Re: DoS with unprivileged mounts

[Eric W. Biederman]

Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:

> On 08/14/2013 10:42 AM, Miklos Szeredi wrote:
>> There's a simple and effective way to prevent unlink(2) and rename(2)
>> from operating on any file or directory by simply mounting something
>> on it.  In any mount instance in any namespace.
>> 
>> Was this considered in the unprivileged mount design?
>> 
>> The solution is also theoretically simple: mounts in unpriv namespaces
>> are marked "volatile" and are dissolved on an unlink type operation.
>
> I'd actually prefer the reverse: unprivileged mounts don't prevent
> unlink and rename.  If the dentry goes away, then the mount could still
> exist, sans underlying file.  (This is already supported on network
> filesystems.)

Of course we do this in network filesystems by pretending the
rename/unlink did not actually happen.  The vfs insists that it be lied
to instead of mirroring what actually happened.

Again all of this is a question about efficient data structures and not
really one of semantics.  Can either semantic be implemented in such a
way that it does not slow down the vfs?

Eric

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