[PATCH 2/5] dirmngr: hkp: avoid race condition when some hosts die

[Daniel Kahn Gillmor]

* 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.

Signed-off-by: Daniel Kahn Gillmor <dkg at fifthhorseman.net>
---
 dirmngr/ks-engine-hkp.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/dirmngr/ks-engine-hkp.c b/dirmngr/ks-engine-hkp.c
index bcc1750..a9d566e 100644
--- a/dirmngr/ks-engine-hkp.c
+++ b/dirmngr/ks-engine-hkp.c
@@ -209,25 +209,24 @@ host_in_pool_p (int *pool, int tblidx)
 static int
 select_random_host (int *table)
 {
-  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; (pidx = table[idx]) != -1; idx++)
+  for (idx=0; (pidx = table[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; (pidx = table[idx]) != -1; idx++)
-    if (hosttable[pidx] && !hosttable[pidx]->dead)
-      tbl[tblsize++] = pidx;
-
   if (tblsize == 1)  /* Save a get_uint_nonce.  */
     pidx = tbl[0];
   else
-- 
2.10.1