Re: FPW compression leaks information

On Wed, Apr 15, 2015 at 9:42 PM, Michael Paquier
<michael.paquier@gmail.com> wrote:
> On Wed, Apr 15, 2015 at 9:20 PM, Michael Paquier
> <michael.paquier@gmail.com> wrote:
>> On Wed, Apr 15, 2015 at 2:22 PM, Fujii Masao wrote:
>>> On Wed, Apr 15, 2015 at 11:55 AM, Michael Paquier wrote:
>>>> 1) Doc patch to mention that it is possible that compression can give
>>>> hints to attackers when working on sensible fields that have a
>>>> non-fixed size.
>>>
>>> I think that this patch is enough as the first step.
>>
>> I'll get something done for that at least, a big warning below the
>> description of wal_compression would do it.

So here is a patch for this purpose, with the following text being used:
+       <warning>
+        <para>
+         When enabling <varname>wal_compression</varname>, there is a risk
+         to leak data similarly to the BREACH and CRIME attacks on SSL where
+         the compression ratio of a full page image gives a hint of what is
+         the existing data of this page.  Tables that contain sensitive
+         information like <structname>pg_authid</structname> with password
+         data could be potential targets to such attacks. Note that as a
+         prerequisite a user needs to be able to insert data on the same page
+         as the data targeted and need to be able to detect checkpoint
+         presence to find out if a compressed full page write is included in
+         WAL to calculate the compression ratio of a page using WAL positions
+         before and after inserting data on the page with data targeted.
+        </para>
+       </warning>

Comments and reformulations are welcome.
Regards,
--
Michael