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Commit fec6c20b authored by Matthew Garrett's avatar Matthew Garrett Committed by Linus Torvalds
Browse files

efi: Validate UEFI boot variables 



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: default avatarMatthew Garrett <mjg@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 41b3254c

drivers/firmware/efivars.c

+182 −0
Original line number Diff line number Diff line
@@ -191,6 +191,176 @@ 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)

{

	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 *entry, const char *buf, size_t count) 
		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 *filp, struct kobject *kobj, 
	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);



	/*