dirmngr: hkp: Avoid potential race condition when some hosts die.

* dirmngr/ks-engine-hkp.c (select_random_host): Use atomic pass
through the host table instead of risking out-of-bounds write.

--

Multiple threads may write to hosttable[x]->dead while
select_random_host() is running.  For example, a housekeeping thread
might clear the ->dead bit on some entries, or another connection to
dirmngr might manually mark a host as alive.

If one or more hosts are resurrected between the two loops over a
given table in select_random_host(), then the allocation of tbl might
not be large enough, resulting in a write past the end of tbl on the
second loop.

This change collapses the two loops into a single loop to avoid this
discrepancy: each host's "dead" bit is now only checked once.

As Werner points out, this isn't currently strictly necessary, since
npth will not switch threads unless a blocking system call is made,
and no blocking system call is made in these two loops.

However, in a subsequent change in this series, we will call a
function in this loop, and that function may sometimes write(2), or
call other functions, which may themselves block.  Keeping this as a
single-pass loop avoids the need to keep track of what might block and
what might not.

Signed-off-by: Daniel Kahn Gillmor <[email protected]>

Gbp-Pq: Topic dirmngr-idling
Gbp-Pq: Name dirmngr-hkp-Avoid-potential-race-condition-when-some.patch

1 files changed, 10 insertions, 13 deletions

diff --git a/dirmngr/ks-engine-hkp.c b/dirmngr/ks-engine-hkp.c
index a8b1ec271..e67861c98 100644
--- a/dirmngr/ks-engine-hkp.c
+++ b/dirmngr/ks-engine-hkp.c
@@ -225,29 +225,26 @@ host_in_pool_p (hostinfo_t hi, int tblidx)
 static int
 select_random_host (hostinfo_t hi)
 {
-  int *tbl;
-  size_t tblsize;
+  int *tbl = NULL;
+  size_t tblsize = 0;
   int pidx, idx;
 
   /* We create a new table so that we randomly select only from
      currently alive hosts.  */
-  for (idx = 0, tblsize = 0;
+  for (idx = 0;
        idx < hi->pool_len && (pidx = hi->pool[idx]) != -1;
        idx++)
     if (hosttable[pidx] && !hosttable[pidx]->dead)
-      tblsize++;
+      {
+        tblsize++;
+        tbl = xtryrealloc(tbl, tblsize * sizeof *tbl);
+        if (!tbl)
+          return -1; /* memory allocation failed! */
+        tbl[tblsize-1] = pidx;
+      }
   if (!tblsize)
     return -1; /* No hosts.  */
 
-  tbl = xtrymalloc (tblsize * sizeof *tbl);
-  if (!tbl)
-    return -1;
-  for (idx = 0, tblsize = 0;
-       idx < hi->pool_len && (pidx = hi->pool[idx]) != -1;
-       idx++)
-    if (hosttable[pidx] && !hosttable[pidx]->dead)
-      tbl[tblsize++] = pidx;
-
   if (tblsize == 1)  /* Save a get_uint_nonce.  */
     pidx = tbl[0];
   else