efivars: Improve variable validation

Ben Hutchings pointed out that the validation in efivars was inadequate -
most obviously, an entry with size 0 would server as a DoS against the
kernel. Improve this based on his suggestions.
Signed-off-by: NMatthew Garrett <mjg@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/firmware/efivars.c

@@ -192,18 +192,21 @@ utf16_strncmp(const efi_char16_t *a, const efi_char16_t *b, size_t len)
 }
 
 static bool
-validate_device_path(struct efi_variable *var, int match, u8 *buffer, int len)
+validate_device_path(struct efi_variable *var, int match, u8 *buffer,
+		     unsigned long len)
 {
 	struct efi_generic_dev_path *node;
 	int offset = 0;
 
 	node = (struct efi_generic_dev_path *)buffer;
 
-	while (offset < len) {
-		offset += node->length;
+	if (len < sizeof(*node))
+		return false;
 
-		if (offset > len)
-			return false;
+	while (offset <= len - sizeof(*node) &&
+	       node->length >= sizeof(*node) &&
+		node->length <= len - offset) {
+		offset += node->length;
 
 		if ((node->type == EFI_DEV_END_PATH ||
 		     node->type == EFI_DEV_END_PATH2) &&
@@ -222,7 +225,8 @@ validate_device_path(struct efi_variable *var, int match, u8 *buffer, int len)
 }
 
 static bool
-validate_boot_order(struct efi_variable *var, int match, u8 *buffer, int len)
+validate_boot_order(struct efi_variable *var, int match, u8 *buffer,
+		    unsigned long len)
 {
 	/* An array of 16-bit integers */
 	if ((len % 2) != 0)
@@ -232,19 +236,27 @@ validate_boot_order(struct efi_variable *var, int match, u8 *buffer, int len)
 }
 
 static bool
-validate_load_option(struct efi_variable *var, int match, u8 *buffer, int len)
+validate_load_option(struct efi_variable *var, int match, u8 *buffer,
+		     unsigned long len)
 {
 	u16 filepathlength;
-	int i, desclength = 0;
+	int i, desclength = 0, namelen;
+
+	namelen = utf16_strnlen(var->VariableName, sizeof(var->VariableName));
 
 	/* 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)
+		if (var->VariableName[i] > 127 ||
+		    hex_to_bin(var->VariableName[i] & 0xff) < 0)
 			return true;
 	}
 
-	/* A valid entry must be at least 6 bytes */
-	if (len < 6)
+	/* Reject it if there's 4 digits of hex and then further content */
+	if (namelen > match + 4)
+		return false;
+
+	/* A valid entry must be at least 8 bytes */
+	if (len < 8)
 		return false;
 
 	filepathlength = buffer[4] | buffer[5] << 8;
@@ -253,7 +265,7 @@ validate_load_option(struct efi_variable *var, int match, u8 *buffer, int len)
 	 * 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;
+	desclength = utf16_strsize((efi_char16_t *)(buffer + 6), len - 6) + 2;
 
 	/* Each boot entry must have a descriptor */
 	if (!desclength)
@@ -275,7 +287,8 @@ validate_load_option(struct efi_variable *var, int match, u8 *buffer, int len)
 }
 
 static bool
-validate_uint16(struct efi_variable *var, int match, u8 *buffer, int len)
+validate_uint16(struct efi_variable *var, int match, u8 *buffer,
+		unsigned long len)
 {
 	/* A single 16-bit integer */
 	if (len != 2)
@@ -285,7 +298,8 @@ validate_uint16(struct efi_variable *var, int match, u8 *buffer, int len)
 }
 
 static bool
-validate_ascii_string(struct efi_variable *var, int match, u8 *buffer, int len)
+validate_ascii_string(struct efi_variable *var, int match, u8 *buffer,
+		      unsigned long len)
 {
 	int i;
 
@@ -303,7 +317,7 @@ validate_ascii_string(struct efi_variable *var, int match, u8 *buffer, int len)
 struct variable_validate {
 	char *name;
 	bool (*validate)(struct efi_variable *var, int match, u8 *data,
-			 int len);
+			 unsigned long len);
 };
 
 static const struct variable_validate variable_validate[] = {
@@ -325,7 +339,7 @@ static const struct variable_validate variable_validate[] = {
 };
 
 static bool
-validate_var(struct efi_variable *var, u8 *data, int len)
+validate_var(struct efi_variable *var, u8 *data, unsigned long len)
 {
 	int i;
 	u16 *unicode_name = var->VariableName;