Re: [PATCH -mm 5/7] add user namespace

Eric W. Biederman
2006-07-14T16:51:04Z

"Serge E. Hallyn" <serue@us.ibm.com> writes:

> Quoting Eric W. Biederman (ebiederm@xmission.com):
>> Dave Hansen <haveblue@us.ibm.com> writes:
>>
>> > On Thu, 2006-07-13 at 21:45 -0600, Eric W. Biederman wrote:
>> >> I think for filesystems like /proc and /sys that there will normally
>> >> be problems. However many of those problems can be rationalized away
>> >> as a reasonable optimization, or are not immediately apparent.
>> >
>> > Could you talk about some of these problems?
>>
>> Already mentioned but. rw permissions on sensitive files are for
>> uid == 0. No capability checks are performed.
>
> As Herbert (IIRC) pointed out that could/should be fixed.

Capabilities have always fitted badly in with the normal unix
permissions. So if we have a solution that works nicely with normal
unix permissions we will have a nice general solution, that is
easy for people to understand.

What I am talking about is making a small tweak to the permission
checking as below. Why do you keep avoiding even considering it?

Eric

int generic_permission(struct inode *inode, int mask,
int (*check_acl)(struct inode *inode, int mask))
{
umode_t mode = inode->i_mode;

- if (current->fsuid == inode->i_uid)
+ if ((current->fsuid == inode->i_uid) &&
+ (current->nsproxy->user_ns == inode->i_sb->user_ns))
mode >>= 6;
else {
if (IS_POSIXACL(inode) && (mode & S_IRWXG) && check_acl) {
int error = check_acl(inode, mask);
if (error == -EACCES)
goto check_capabilities;
else if (error != -EAGAIN)
return error;
}

- if (in_group_p(inode->i_gid))
+ if (in_group_p(inode->i_sb->user_ns, inode->i_gid))
mode >>= 3;
}

/*
* If the DACs are ok we don't need any capability check.
*/
if (((mode & mask & (MAY_READ|MAY_WRITE|MAY_EXEC)) == mask))
return 0;

check_capabilities:
/*
* Read/write DACs are always overridable.
* Executable DACs are overridable if at least one exec bit is set.
*/
if (!(mask & MAY_EXEC) ||
(inode->i_mode & S_IXUGO) || S_ISDIR(inode->i_mode))
if (capable(CAP_DAC_OVERRIDE))
return 0;

/*
* Searching includes executable on directories, else just read.
*/
if (mask == MAY_READ || (S_ISDIR(inode->i_mode) && !(mask & MAY_WRITE)))
if (capable(CAP_DAC_READ_SEARCH))
return 0;

return -EACCES;
}