Linux内核源代码情景分析-访问权限与文件安全性

    在Linux内核源代码情景分析-从路径名到目标节点，一文中path_walk代码中，err = permission(inode, MAY_EXEC)当前进程是否可以访问这个节点，代码如下：

int permission(struct inode * inode,int mask)
{
	if (inode->i_op && inode->i_op->permission) {
		int retval;
		lock_kernel();
		retval = inode->i_op->permission(inode, mask);
		unlock_kernel();
		return retval;
	}
	return vfs_permission(inode, mask);
}

    在ext2_read_inode中，i_op可以设置为ext2_file_inode_operations，ext2_dir_inode_operations，ext2_fast_symlink_inode_operations，page_symlink_inode_operations，均没有permission指针。所以执行vfs_permission，代码如下：

int vfs_permission(struct inode * inode,int mask)
{
	int mode = inode->i_mode;

	if ((mask & S_IWOTH) && IS_RDONLY(inode) &&
		 (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))
		return -EROFS; //如果要求的写文件，并且是只读系统，而且是常规文件，目录，或者链接时，返回-EROFS，表示不能访问

	if ((mask & S_IWOTH) && IS_IMMUTABLE(inode)) //Nobody gets write access to an immutable file
		return -EACCES; 

	if (current->fsuid == inode->i_uid)//如果当前进程的fsuid和inode结构的i_uid相等，那么应该比较mode中S_IRUSR、S_IWUSR、S_IXUSR位
		mode >>= 6;
	else if (in_group_p(inode->i_gid))//如果当前进程的fsgid和inode结构的i_gid相等，那么应该比较mode中S_IRGRP、S_IWGRP、S_IXGRP位
		mode >>= 3;

	if (((mode & mask & S_IRWXO) == mask) || capable(CAP_DAC_OVERRIDE))//mask相当于要求，mode是现有的当前用户可以访问的权限，如果相等就返回0。假如不相等，如果当前进程得到了授权，允许其CAP_DAC_OVERRIDE，即可以凌驾于文件系统的访问权限控制机制DAC之上。
		return 0;

	/* read and search access */
	if ((mask == S_IROTH) ||
	    (S_ISDIR(inode->i_mode)  && !(mask & ~(S_IROTH | S_IXOTH))))
		if (capable(CAP_DAC_READ_SEARCH))
			return 0;

	return -EACCES;
}

     其中mask为：

#define MAY_EXEC 1
#define MAY_WRITE 2
#define MAY_READ 4

    inode->i_mode为：

    用于对三种不同用户的访问权限：

#define S_IRWXU 00700
#define S_IRUSR 00400
#define S_IWUSR 00200
#define S_IXUSR 00100

#define S_IRWXG 00070
#define S_IRGRP 00040
#define S_IWGRP 00020
#define S_IXGRP 00010

#define S_IRWXO 00007
#define S_IROTH 00004
#define S_IWOTH 00002
#define S_IXOTH 00001

    参考这张图就好理解了。

    

    还有三个标志位，是一个状态占一个位：

#define S_ISUID  0004000
#define S_ISGID  0002000
#define S_ISVTX  0001000

    现在16位只剩下4位了，表示文件类型，要为每种文件类型都分配一个标志位就不够了，所以表示文件的类型的这4位是编码的。

#define S_IFMT  00170000
#define S_IFSOCK 0140000
#define S_IFLNK	 0120000
#define S_IFREG  0100000
#define S_IFBLK  0060000
#define S_IFDIR  0040000
#define S_IFCHR  0020000
#define S_IFIFO  0010000

    capable，代码如下：

static inline int capable(int cap)
{
#if 1 /* ok now */
	if (cap_raised(current->cap_effective, cap))
#else
	if (cap_is_fs_cap(cap) ? current->fsuid == 0 : current->euid == 0)
#endif
	{
		current->flags |= PF_SUPERPRIV;
		return 1;
	}
	return 0;
}

#define cap_raised(c, flag)  (cap_t(c) & CAP_TO_MASK(flag))

#define cap_t(x) (x)

#define CAP_TO_MASK(x) (1 << (x))

   

    current->cap_effective是在sys_execve->do_execve->prepare_binprm初始设置的。

int prepare_binprm(struct linux_binprm *bprm)
{
	int mode;
	struct inode * inode = bprm->file->f_dentry->d_inode;

	mode = inode->i_mode;
	/* Huh? We had already checked for MAY_EXEC, WTF do we check this? */
	if (!(mode & 0111))	/* with at least _one_ execute bit set */
		return -EACCES;
	if (bprm->file->f_op == NULL)
		return -EACCES;

	bprm->e_uid = current->euid;
	bprm->e_gid = current->egid;

	if(!IS_NOSUID(inode)) {
		/* Set-uid? */
		if (mode & S_ISUID)
			bprm->e_uid = inode->i_uid;

		/* Set-gid? */
		/*
		 * If setgid is set but no group execute bit then this
		 * is a candidate for mandatory locking, not a setgid
		 * executable.
		 */
		if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
			bprm->e_gid = inode->i_gid;
	}

	/* We don't have VFS support for capabilities yet */
	cap_clear(bprm->cap_inheritable);
	cap_clear(bprm->cap_permitted);
	cap_clear(bprm->cap_effective);

	/*  To support inheritance of root-permissions and suid-root
         *  executables under compatibility mode, we raise all three
         *  capability sets for the file.
         *
         *  If only the real uid is 0, we only raise the inheritable
         *  and permitted sets of the executable file.
         */

	if (!issecure(SECURE_NOROOT)) {
		if (bprm->e_uid == 0 || current->uid == 0) {
			cap_set_full(bprm->cap_inheritable);
			cap_set_full(bprm->cap_permitted);
		}
		if (bprm->e_uid == 0) 
			cap_set_full(bprm->cap_effective);//这里设置的
	}

	memset(bprm->buf,0,BINPRM_BUF_SIZE);
	return kernel_read(bprm->file,0,bprm->buf,BINPRM_BUF_SIZE);
}