Re: [PATCH] private mounts

From: Bill Davidsen
Date: Wed Apr 27 2005 - 15:58:01 EST

Ram wrote:
On Wed, 2005-04-27 at 11:09, Miklos Szeredi wrote:


user 1 does a invisible mount on /mnt/mnt1
root does a visible mount on /mnt/mnt1

user 1 will no longer be able to access his /mnt/mnt1

in fact even if root mounts something on /mnt, the problem still exists.

This is not something specific to FUSE. Root can overmount any of
your directories after which you won't be able to access it (unless
some of your processes have a CWD there).

sorry, I think I have not raised by concern clearly.

I am mostly talking about the semantics of 'invisible/private mount' not
FUSE in particular, since the kernel patch brings in new feature
to VFS.

My understanding of private mount is:
1. The contents of the private mount is visible only to the mount owner.
2. The vfsmount of the private mount is only accessible to
the mount owner, and only the mount owner can mount anything
on top of it.

But I dont see (2) is being checked for.

I can overmount something on top of a private mount owned by someother
user. I verified that with your patch.

1. do a invisible mount as user 'x' on /mnt
2. do a visible mount as root on /mnt and it *succeeds* and also masks
the earlier mount to the user 'x'.

I am not concerned about the masking effect so much. But I am concerned
that the private vfsmount at /mnt is accessible to someother user to mount something else on top of it. **The dentry on top of which the
new vfsmount is done belongs to the private vfsmount**.

Am I making sense? If I do make sense, than all we need is a patch on
top of your patch which disallows non-owner to mount something on top of
a private/invisible vfsmount owned by some owner.

If I am not making sense, I keep quite :)

I think you point out a solution could be worse that what it cures. There are clearly problems with mount over, but imagine that a user does an invisible mount over /mnt, doesn't that prevent other mounts which are usually made, like /mnt/cdrom, /mnt/loopN, etc?

Every time someone suggests a solution it seems to open a new path to possible abuse. And features which only work with a monotonic kernel rather than modules would seem to indicate that the feature is nice but the implementation might benefit from more thinking time.

Frankly the whole statement that the controversial code MUST go in now and could be removed later sounds like a salesman telling me I MUST sign the contract today, but he will let me out of it if I decide it was a mistake.

I'm not against the feature, but a lot of people I consider competent seem to find the implementation controversial, which argues for waiting until more eyes are on the code. If the rest of the code is useless without the controversial part, maybe it should all stay a patch to use or not as people decide.

-bill davidsen (davidsen@xxxxxxx)
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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