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SubjectRe: [Question] mount points on knfsd.
Date
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Bill,

>>>Bill Hawes said:
>
> Returning the underlying directory avoids inode aliasing problems, but
> is otherwise a security problem in that it exposes part of a filesystem
> not otherwise visible. (How many sysadmins know what's below the
> mountpoints for something that's routinely coverd?)

I think that the solution to this problem is to make sure the mount
points are empty and unwritable. On ext2 filesystems the `i' flag
can be set for the mount points to solve this security problem.

I really believe that this is the correct way to handle `covered'
files. It gives the system administrator more flexability.

As an example: if computer A has /usr all on one partition
Computer B mount A:/usr on /usr
Computer A then mounts C:/usr/X11R6 on /usr/X11R6

What should a user on computer B see when they execute ls -la /usr/X11R6?

With this patch they will continue to see what they have always seen
(A:/usr/X11R6).

With your idea they would see (I think) `.' and `..' and nothing else.

And then some would suggest that C:/usr/X11R6 should be displayed.

> This issue has come up several times before, and people have sent in
> patches to let nfsd access the underlying directory. I don't think it's
> a good thing to do though.

I hope I have convienced you ;-).

> What would be acceptable is to allow the mounted-over inode to be
> visible, but nothing further, so that it could become a mountpoint but
> not serve to access any normally invisible directories.
>

Also, I think this would be hard!


---------------------------------
G. Allen Morris III

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