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Commit4203842

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Use pg_strong_random() to select each server process's random seed.
Previously we just set the seed based on process ID and start timestamp.Both those values are directly available within the session, and canbe found out or guessed by other users too, making the session's seriesof random(3) values fairly predictable. Up to now, our backend-internaluses of random(3) haven't seemed security-critical, but commit88bdbd3added one that potentially is: when using log_statement_sample_rate, auser might be able to predict which of his SQL statements will get logged.To improve this situation, upgrade the per-process seed initializationmethod to use pg_strong_random() if available, greatly reducing thepredictability of the initial seed value. This adds a few tens ofmicroseconds to process start time, but since backend startup time isat least a couple of milliseconds, that seems an acceptable price.This means that pg_strong_random() needs to be able to run withoutreliance on any backend infrastructure, since it will be invokedbefore any of that is up. It was safe for that already, but adjustcomments and #include commands to make it clearer.Discussion:https://postgr.es/m/3859.1545849900@sss.pgh.pa.us
1 parent6645ad6 commit4203842

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2 files changed

+31
-14
lines changed

2 files changed

+31
-14
lines changed

‎src/backend/postmaster/postmaster.c

Lines changed: 25 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -2520,11 +2520,13 @@ ClosePostmasterPorts(bool am_syslogger)
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/*
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* InitProcessGlobals -- set MyProcPid, MyStartTime[stamp], random seeds
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*
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* Called early in every backend.
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* Called early inthe postmaster andevery backend.
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*/
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void
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InitProcessGlobals(void)
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{
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unsignedintrseed;
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MyProcPid=getpid();
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MyStartTimestamp=GetCurrentTimestamp();
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MyStartTime=timestamptz_to_time_t(MyStartTimestamp);
@@ -2539,15 +2541,30 @@ InitProcessGlobals(void)
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#endif
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/*
2542-
* Set a different seed for random() in every backend. Since PIDs and
2543-
* timestamps tend to change more frequently in their least significant
2544-
* bits, shift the timestamp left to allow a larger total number of seeds
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* in a given time period. Since that would leave only 20 bits of the
2546-
* timestamp that cycle every ~1 second, also mix in some higher bits.
2544+
* Set a different seed for random() in every process. We want something
2545+
* unpredictable, so if possible, use high-quality random bits for the
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* seed. Otherwise, fall back to a seed based on timestamp and PID.
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*
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* Note we can't use pg_backend_random here, since this is used in the
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* postmaster, and even in a backend we might not be attached to shared
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* memory yet.
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*/
2548-
srandom(((uint64)MyProcPid) ^
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#ifdefHAVE_STRONG_RANDOM
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if (!pg_strong_random(&rseed,sizeof(rseed)))
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#endif
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{
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/*
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* Since PIDs and timestamps tend to change more frequently in their
2558+
* least significant bits, shift the timestamp left to allow a larger
2559+
* total number of seeds in a given time period. Since that would
2560+
* leave only 20 bits of the timestamp that cycle every ~1 second,
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* also mix in some higher bits.
2562+
*/
2563+
rseed= ((uint64)MyProcPid) ^
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((uint64)MyStartTimestamp <<12) ^
2550-
((uint64)MyStartTimestamp >>20));
2565+
((uint64)MyStartTimestamp >>20);
2566+
}
2567+
srandom(rseed);
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}
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‎src/port/pg_strong_random.c

Lines changed: 6 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -6,6 +6,10 @@
66
* Our definition of "strong" is that it's suitable for generating random
77
* salts and query cancellation keys, during authentication.
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*
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* Note: this code is run quite early in postmaster and backend startup;
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* therefore, even when built for backend, it cannot rely on backend
11+
* infrastructure such as elog() or palloc().
12+
*
913
* Copyright (c) 1996-2018, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
@@ -14,11 +18,7 @@
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*-------------------------------------------------------------------------
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*/
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17-
#ifndefFRONTEND
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#include"postgres.h"
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#else
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#include"postgres_fe.h"
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#endif
21+
#include"c.h"
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2323
#include<fcntl.h>
2424
#include<unistd.h>
@@ -44,7 +44,7 @@ static HCRYPTPROV hProvider = 0;
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* Read (random) bytes from a file.
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*/
4646
staticbool
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random_from_file(char*filename,void*buf,size_tlen)
47+
random_from_file(constchar*filename,void*buf,size_tlen)
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{
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intf;
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char*p=buf;

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