[PATCH review 0/7] Bind mount escape fixes

Eric W. Biederman ebiederm at xmission.com
Sat Aug 15 21:07:41 UTC 2015



On August 15, 2015 2:48:34 PM CDT, Linus Torvalds <torvalds at linux-foundation.org> wrote:
>On Sat, Aug 15, 2015 at 12:36 PM, Linus Torvalds
><torvalds at linux-foundation.org> wrote:
>>
>> Can we really not validate ".." some clever way _without_ adding all
>> those "mount escape" flags? And by "clever" I potentially mean "not
>> clever" and in fact just fairly brute force. I'd almost prefer to
>just
>> walk the parent chains all the way to the root and validate the ".."
>> that way..
>
>For example: while it's true that walking a logn chain of parents (to
>validate that we hit root etc) would be expensive, I don't think we'd
>necessarily need to do it for the common case.
>
>For example, if out current "mnt->mnt_root" is a _real_ root (so
>IS_ROOT() is true), then we know we're not in some possibly partial
>bind mount, so we don't need to check anything else, and we can
>happily move to the parent dentry *without* having to be particularly
>careful.
>
>Otherwise we might need to walk the dentry parent chain to check that
>yes, we will hit that mnt->mnt_root" entry, and that we're not
>possibly escaping the bind mount. But even that walk is "just"
>following a chain of pointers. It's not *that* expensive.
>
>I'd much rather make ".." more expensive, if it means that we don't
>have to track the status of whether a mount has a potentially escaped
>directory in it or not.  Because I think we can avoid the costs for
>traditional non-bind mounts.
>
>No?

Yes we can compare s_root and mnt_root and only call is_subir  if they don't match.

At this point it is a matter of trade offs.

If there is not an escape I do not expect my current implementation will have a measurable cost.   And I don't expect there will be any escapes.

That said if you and Al would be happy with what you are proposing I can easily implement it.

My only concern at this point is that I know some containers run  with a bind mount for their root directory so it might be a change with a measurable cost.  At the same time shallow directory paths are the norm so I don't expect there to be much of a cost.

Eric




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