[ 46/75] efi: Validate UEFI boot variables

Greg KH
2012-05-04T20:46:24Z

From: Matthew Garrett <mjg@redhat.com>

commit fec6c20b570bcf541e581fc97f2e0cbdb9725b98 upstream.

A common flaw in UEFI systems is a refusal to POST triggered by a malformed
boot variable. Once in this state, machines may only be restored by
reflashing their firmware with an external hardware device. While this is
obviously a firmware bug, the serious nature of the outcome suggests that
operating systems should filter their variable writes in order to prevent
a malicious user from rendering the machine unusable.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/firmware/efivars.c | 182 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 182 insertions(+)

--- a/drivers/firmware/efivars.c
+++ b/drivers/firmware/efivars.c
@@ -191,6 +191,176 @@ utf16_strncmp(const efi_char16_t *a, con
}
}

+static bool
+validate_device_path(struct efi_variable *var, int match, u8 *buffer, int len)
+{
+ struct efi_generic_dev_path *node;
+ int offset = 0;
+
+ node = (struct efi_generic_dev_path *)buffer;
+
+ while (offset < len) {
+ offset += node->length;
+
+ if (offset > len)
+ return false;
+
+ if ((node->type == EFI_DEV_END_PATH ||
+ node->type == EFI_DEV_END_PATH2) &&
+ node->sub_type == EFI_DEV_END_ENTIRE)
+ return true;
+
+ node = (struct efi_generic_dev_path *)(buffer + offset);
+ }
+
+ /*
+ * If we're here then either node->length pointed past the end
+ * of the buffer or we reached the end of the buffer without
+ * finding a device path end node.
+ */
+ return false;
+}
+
+static bool
+validate_boot_order(struct efi_variable *var, int match, u8 *buffer, int len)
+{
+ /* An array of 16-bit integers */
+ if ((len % 2) != 0)
+ return false;
+
+ return true;
+}
+
+static bool
+validate_load_option(struct efi_variable *var, int match, u8 *buffer, int len)
+{
+ u16 filepathlength;
+ int i, desclength = 0;
+
+ /* Either "Boot" or "Driver" followed by four digits of hex */
+ for (i = match; i < match+4; i++) {
+ if (hex_to_bin(var->VariableName[i] & 0xff) < 0)
+ return true;
+ }
+
+ /* A valid entry must be at least 6 bytes */
+ if (len < 6)
+ return false;
+
+ filepathlength = buffer[4] | buffer[5] << 8;
+
+ /*
+ * There's no stored length for the description, so it has to be
+ * found by hand
+ */
+ desclength = utf16_strsize((efi_char16_t *)(buffer + 6), len) + 2;
+
+ /* Each boot entry must have a descriptor */
+ if (!desclength)
+ return false;
+
+ /*
+ * If the sum of the length of the description, the claimed filepath
+ * length and the original header are greater than the length of the
+ * variable, it's malformed
+ */
+ if ((desclength + filepathlength + 6) > len)
+ return false;
+
+ /*
+ * And, finally, check the filepath
+ */
+ return validate_device_path(var, match, buffer + desclength + 6,
+ filepathlength);
+}
+
+static bool
+validate_uint16(struct efi_variable *var, int match, u8 *buffer, int len)
+{
+ /* A single 16-bit integer */
+ if (len != 2)
+ return false;
+
+ return true;
+}
+
+static bool
+validate_ascii_string(struct efi_variable *var, int match, u8 *buffer, int len)
+{
+ int i;
+
+ for (i = 0; i < len; i++) {
+ if (buffer[i] > 127)
+ return false;
+
+ if (buffer[i] == 0)
+ return true;
+ }
+
+ return false;
+}
+
+struct variable_validate {
+ char *name;
+ bool (*validate)(struct efi_variable *var, int match, u8 *data,
+ int len);
+};
+
+static const struct variable_validate variable_validate[] = {
+ { "BootNext", validate_uint16 },
+ { "BootOrder", validate_boot_order },
+ { "DriverOrder", validate_boot_order },
+ { "Boot*", validate_load_option },
+ { "Driver*", validate_load_option },
+ { "ConIn", validate_device_path },
+ { "ConInDev", validate_device_path },
+ { "ConOut", validate_device_path },
+ { "ConOutDev", validate_device_path },
+ { "ErrOut", validate_device_path },
+ { "ErrOutDev", validate_device_path },
+ { "Timeout", validate_uint16 },
+ { "Lang", validate_ascii_string },
+ { "PlatformLang", validate_ascii_string },
+ { "", NULL },
+};
+
+static bool
+validate_var(struct efi_variable *var, u8 *data, int len)
+{
+ int i;
+ u16 *unicode_name = var->VariableName;
+
+ for (i = 0; variable_validate[i].validate != NULL; i++) {
+ const char *name = variable_validate[i].name;
+ int match;
+
+ for (match = 0; ; match++) {
+ char c = name[match];
+ u16 u = unicode_name[match];
+
+ /* All special variables are plain ascii */
+ if (u > 127)
+ return true;
+
+ /* Wildcard in the matching name means we've matched */
+ if (c == '*')
+ return variable_validate[i].validate(var,
+ match, data, len);
+
+ /* Case sensitive match */
+ if (c != u)
+ break;
+
+ /* Reached the end of the string while matching */
+ if (!c)
+ return variable_validate[i].validate(var,
+ match, data, len);
+ }
+ }
+
+ return true;
+}
+
static efi_status_t
get_var_data_locked(struct efivars *efivars, struct efi_variable *var)
{
@@ -324,6 +494,12 @@ efivar_store_raw(struct efivar_entry *en
return -EINVAL;
}

+ if ((new_var->Attributes & ~EFI_VARIABLE_MASK) != 0 ||
+ validate_var(new_var, new_var->Data, new_var->DataSize) == false) {
+ printk(KERN_ERR "efivars: Malformed variable content\n");
+ return -EINVAL;
+ }
+
spin_lock(&efivars->lock);
status = efivars->ops->set_variable(new_var->VariableName,
&new_var->VendorGuid,
@@ -626,6 +802,12 @@ static ssize_t efivar_create(struct file
if (!capable(CAP_SYS_ADMIN))
return -EACCES;

+ if ((new_var->Attributes & ~EFI_VARIABLE_MASK) != 0 ||
+ validate_var(new_var, new_var->Data, new_var->DataSize) == false) {
+ printk(KERN_ERR "efivars: Malformed variable content\n");
+ return -EINVAL;
+ }
+
spin_lock(&efivars->lock);

/*