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Fix use-after-free in pgstat_fetch_stat_backend_by_pid()

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stats_fetch_consistency set to "snapshot" causes the backend entry
"beentry" retrieved by pgstat_get_beentry_by_proc_number() to be reset
at the beginning of pgstat_fetch_stat_backend() when fetching the
backend pgstats entry.  As coded, "beentry" was being accessed after
being freed.  This commit moves all the accesses to "beentry" to happen
before calling pgstat_fetch_stat_backend(), fixing the problem.

This problem could be reached by calling the SQL functions
pg_stat_get_backend_io() or pg_stat_get_backend_wal().

Issue caught by valgrind.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/f1788cc0-253a-4a3a-aee0-1b8ab9538736@gmail.com
This commit is contained in:
Michael Paquier
2025-04-07 09:51:40 +09:00
parent 173c97812f
commit 3191a593d6
1 changed files with 12 additions and 4 deletions
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16
src/backend/utils/activity/pgstat_backend.c
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@ -133,10 +133,6 @@ pgstat_fetch_stat_backend_by_pid(int pid, BackendType *bktype)
if (!pgstat_tracks_backend_bktype(beentry->st_backendType))
return NULL;
backend_stats = pgstat_fetch_stat_backend(procNumber);
if (!backend_stats)
return NULL;
/* if PID does not match, leave */
if (beentry->st_procpid != pid)
return NULL;
@ -144,6 +140,18 @@ pgstat_fetch_stat_backend_by_pid(int pid, BackendType *bktype)
if (bktype)
*bktype = beentry->st_backendType;
/*
* Retrieve the entry. Note that "beentry" may be freed depending on the
* value of stats_fetch_consistency, so do not access it from this point.
*/
backend_stats = pgstat_fetch_stat_backend(procNumber);
if (!backend_stats)
{
if (bktype)
*bktype = B_INVALID;
return NULL;
}
return backend_stats;
}