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File 0175-erts-Fix-write-after-free-bug-for-DistEntry-AGAIN.patch of Package erlang (Revision 9fe15014952aba0ee82351ad543c0798)

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From fcd81a3c4ce7b1ceceb73d030723d2d4a0985873 Mon Sep 17 00:00:00 2001
From: Sverker Eriksson <sverker@erlang.org>
Date: Tue, 15 Jun 2021 12:56:15 +0200
Subject: [PATCH] erts: Fix write-after-free bug for DistEntry (AGAIN)

Symptom:
VM crash caused by write-after-free on reference counter of DistEntry
magic binary (GH-4964).

Problem:
DistEntry reference counter too low and is therefore decremented after
deallocation.

This problem was earlier tried to be fixed by 348b9ba8045c3d71d87f826d15fca145999a1593
with a call to erts_refc_inc_if() in erts_build_dhandle(), but that
was not race-free as this scenario shows:

Scenario:
0. DistEntry refc = 1
1. Thread A do lookup of DistEntry without refc++
2. Thread A calls erts_build_dhandle()
       erts_refc_inc_if(0) does nothing as refc=1
3. Thread B do refc-- from 1 to 0 and schedules pending delete.
4. Thread A calls erts_mk_magic_ref() that does refc++ from 0 to 1

5. Thread B executes pending delete and does refc-- from 1 to 0.

DistEntry now has refc==0 while it's referred by a live dhandle term
which may later lead to write-after-free on the reference counter.

Solution:
Replace erts_refc_inc_if(0) in erts_build_dhandle()
with a combination of erts_refc_inctest()
followed by a conditional erts_refc_inc().
This is similar to the other places where we do an extra refc bump
for pending delete.
---
 erts/emulator/beam/erl_bif_unique.h       | 14 ++++++-
 erts/emulator/beam/erl_node_tables.c      |  7 +++-
 erts/emulator/test/distribution_SUITE.erl | 45 +++++++++++++++++++++++
 3 files changed, 62 insertions(+), 4 deletions(-)

diff --git a/erts/emulator/beam/erl_bif_unique.h b/erts/emulator/beam/erl_bif_unique.h
index 41fce533d6..bda9a50d25 100644
--- a/erts/emulator/beam/erl_bif_unique.h
+++ b/erts/emulator/beam/erl_bif_unique.h
@@ -87,6 +87,7 @@ ERTS_GLB_INLINE void erts_sched_make_magic_ref_in_array(ErtsSchedulerData *esdp,
 							Uint32 ref[ERTS_REF_NUMBERS]);
 ERTS_GLB_INLINE Eterm erts_sched_make_ref_in_buffer(ErtsSchedulerData *esdp,
 						    Eterm buffer[ERTS_REF_THING_SIZE]);
+ERTS_GLB_INLINE Eterm erts_mk_magic_ref_get_refc(Eterm * *hpp, ErlOffHeap * ohp, Binary*, erts_aint_t*);
 ERTS_GLB_INLINE Eterm erts_mk_magic_ref(Eterm **hpp, ErlOffHeap *ohp, Binary *mbp);
 ERTS_GLB_INLINE Binary *erts_magic_ref2bin(Eterm mref);
 ERTS_GLB_INLINE void erts_magic_ref_save_bin(Eterm ref);
@@ -175,17 +176,26 @@ erts_sched_make_ref_in_buffer(ErtsSchedulerData *esdp,
 }
 
 ERTS_GLB_INLINE Eterm
-erts_mk_magic_ref(Eterm **hpp, ErlOffHeap *ohp, Binary *bp)
+erts_mk_magic_ref_get_refc(Eterm **hpp, ErlOffHeap *ohp, Binary *bp, erts_aint_t* refcp)
 {
     Eterm *hp = *hpp;
     ASSERT(bp->intern.flags & BIN_FLAG_MAGIC);
     write_magic_ref_thing(hp, ohp, (ErtsMagicBinary *) bp);
     *hpp += ERTS_MAGIC_REF_THING_SIZE;
-    erts_refc_inc(&bp->intern.refc, 1);
+    if (refcp)
+        *refcp = erts_refc_inctest(&bp->intern.refc, 1);
+    else
+        erts_refc_inc(&bp->intern.refc, 1);
     OH_OVERHEAD(ohp, bp->orig_size / sizeof(Eterm));
     return make_internal_ref(hp);
 }
 
+ERTS_GLB_INLINE Eterm
+erts_mk_magic_ref(Eterm **hpp, ErlOffHeap *ohp, Binary *bp)
+{
+    return erts_mk_magic_ref_get_refc(hpp, ohp, bp, NULL);
+}
+
 ERTS_GLB_INLINE Binary *
 erts_magic_ref2bin(Eterm mref)
 {
diff --git a/erts/emulator/beam/erl_node_tables.c b/erts/emulator/beam/erl_node_tables.c
index b8c36f4ecd..3c806944eb 100644
--- a/erts/emulator/beam/erl_node_tables.c
+++ b/erts/emulator/beam/erl_node_tables.c
@@ -394,10 +394,13 @@ erts_build_dhandle(Eterm **hpp, ErlOffHeap* ohp,
 {
     Binary *bin = ErtsDistEntry2Bin(dep);
     Eterm mref, dhandle;
+    erts_aint_t refc;
     ASSERT(bin);
     ASSERT(ERTS_MAGIC_BIN_DESTRUCTOR(bin) == erts_dist_entry_destructor);
-    erts_refc_inc_if(&bin->intern.refc, 0, 0); /* inc for pending delete */
-    mref = erts_mk_magic_ref(hpp, ohp, bin);
+    mref = erts_mk_magic_ref_get_refc(hpp, ohp, bin, &refc);
+    if (refc < 2) {
+        erts_refc_inc(&bin->intern.refc, 2); /* inc for pending delete */
+    }
     dhandle = TUPLE2(*hpp, make_small(conn_id), mref);
     *hpp += 3;
     return dhandle;
diff --git a/erts/emulator/test/distribution_SUITE.erl b/erts/emulator/test/distribution_SUITE.erl
index 25e6ea89c8..2b8f69b2b9 100644
--- a/erts/emulator/test/distribution_SUITE.erl
+++ b/erts/emulator/test/distribution_SUITE.erl
@@ -72,6 +72,7 @@
          message_latency_large_link_exit/0,
          message_latency_large_monitor_exit/0,
          message_latency_large_exit2/0,
+         dist_entry_refc_race/1,
          system_limit/1,
          hopefull_data_encoding/1,
          hopefull_export_fun_bug/1,
@@ -83,6 +84,7 @@
          optimistic_dflags_echo/0, optimistic_dflags_sender/1,
          roundtrip/1, bounce/1, do_dist_auto_connect/1, inet_rpc_server/1,
          dist_parallel_sender/3, dist_parallel_receiver/0,
+         derr_run/1,
          dist_evil_parallel_receiver/0, make_busy/2]).
 
 %% epmd_module exports
@@ -104,6 +106,7 @@ all() ->
      contended_atom_cache_entry, contended_unicode_atom_cache_entry,
      {group, message_latency},
      {group, bad_dist}, {group, bad_dist_ext},
+     dist_entry_refc_race,
      start_epmd_false, no_epmd, epmd_module, system_limit,
      hopefull_data_encoding, hopefull_export_fun_bug,
      huge_iovec].
@@ -2816,6 +2819,48 @@ address_please(_Name, _Address, _AddressFamily) ->
     mk_rand_bin(N-1, [rand:uniform(256) - 1 | Data]).
 
 
+%% Try provoke DistEntry refc bugs (OTP-17513).
+dist_entry_refc_race(_Config) ->
+    {ok, Node} = start_node(dist_entry_refc_race, "+zdntgc 1"),
+    Pid = spawn_link(Node, ?MODULE, derr_run, [self()]),
+    {Pid, done} = receive M -> M end,
+    stop_node(Node),
+    ok.
+
+derr_run(Papa) ->
+    inet_db:set_lookup([file]), % make connection attempt fail fast
+    NScheds = erlang:system_info(schedulers_online),
+    SeqList = lists:seq(1, 25 * NScheds),
+    Nodes = [list_to_atom("none@host" ++ integer_to_list(Seq))
+             || Seq <- SeqList],
+    Self = self(),
+    Pids = [spawn_link(fun () -> derr_sender(Self, Nodes) end)
+            || _ <- SeqList],
+    derr_count(1, 8000),
+    [begin unlink(P), exit(P,kill) end || P <- Pids],
+    Papa ! {self(), done},
+    ok.
+
+derr_count(Max, Max) ->
+    done;
+derr_count(N, Max) ->
+    receive
+        count -> ok
+    end,
+    case N rem 1000 of
+        0 ->
+            io:format("Total attempts: ~bk~n", [N div 1000]);
+        _ -> ok
+    end,
+    derr_count(N+1, Max).
+
+
+derr_sender(Main, Nodes) ->
+    [{none, Node} ! msg || Node <- Nodes],
+    Main ! count,
+    derr_sender(Main, Nodes).
+
+
 %%% Utilities
 
 timestamp() ->
-- 
2.26.2

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File 0175-erts-Fix-write-after-free-bug-for-DistEntry-AGAIN.patch of Package erlang (Revision 9fe15014952aba0ee82351ad543c0798)

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