From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Peter Geoghegan <pg@bowt.ie>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Adrien Nayrat <adrien.nayrat@anayrat.info>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Vik Fearing <vik.fearing@2ndquadrant.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: David Rowley <david.rowley@2ndquadrant.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: random() (was Re: New GUC to sample log queries)
Date: Fri, 28 Dec 2018 18:15:05 -0500
Message-ID: <10913.1546038905@sss.pgh.pa.us> (raw)
In-Reply-To: <9703.1545939393@sss.pgh.pa.us>
References: <20181129214850.kiq6mvofsgplqjdr@alvherre.pgsql>
<3859.1545849900@sss.pgh.pa.us>
<20181227013458.GF2106@paquier.xyz>
<32696.1545875185@sss.pgh.pa.us>
<CAH2-Wz=xJiy+BgU7M49L6EHSkMbJWbzbprge=AqyOC8aDXRj8A@mail.gmail.com>
<2820.1545878340@sss.pgh.pa.us>
<CAH2-WznuXsWmjnvbt=odKMVxpj1zutgzRb7s6Vu3VPXCjejS5g@mail.gmail.com>
<CAEepm=1tJB2=w_GdXHRLkWQcCnFWGdmNiEdd5rupShKT=DwWyQ@mail.gmail.com>
<9703.1545939393@sss.pgh.pa.us>
I wrote:
> Thomas Munro <thomas.munro@enterprisedb.com> writes:
>> +1, but I wonder if just separating them is enough. Is our seeding
>> algorithm good enough for this new purpose? The initial seed is 100%
>> predictable to a logged in user (it's made from the backend PID and
>> backend start time, which we tell you), and not even that hard to
>> guess from the outside, so I think Coverity's warning is an
>> understatement in this case. Even if we separate the PRNG state used
>> for internal stuff so that users can't clobber its seed from SQL,
>> wouldn't it be possible to predict which statements will survive the
>> log sampling filter given easily available information and a good
>> guess at how many times random() (or whatever similar thing) has been
>> called so far?
> Yeah, that's a good point. Maybe we should upgrade the per-process
> seed initialization to make it less predictable. I could see expending
> a call of the strong RNG to contribute some more noise to the seeds
> selected in InitProcessGlobals().
Here's a simple patch to do so.
Looking at this, I seem to remember that we considered doing exactly this
awhile ago, but refrained because there was concern about depleting the
system's reserve of entropy if we have a high backend spawn rate, and it
didn't seem like there was a security reason to insist on unpredictable
random() results. However, the log-sampling patch destroys the latter
argument. As for the former argument, I'm not sure how big a deal that
really is. Presumably, the act of spawning a backend would itself
contribute some more entropy to the pool (particularly if a network
connection is involved), so the depletion problem might be fictitious
in the first place. Also, a few references I consulted, such as the
Linux urandom(4) man page, suggest that even in a depleted-entropy
state the results of reading /dev/urandom should be random enough
for all but the very strictest security requirements.
Thoughts?
regards, tom lane
Attachments:
[text/x-diff] make-random-seed-more-random-1.patch (2.5K, ../10913.1546038905@sss.pgh.pa.us/2-make-random-seed-more-random-1.patch)
download | inline diff:diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.cindex eedc617..2d5a0ac 100644
*** a/src/backend/postmaster/postmaster.c
--- b/src/backend/postmaster/postmaster.c
*************** ClosePostmasterPorts(bool am_syslogger)
*** 2520,2530 ****
/*
* InitProcessGlobals -- set MyProcPid, MyStartTime[stamp], random seeds
*
! * Called early in every backend.
*/
void
InitProcessGlobals(void)
{
MyProcPid = getpid();
MyStartTimestamp = GetCurrentTimestamp();
MyStartTime = timestamptz_to_time_t(MyStartTimestamp);
--- 2520,2532 ----
/*
* InitProcessGlobals -- set MyProcPid, MyStartTime[stamp], random seeds
*
! * Called early in the postmaster and every backend.
*/
void
InitProcessGlobals(void)
{
+ unsigned int rseed;+
MyProcPid = getpid();
MyStartTimestamp = GetCurrentTimestamp();
MyStartTime = timestamptz_to_time_t(MyStartTimestamp);
*************** InitProcessGlobals(void)
*** 2539,2553 ****
#endif
/*
! * Set a different seed for random() in every backend. Since PIDs and
! * timestamps tend to change more frequently in their least significant
! * bits, shift the timestamp left to allow a larger total number of seeds
! * in a given time period. Since that would leave only 20 bits of the
! * timestamp that cycle every ~1 second, also mix in some higher bits.
*/
! srandom(((uint64) MyProcPid) ^
((uint64) MyStartTimestamp << 12) ^
! ((uint64) MyStartTimestamp >> 20));
}
--- 2541,2570 ----
#endif
/*
! * Set a different seed for random() in every process. We want something
! * unpredictable, so if possible, use high-quality random bits for the
! * seed. Otherwise, fall back to a seed based on timestamp and PID.
! *
! * Note we can't use pg_backend_random here, since this is used in the
! * postmaster, and even in a backend we might not be attached to shared
! * memory yet.
*/
! #ifdef HAVE_STRONG_RANDOM
! if (!pg_strong_random(&rseed, sizeof(rseed)))
! #endif
! {
! /*
! * Since PIDs and timestamps tend to change more frequently in their
! * least significant bits, shift the timestamp left to allow a larger
! * total number of seeds in a given time period. Since that would
! * leave only 20 bits of the timestamp that cycle every ~1 second,
! * also mix in some higher bits.
! */
! rseed = ((uint64) MyProcPid) ^
((uint64) MyStartTimestamp << 12) ^
! ((uint64) MyStartTimestamp >> 20);
! }
! srandom(rseed);
}
=
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, thomas.munro@enterprisedb.com, hlinnaka@iki.fi, pg@bowt.ie, michael@paquier.xyz, alvherre@2ndquadrant.com, adrien.nayrat@anayrat.info, 9erthalion6@gmail.com, tomas.vondra@2ndquadrant.com, vik.fearing@2ndquadrant.com, robertmhaas@gmail.com, david.rowley@2ndquadrant.com
Subject: Re: random() (was Re: New GUC to sample log queries)
In-Reply-To: <10913.1546038905@sss.pgh.pa.us>
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