Cyrus Sieve futures

Anatoli me at
Tue Feb 7 02:36:56 EST 2017

Hi Ken,

I don't have any special uses for Sieve apart from the most basic ones, 
but I would like to ask you if you see it feasible, as part of this 
work, to separate the "numbers crunching" (e.g. rules matching, 
transformations, etc.) code from the I/O code (that is responsible for 
manipulating files, communicating with other processes, etc.)?

My idea is to isolate and safeguard Sieve's "numbers crunching" code 
with <> seccomp 
<>, which basically 
only allows to execute userland code (i.e. no syscalls, etc.) and 
read/write to already-open file descriptors. This should reduce 
currently broad attack surface to some really exotic bugs in the Linux 
kernel (up to now just one significant bug for strict seccomp bypass was 
discovered (CVE-2009-0835 in 2.6 kernel 8 years ago), soon after this 
functionality was implemented).

One of possible architectures could be described this way (similar to 
Postfix architecture <>):

 1. Sieve daemon receives an incoming data for processing and stores it
    in a memory buffer (without touching it)
 2. It creates a new "results" file and opens it for writing
 3. It forks a process
 4. The newly created child process switches to seccomp
 5. It processes the data and writes the results (in a special format)
    to the "results" file, then exits
 6. Sieve daemon detects the exit of the child process and reads the
    "results" file to perform the requested actions, then deletes the file
 7. If the child is killed (as the result of seccomp restrictions
    violation) or something is wrong with the format of the "results"
    file, Sieve daemon quarantines the data and writes an error to the logs

The format for the "results" file should be simple and well defined, and 
the code to interpret it should be carefully written. This could be 
started as a mere adaptation for the new architecture of the current 
actions processing logic and be progressively improved later. This would 
be much easier than making the entire Sieve code base and the libraries 
it uses (e.g. PCRE) reasonably safe (PCRE alone is a huge bag of 
including lots of RCEs).

Another question (just wondering if it's in your (or other devs) plans 
and its feasibility): is it practically possible to implement for Sieve 
something like "run the rules on X folder (+ subfolders)" same way as 
local rules in most MUAs could be applied to already stored mails? I 
find lack of this feature as the only (but notable) downside to Sieve vs 
local rules.


*From:* Ken Murchison Via Cyrus-devel
*Sent:* Monday, February 06, 2017 19:34
*To:* Cyrus-devel
*Subject:* Cyrus Sieve futures

> All,
> I'm in the process of rewriting the Sieve parser and adding new 
> extensions for what will become part of Cyrus v3.1.  We currently 
> support deprecated and non-standardized extensions "imapflags" 
> (standardized as "imap4flags) and "notify" (standardized as 
> "enotify").  I'd like to rip out the parser and bytecode generator for 
> these extensions, and leave just the bytecode executing code for the 
> deprecated actions "mark", "unmark", and "denotify".
> Any existing scripts using these actions (or the older "notify" 
> syntax) would continue to run.  New/updated scripts would have to 
> switch to using the updated "notify" syntax and replace "mark" and 
> "unmark" with "setflag"/"addflag" and "removeflag".  Does anyone have 
> an issue with these changes?
> Does anyone have any requests for standard extensions that we don't 
> currently support?  Note that "variables", "mailbox" and "*metadata" 
> will be in Cyrus 3.0 and "ereject", "editheader", and "extlists" are 
> already in what will be the 3.1 branch.
> Extensions that I'm looking at implementing (pretty much because they 
> are low-hanging fruit) are "duplicate", "environment", and "ihave".  I 
> may also look at "replace" and "extracttext" which would be useful if 
> we add handling of calendar events to Sieve.

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