linux-kernel - Re: [PATCH v2 0/3] fuse: Add support for mounts from pid/user namespaces

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Date:	Thu, 25 Sep 2014 11:05:36 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] fuse: Add support for mounts from pid/user namespaces

Miklos Szeredi <miklos@...redi.hu> writes:

> On Wed, Sep 24, 2014 at 7:10 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>
>
>> So in summary I see:
>> - Low utility in being able to manipulate files with bad uids.
>> - Bad uids are mostly likely malicious action.
>> - make_bad_inode is trivial to analyze.
>> - No impediments to change if I am wrong.
>>
>> So unless there is a compelling case, right now I would recommend
>> returning -EIO initially.   That allows us to concentrate on the easier
>> parts of this and it leaves the changes only in fuse.
>
> The problem with marking the inode bad is that it will mark it bad for
> all instances of this filesystem.  Including ones which are in a
> namespace where the UIDs make perfect sense.

There are two cases:
app <-> fuse
fuse <-> server

I proposed mark_bad_inode for "userspace server -> fuse".
Where we have one superblock and one server so and one namespace that
they decide to talk in when the filesystem was mounted.

I think bad_inode is a reasonable response when the filesystem server
starts spewing non-sense.

> So that really doesn't look like a good solution.
>
> Doing the check in inode_permission() might be too heavyweight, but
> it's still the only one that looks sane.

For the "app <-> fuse" case we already have checks in inode_permision
that are kuid based that handle that case.  We use kuids not for
performance (although there is a small advatnage) but to much more to
keep the logic simple and maintainable.

For the "app -> fuse" case in .setattr we do need a check to verify
that the uid and gid are valid.  However that check was added with
the basic user namespace support and fuse current returns -EOVERFLOW
when that happens.

Eric
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