[tahoe-dev] CRISP Advisory 2008-01

[zooko]

Okay I updated the known_issues.txt file to describe this issue.  The  
purpose of the known_issues.txt file is to enable users of Tahoe to  
know what the important issues are in released versions of Tahoe,  
whether those issues affect them, how to work-around them, and  
whether they are fixed in newer releases of Tahoe.  Please let me  
know if this text fails that purpose.

Regards,

Zooko

= Known Issues =

Below is a list of known issues in recent releases of Tahoe, and how
to manage them.


== issues in Tahoe v1.1.0, released 2008-06-11 ==

=== issue 9: more than one file can match an immutable file cap ===

In Tahoe v1.0 and v1.1.0, a flaw in the cryptographic integrity check
makes it possible for the original uploader of an immutable file to
produce more than one immutable file matching the same capability, so
that different downloads using the same capability could result in
different files.  This flaw can be exploited only by the original
uploader of an immutable file, which means that it is not a severe
vulnerability: you can still rely on the integrity check to make sure
that the file you download with a given capability is a file that the
original uploader intended.  The flaw is that the integrity check does
not also provide the guarantee that the original uploader could have
uploaded only one file per capability.

==== how to manage it ====

This was fixed in Tahoe v1.1.1, released 2008-07-21, under ticket
#491.  Upgrade to that release of Tahoe and then you can rely on the
property that there is only one file that you can download using a
given capability.  If you are still using Tahoe v1.0.0 or v1.1.0, then
remember that the original uploader could produce multiple files that
match the same capability, so for example if someone gives you a
capability, and you use it to download a file, and you give that
capability to your friend, and he uses it to download a file, you and
your friend could get different files.


=== issue 8: server out of space when writing mutable file ===

If a v1.0 or v1.1.0 storage server runs out of disk space or is
otherwise unable to write to its local filesystem, then problems can
ensue.  For immutable files, this will not lead to any problem (the
attempt to upload that share to that server will fail, the partially
uploaded share will be deleted from the storage server's "incoming
shares" directory, and the client will move on to using another
storage server instead).

If the write was an attempt to modify an existing mutable file,
however, a problem will result: when the attempt to write the new
share fails (e.g. due to insufficient disk space), then it will be
aborted and the old share will be left in place.  If enough such old
shares are left, then a subsequent read may get those old shares and
see the file in its earlier state, which is a "rollback" failure.
With the default parameters (3-of-10), six old shares will be enough
to potentially lead to a rollback failure.

==== how to manage it ====

Make sure your Tahoe storage servers don't run out of disk space.
This means refusing storage requests before the disk fills up. There
are a couple of ways to do that with v1.1.

First, there is a configuration option named "sizelimit" which will
cause the storage server to do a "du" style recursive examination of
its directories at startup, and then if the sum of the size of files
found therein is greater than the "sizelimit" number, it will reject
requests by clients to write new immutable shares.

However, that can take a long time (something on the order of a minute
of examination of the filesystem for each 10 GB of data stored in the
Tahoe server), and the Tahoe server will be unavailable to clients
during that time.

Another option is to set the "readonly_storage" configuration option
on the storage server before startup.  This will cause the storage
server to reject all requests to upload new immutable shares.

Note that neither of these configurations affect mutable shares: even
if sizelimit is configured and the storage server currently has
greater space used than allowed, or even if readonly_storage is
configured, servers will continue to accept new mutable shares and
will continue to accept requests to overwrite existing mutable shares.

Mutable files are typically used only for directories, and are usually
much smaller than immutable files, so if you use one of these
configurations to stop the influx of immutable files while there is
still sufficient disk space to receive an influx of (much smaller)
mutable files, you may be able to avoid the potential for "rollback"
failure.

A future version of Tahoe will include a fix for this issue.  Here is
[http://allmydata.org/pipermail/tahoe-dev/2008-May/000630.html the
mailing list discussion] about how that future version will work.


=== issue 7: pyOpenSSL/Twisted defect causes false alarms in tests ===

The combination of Twisted v8 and pyOpenSSL v0.7 causes the Tahoe v1.1
unit tests to fail, even though the behavior of Tahoe itself which is
being tested is correct.

==== how to manage it ====

If you are using Twisted v8 and pyOpenSSL v0.7, then please ignore
ERROR "Reactor was unclean" in test_system and test_introducer.
Downgrading to an older version of Twisted or pyOpenSSL will cause
those false alarms to stop happening.


== issues in Tahoe v1.0.0, released 2008-03-25 ==

(Tahoe v1.0 was superceded by v1.1 which was released 2008-06-11.)

=== issue 6: server out of space when writing mutable file ===

In addition to the problems caused by insufficient disk space
described above, v1.0 clients which are writing mutable files when the
servers fail to write to their filesystem are likely to think the
write succeeded, when it in fact failed. This can cause data loss.

==== how to manage it ====

Upgrade client to v1.1, or make sure that servers are always able to
write to their local filesystem (including that there is space
available) as described in "issue 1" above.


=== issue 5: server out of space when writing immutable file ===

Tahoe v1.0 clients are using v1.0 servers which are unable to write to
their filesystem during an immutable upload will correctly detect the
first failure, but if they retry the upload without restarting the
client, or if another client attempts to upload the same file, the
second upload may appear to succeed when it hasn't, which can lead to
data loss.

==== how to manage it ====

Upgrading either or both of the client and the server to v1.1 will fix
this issue.  Also it can be avoided by ensuring that the servers are
always able to write to their local filesystem (including that there
is space available) as described in "issue 1" above.


=== issue 4: large directories or mutable files of certain sizes ===

If a client attempts to upload a large mutable file with a size
greater than about 3,139,000 and less than or equal to 3,500,000 bytes
then it will fail but appear to succeed, which can lead to data loss.

(Mutable files larger than 3,500,000 are refused outright).  The
symptom of the failure is very high memory usage (3 GB of memory) and
100% CPU for about 5 minutes, before it appears to succeed, although
it hasn't.

Directories are stored in mutable files, and a directory of
approximately 9000 entries may fall into this range of mutable file
sizes (depending on the size of the filenames or other metadata
associated with the entries).

==== how to manage it ====

This was fixed in v1.1, under ticket #379.  If the client is upgraded
to v1.1, then it will fail cleanly instead of falsely appearing to
succeed when it tries to write a file whose size is in this range.  If
the server is also upgraded to v1.1, then writes of mutable files
whose size is in this range will succeed.  (If the server is upgraded
to v1.1 but the client is still v1.0 then the client will still suffer
this failure.)


=== issue 3: uploading files greater than 12 GiB ===

If a Tahoe v1.0 client uploads a file greater than 12 GiB in size,  
the file will
be silently corrupted so that it is not retrievable, but the client  
will think
that it succeeded.  This is a "data loss" failure.

==== how to manage it ====

Don't upload files larger than 12 GiB.  If you have previously  
uploaded files of
that size, assume that they have been corrupted and are not  
retrievable from the
Tahoe storage grid.  Tahoe v1.1 clients will refuse to upload files  
larger than
12 GiB with a clean failure.  A future release of Tahoe will remove this
limitation so that larger files can be uploaded.


=== issue 2: pycryptopp defect resulting in data corruption ===

Versions of pycryptopp earlier than pycryptopp-0.5.0 had a defect
which, when compiled with some compilers, would cause AES-256
encryption and decryption to be computed incorrectly.  This could
cause data corruption.  Tahoe v1.0 required, and came with a bundled
copy of, pycryptopp v0.3.

==== how to manage it ====

You can detect whether pycryptopp-0.3 has this failure when it is
compiled by your compiler.  Run the unit tests that come with
pycryptopp-0.3: unpack the "pycryptopp-0.3.tar" file that comes in the
Tahoe v1.0 {{{misc/dependencies}}} directory, cd into the resulting
{{{pycryptopp-0.3.0}}} directory, and execute {{{python ./setup.py
test}}}.  If the tests pass, then your compiler does not trigger this
failure.


=== issue 1: potential disclosure of a file through embedded
hyperlinks or JavaScript in that file ===

If there is a file stored on a Tahoe storage grid, and that file gets
downloaded and displayed in a web browser, then JavaScript or
hyperlinks within that file can leak the capability to that file to a
third party, which means that third party gets access to the file.

If there is JavaScript in the file, then it could deliberately leak
the capability to the file out to some remote listener.

If there are hyperlinks in the file, and they get followed, then
whichever server they point to receives the capability to the
file. Note that IMG tags are typically followed automatically by web
browsers, so being careful which hyperlinks you click on is not
sufficient to prevent this from happening.

==== how to manage it ====

For future versions of Tahoe, we are considering ways to close off
this leakage of authority while preserving ease of use -- the
discussion of this issue is ticket #127.

For the present, a good work-around is that if you want to store and
view a file on Tahoe and you want that file to remain private, then
remove from that file any hyperlinks pointing to other people's
servers and remove any JavaScript unless you are sure that the
JavaScript is not written to maliciously leak access.