The Samba-Bugzilla – Attachment 16464 Details for
Bug 14595
CVE-2020-27840 [SECURITY] Unauthenticated remote heap corruption via bad DNs
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[patch]
patch for 4.11 v2
CVE-2020-27840-patch-for-4-11-v2.patch (text/plain), 9.47 KB, created by
Douglas Bagnall
on 2021-02-16 22:42:25 UTC
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Description:
patch for 4.11 v2
Filename:
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Creator:
Douglas Bagnall
Created:
2021-02-16 22:42:25 UTC
Size:
9.47 KB
patch
obsolete
>From 4f2cf13ea2f290c678c60f756c2668fb54c72240 Mon Sep 17 00:00:00 2001 >From: Douglas Bagnall <douglas.bagnall@catalyst.net.nz> >Date: Thu, 11 Feb 2021 17:05:14 +1300 >Subject: [PATCH 1/3] CVE-2020-27840: pytests:segfault: add ldb.Dn validate > test > >ldb.Dn.validate wraps ldb_dn_explode. > >BUG: https://bugzilla.samba.org/show_bug.cgi?id=14595 > >Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz> >Reviewed-by: Andrew Bartlett <abartlet@samba.org> >--- > python/samba/tests/segfault.py | 6 ++++++ > selftest/knownfail.d/python-segfaults | 1 + > 2 files changed, 7 insertions(+) > >diff --git a/python/samba/tests/segfault.py b/python/samba/tests/segfault.py >index b95c1ef5fa9..7e306a61549 100644 >--- a/python/samba/tests/segfault.py >+++ b/python/samba/tests/segfault.py >@@ -134,3 +134,9 @@ class SegfaultTests(samba.tests.TestCase): > except ldb.LdbError: > pass > str(m) >+ >+ @segfault_detector >+ def test_ldb_dn_explode_crash(self): >+ for i in range(106, 150): >+ dn = ldb.Dn(ldb.Ldb(), "a=b%s,c= " % (' ' * i)) >+ dn.validate() >diff --git a/selftest/knownfail.d/python-segfaults b/selftest/knownfail.d/python-segfaults >index c8331d941ef..5a440437c4d 100644 >--- a/selftest/knownfail.d/python-segfaults >+++ b/selftest/knownfail.d/python-segfaults >@@ -4,3 +4,4 @@ samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_hive_open_ldb > samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_net_replicate_chunk_1 > samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_net_replicate_init__1 > samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_net_replicate_init__3 >+samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_ldb_dn_explode_crash >-- >2.25.1 > > >From 8f6086f66f249e7569e7157b479e880988d67547 Mon Sep 17 00:00:00 2001 >From: Douglas Bagnall <douglas.bagnall@catalyst.net.nz> >Date: Fri, 11 Dec 2020 16:32:25 +1300 >Subject: [PATCH 2/3] CVE-2020-27840 ldb_dn: avoid head corruption in > ldb_dn_explode > >A DN string with lots of trailing space can cause ldb_dn_explode() to >put a zero byte in the wrong place in the heap. > >When a DN string has a value represented with trailing spaces, >like this > > "CN=foo ,DC=bar" > >the whitespace is supposed to be ignored. We keep track of this in the >`t` pointer, which is NULL when we are not walking through trailing >spaces, and points to the first space when we are. We are walking with >the `p` pointer, writing the value to `d`, and keeping the length in >`l`. > > "CN=foo ,DC= " ==> "foo " > ^ ^ ^ > t p d > --l--- > >The value is finished when we encounter a comma or the end of the >string. If `t` is not NULL at that point, we assume there are trailing >spaces and wind `d and `l` back by the correct amount. Then we switch >to expecting an attribute name (e.g. "CN"), until we get to an "=", >which puts us back into looking for a value. > >Unfortunately, we forget to immediately tell `t` that we'd finished >the last value, we can end up like this: > > "CN=foo ,DC= " ==> "" > ^ ^ ^ > t p d > l=0 > >where `p` is pointing to a new value that contains only spaces, while >`t` is still referring to the old value. `p` notices the value ends, >and we subtract `p - t` from `d`: > > "CN=foo ,DC= " ==> ? "" > ^ ^ ^ > t p d > l ~= SIZE_MAX - 8 > >At that point `d` wants to terminate its string with a '\0', but >instead it terminates someone else's byte. This does not crash if the >number of trailing spaces is small, as `d` will point into a previous >value (a copy of "foo" in this example). Corrupting that value will >ultimately not matter, as we will soon try to allocate a buffer `l` >long, which will be greater than the available memory and the whole >operation will fail properly. > >However, with more spaces, `d` will point into memory before the >beginning of the allocated buffer, with the exact offset depending on >the length of the earlier attributes and the number of spaces. > >What about a longer DN with more attributes? For example, >"CN=foo ,DC= ,DC=example,DC=com" -- since `d` has moved out of >bounds, won't we continue to use it and write more DN values into >mystery memory? Fortunately not, because the aforementioned allocation >of `l` bytes must happen first, and `l` is now huge. The allocation >happens in a talloc_memdup(), which is by default restricted to >allocating 256MB. > >So this allows a person who controls a string parsed by ldb_dn_explode >to corrupt heap memory by placing a single zero byte at a chosen >offset before the allocated buffer. > >An LDAP bind request can send a string DN as a username. This DN is >necessarily parsed before the password is checked, so an attacker does >not need proper credentials. The attacker can easily cause a denial of >service and we cannot rule out more subtle attacks. > >The immediate solution is to reset `t` to NULL when a comma is >encountered, indicating that we are no longer looking at trailing >whitespace. > >Found with the help of Honggfuzz. > >BUG: https://bugzilla.samba.org/show_bug.cgi?id=14595 > >Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz> >Reviewed-by: Andrew Bartlett <abartlet@samba.org> >--- > lib/ldb/common/ldb_dn.c | 1 + > selftest/knownfail.d/python-segfaults | 1 - > 2 files changed, 1 insertion(+), 1 deletion(-) > >diff --git a/lib/ldb/common/ldb_dn.c b/lib/ldb/common/ldb_dn.c >index 83f94e3b913..047244287f5 100644 >--- a/lib/ldb/common/ldb_dn.c >+++ b/lib/ldb/common/ldb_dn.c >@@ -570,6 +570,7 @@ static bool ldb_dn_explode(struct ldb_dn *dn) > /* trim back */ > d -= (p - t); > l -= (p - t); >+ t = NULL; > } > > in_attr = true; >diff --git a/selftest/knownfail.d/python-segfaults b/selftest/knownfail.d/python-segfaults >index 5a440437c4d..c8331d941ef 100644 >--- a/selftest/knownfail.d/python-segfaults >+++ b/selftest/knownfail.d/python-segfaults >@@ -4,4 +4,3 @@ samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_hive_open_ldb > samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_net_replicate_chunk_1 > samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_net_replicate_init__1 > samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_net_replicate_init__3 >-samba.tests.segfault.samba.tests.segfault.SegfaultTests.test_ldb_dn_explode_crash >-- >2.25.1 > > >From aecdb95ab26845f0993c3bd7a354ab7554120cab Mon Sep 17 00:00:00 2001 >From: Douglas Bagnall <douglas.bagnall@catalyst.net.nz> >Date: Thu, 11 Feb 2021 16:28:43 +1300 >Subject: [PATCH 3/3] CVE-2020-27840: pytests: move Dn.validate test to ldb > >We had the test in the Samba Python segfault suite because >a) the signal catching infrastructure was there, and >b) the ldb tests lack Samba's knownfail mechanism, which allowed us to > assert the failure. > >BUG: https://bugzilla.samba.org/show_bug.cgi?id=14595 > >Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz> >Reviewed-by: Andrew Bartlett <abartlet@samba.org> >--- > lib/ldb/tests/python/crash.py | 45 ++++++++++++++++++++++++++++++++++ > lib/ldb/wscript | 1 + > python/samba/tests/segfault.py | 6 ----- > 3 files changed, 46 insertions(+), 6 deletions(-) > create mode 100644 lib/ldb/tests/python/crash.py > >diff --git a/lib/ldb/tests/python/crash.py b/lib/ldb/tests/python/crash.py >new file mode 100644 >index 00000000000..32839814552 >--- /dev/null >+++ b/lib/ldb/tests/python/crash.py >@@ -0,0 +1,45 @@ >+#!/usr/bin/env python3 >+# >+# Tests for crashing functions >+ >+import os >+from unittest import TestCase >+import os >+import sys >+import traceback >+ >+import ldb >+ >+ >+def segfault_detector(f): >+ def wrapper(*args, **kwargs): >+ pid = os.fork() >+ if pid == 0: >+ # child, crashing? >+ try: >+ f(*args, **kwargs) >+ except Exception as e: >+ traceback.print_exc() >+ sys.stderr.flush() >+ sys.stdout.flush() >+ os._exit(0) >+ >+ # parent, waiting >+ pid2, status = os.waitpid(pid, 0) >+ if os.WIFSIGNALED(status): >+ signal = os.WTERMSIG(status) >+ raise AssertionError("Failed with signal %d" % signal) >+ >+ return wrapper >+ >+ >+class LdbDnCrashTests(TestCase): >+ @segfault_detector >+ def test_ldb_dn_explode_crash(self): >+ for i in range(106, 150): >+ dn = ldb.Dn(ldb.Ldb(), "a=b%s,c= " % (' ' * i)) >+ dn.validate() >+ >+if __name__ == '__main__': >+ import unittest >+ unittest.TestProgram() >diff --git a/lib/ldb/wscript b/lib/ldb/wscript >index da2b935d102..7e0179419df 100644 >--- a/lib/ldb/wscript >+++ b/lib/ldb/wscript >@@ -614,6 +614,7 @@ def test(ctx): > os.mkdir(tmp_dir) > pyret = samba_utils.RUN_PYTHON_TESTS( > ['tests/python/api.py', >+ 'tests/python/crash.py', > 'tests/python/index.py', > 'tests/python/repack.py'], > extra_env={'SELFTEST_PREFIX': test_prefix}) >diff --git a/python/samba/tests/segfault.py b/python/samba/tests/segfault.py >index 7e306a61549..b95c1ef5fa9 100644 >--- a/python/samba/tests/segfault.py >+++ b/python/samba/tests/segfault.py >@@ -134,9 +134,3 @@ class SegfaultTests(samba.tests.TestCase): > except ldb.LdbError: > pass > str(m) >- >- @segfault_detector >- def test_ldb_dn_explode_crash(self): >- for i in range(106, 150): >- dn = ldb.Dn(ldb.Ldb(), "a=b%s,c= " % (' ' * i)) >- dn.validate() >-- >2.25.1 >
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