Commit message: apparmor: fix oops, validate buffer size in apparmor_setprocattr()

When proc_pid_attr_write() was changed to use memdup_user apparmor's
(interface violating) assumption that the setprocattr buffer was always
a single page was violated.

The size test is not strictly speaking needed as proc_pid_attr_write()
will reject anything larger, but for the sake of robustness we can keep
it in.

SMACK and SELinux look safe to me, but somebody else should probably
have a look just in case.

Based on original patch from Vegard Nossum <vegard.nossum@oracle.com>
modified for the case that apparmor provides null termination.

Fixes: bb646cdb12e75d82258c2f2e7746d5952d3e321a
Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: John Johansen <john.johansen@canonical.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Eric Paris <eparis@parisplace.org>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: stable@kernel.org
Signed-off-by: John Johansen <john.johansen@canonical.com>
Reviewed-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>
CVE: CVE-2016-6187
CWE: CWE-119
Vulnerability description: The apparmor_setprocattr function in security/apparmor/lsm.c in the Linux kernel before 4.6.5 does not validate the buffer size, which allows local users to gain privileges by triggering an AppArmor setprocattr hook.

Project: linux
File: lsm.c

static int apparmor_setprocattr(struct task_struct *task, char *name,
				void *value, size_t size)
 {
 	struct common_audit_data sa;
 	struct apparmor_audit_data aad = {0,};
	char *command, *args = value;
 	size_t arg_size;
 	int error;
 
 	if (size == 0)
 		return -EINVAL;
	/* args points to a PAGE_SIZE buffer, AppArmor requires that
	 * the buffer must be null terminated or have size <= PAGE_SIZE -1
	 * so that AppArmor can null terminate them
	 */
	if (args[size - 1] != '\0') {
		if (size == PAGE_SIZE)
			return -EINVAL;
		args[size] = '\0';
	}
 	/* task can only write its own attributes */
 	if (current != task)
 		return -EACCES;
 
	args = value;
 	args = strim(args);
 	command = strsep(&args, " ");
 	if (!args)
		return -EINVAL;
 	args = skip_spaces(args);
 	if (!*args)
		return -EINVAL;
 
 	arg_size = size - (args - (char *) value);
 	if (strcmp(name, "current") == 0) {
		if (strcmp(command, "changehat") == 0) {
			error = aa_setprocattr_changehat(args, arg_size,
							 !AA_DO_TEST);
		} else if (strcmp(command, "permhat") == 0) {
			error = aa_setprocattr_changehat(args, arg_size,
							 AA_DO_TEST);
		} else if (strcmp(command, "changeprofile") == 0) {
			error = aa_setprocattr_changeprofile(args, !AA_ONEXEC,
							     !AA_DO_TEST);
		} else if (strcmp(command, "permprofile") == 0) {
			error = aa_setprocattr_changeprofile(args, !AA_ONEXEC,
							     AA_DO_TEST);
		} else
			goto fail;
	} else if (strcmp(name, "exec") == 0) {
		if (strcmp(command, "exec") == 0)
			error = aa_setprocattr_changeprofile(args, AA_ONEXEC,
							     !AA_DO_TEST);
		else
 			goto fail;
 	} else
 		/* only support the "current" and "exec" process attributes */
		return -EINVAL;
 
 	if (!error)
 		error = size;
 	return error;
 
 fail:
	sa.type = LSM_AUDIT_DATA_NONE;
	sa.aad = &aad;
 	aad.profile = aa_current_profile();
 	aad.op = OP_SETPROCATTR;
 	aad.info = name;
	aad.error = -EINVAL;
 	aa_audit_msg(AUDIT_APPARMOR_DENIED, &sa, NULL);
	return -EINVAL;
 }