Re: postgres zeroization of dead tuples ? i.e scrubbing dead tuples with sensitive data.

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От David G. Johnston
Тема Re: postgres zeroization of dead tuples ? i.e scrubbing dead tuples with sensitive data.
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Msg-id CAKFQuwY+HB1hUH3av8JNK8yS_Q-VWwQTernQLswFrtNFDFb06Q@mail.gmail.com
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Ответ на postgres zeroization of dead tuples ? i.e scrubbing dead tuples with sensitive data.  ("Day, David" <dday@redcom.com>)
Ответы Re: postgres zeroization of dead tuples ? i.e scrubbing dead tuples with sensitive data.  (Alvaro Herrera <alvherre@2ndquadrant.com>)
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On Wed, Nov 18, 2015 at 12:45 PM, Day, David <dday@redcom.com> wrote:

Hi,

 

One of my co-workers came out of a NIST cyber-security type meeting today and asked me to delve into postgres and zeroization.

 

I am casually aware of mvcc issues and vacuuming

 

I believe the   concern,  based on my current understanding  of postgres inner workings,  is  that when a dead tuple is reclaimed by vacuuming:  Is that reclaimed space initialized in some fashion that would  shred any sensitive data that was formerly there to any  inspection by  the subsequent owner of  that disk page ? ( zeroization )

 

Not sure that is the exact question to ask but hopefully you get a feel for the requirement is  not to  leave any sensitive data laying about for

recovery by a hacker,  or at least minimize the places it could be obtained without actually being able to log into postgres or having raw disk access privileges.  

 

Thanks for any comments/instruction/links on the matter.



​"""
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Plain VACUUM (without FULL) simply reclaims space and makes it available for re-use. This form of the command can operate in parallel with normal reading and writing of the table, as an exclusive lock is not obtained. However, extra space is not returned to the operating system (in most cases); it's just kept available for re-use within the same table. VACUUM FULL rewrites the entire contents of the table into a new disk file with no extra space, allowing unused space to be returned to the operating system. This form is much slower and requires an exclusive lock on each table while it is being processed.
​"""​

​So:

1) Does VACUUM FULL perform any post-rewrite action to obliterate previous disk file?
2) Does the ready to be reused space get initialized to "zeros" during a normal VACUUM or do the previous tuple contents exist there until they are next overwritten?

Unfortunately I do not know the answers and don't wish to hazard a guess.

I'm not certain what mechanics you envision that would allow one to access this dead space without having raw disk access privileges.  In the case of VACUUM FULL PostgreSQL gives back control of the relevant file to the O/S and supposedly cannot regain access to it in any reliable (i.e., interpretable as PostgreSQL data) sense.

David J.


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