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Date: Mon, 14 Mar 2011 09:12:34 +0100
From: Stephan Mueller <stephan.mueller@...ec.com>
To: oss-security@...ts.openwall.com
Cc: Vasiliy Kulikov <segoon@...nwall.com>
Subject: Re: Untrusted fs and invalid filenames
Am Samstag, 12. März 2011, um 18:03:45 schrieb Vasiliy Kulikov:
Hi Vasiliy,
>
> What I suggest is something like "-o untrusted" option to mount. This
> would mean that the system considers the input from such fs as a malicious
> input. Such mounted fs would try to consider the data on disk as
> untrusted and to be as robust as possible, e.g. check against
> "/"-filenames, against corrupted fs structures, etc. I'd be happy to
> hear opinions about the usefulness of this feature.
I completely second your concerns.
However, how do you propose to implement that "untrusted" option? The core
problem IMHO is that the physical layout and structure in a file system is
assumed to be correct in general by the kernel. The physical file system
implementations (including any depending code, like the LSMs for interpreting
XATTRs) have some checks for an input validation. But I highly doubt that all
checks necessary for an untrusted file system layout are implemented - to have
all such checks would cause some speed penalties nobody wants to carry.
For example, the more sophisticated physical file systems (ext3/4, btrfs or
xfs come to mind) use pointers to the different blocks/extends. Is it really
ensured that misalignment of these pointers cannot cause adverse consequences
- at least crash the system?
Therefore, if you consider a file system untrusted, a simple flag "untrusted"
which disables some high-level logic (like symlinks across partitions or funky
file names) may just be window-dressing until the entire parsing of the
physical data structure layout is hardened.
Ciao
Stephan
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