 UEFI vendor string is copied from the system
table into a char array with a size of 100 bytes. However, this
size of 100 bytes is also used for memremapping() the source,
which may not be sufficient if the vendor string exceeds 50
UTF-16 characters, and the placement of the vendor string inside
a 4 KB page happens to leave the end unmapped.

So use the correct '100 * sizeof(efi_char16_t)' for the size of
the mapping.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Fixes: f84d0275 ("arm64: add EFI runtime services")
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

arch/arm64/kernel/efi.c

@@ -98,7 +98,7 @@ static int __init uefi_init(void)
 
 	/* Show what we know for posterity */
 	c16 = early_memremap(efi.systab->fw_vendor,
-			     sizeof(vendor));
+			     sizeof(vendor) * sizeof(efi_char16_t));
 	if (c16) {
 		for (i = 0; i < (int) sizeof(vendor) - 1 && *c16; ++i)
 			vendor[i] = c16[i];
@@ -113,7 +113,7 @@ static int __init uefi_init(void)
 	if (retval == 0)
 		set_bit(EFI_CONFIG_TABLES, &efi.flags);
 
-	early_memunmap(c16, sizeof(vendor));
+	early_memunmap(c16, sizeof(vendor) * sizeof(efi_char16_t));
 	early_memunmap(efi.systab,  sizeof(efi_system_table_t));
 
 	return retval;