[SSSD] [PATCH] NSS: fix a use-after-free issue

Lukas Slebodnik lslebodn at redhat.com
Mon Nov 2 09:30:51 UTC 2015


On (02/11/15 10:05), Sumit Bose wrote:
>On Mon, Nov 02, 2015 at 09:42:51AM +0100, Lukas Slebodnik wrote:
>> On (30/10/15 17:35), Sumit Bose wrote:
>> >Hi,
>> >
>> >I found this accidentally because I was still running SSSD with
>> >MALLOC_PERTURB_ set which I used some time ago to hunt a glibc issue.
>> >
>> >This issue wasn't caught before by the unit-tests because
>> >sss_cmd_done() which frees the context is overwritten in the tests and
>> >so far didn't free the context. Additionally, since the use after free
>> >was in a debug message, the unit tests must be run with debugging enable
>> >to run over it. I think this is also the reason why Coverity or Clang
>> >didn't found this issue before. Does anyone know if it is possible to
>> >tell Converity to assume debug_level is set to 10?
>> >
>> >bye,
>> >Sumit
>> >
>> 
>> >From 309f59199c49d3d4dc4fa42229f6b2fa1e82e72a Mon Sep 17 00:00:00 2001
>> >From: Sumit Bose <sbose at redhat.com>
>> >Date: Fri, 30 Oct 2015 16:28:37 +0100
>> >Subject: [PATCH] NSS: fix a use-after-free issue
>> >
>> >---
>> > src/responder/nss/nsssrv_cmd.c  | 10 ++++++----
>> > src/tests/cmocka/test_nss_srv.c |  1 +
>> > 2 files changed, 7 insertions(+), 4 deletions(-)
>> >
>> >diff --git a/src/responder/nss/nsssrv_cmd.c b/src/responder/nss/nsssrv_cmd.c
>> >index b8bd6425e2c937ce6008fd6663fe0312ad68f01e..58d20fab11eea6f419031f439083ca5f6d8c61bb 100644
>> >--- a/src/responder/nss/nsssrv_cmd.c
>> >+++ b/src/responder/nss/nsssrv_cmd.c
>> >@@ -5455,11 +5455,13 @@ static int nss_cmd_getbysid(enum sss_cli_command cmd, struct cli_ctx *cctx)
>> >     ret = nss_check_well_known_sid(cmdctx);
>> >     if (ret != ENOENT) {
>> >         if (ret == EOK) {
>> >-            DEBUG(SSSDBG_TRACE_ALL, "SID [%s] is a Well-Known SID.\n",
>> >-                                         cmdctx->secid);
>> >-        } else {
>> >-            DEBUG(SSSDBG_OP_FAILURE, "nss_check_well_known_sid failed.\n");
>> >+            DEBUG(SSSDBG_TRACE_ALL, "SID [%s] is a Well-Known SID.\n", sid_str);
>> >+            /* message is already send and cmdctx is freed,
>> >+             * we can just return */
>> >+            return EOK;
>> >         }
>> >+
>> >+        DEBUG(SSSDBG_OP_FAILURE, "nss_check_well_known_sid failed.\n");
>>                 it would be good to log error code here.
>> >         goto done;
>> 
>> Was "use-after-free" caused only by logging sid or was it used elsewhere?
>
>ah, sorry. Yes, I made two changes here, replacing the variable in the
>debug message and returning immediately and no go to done. The
>use-after-free was in the debug message. cmdctx is not touched in the
>following code path anymore but since we are all set I think it is
>safer to just return here.
>
>> 
>> Could you append valgrind error to commit mesage?
>
>I found the issue via a core dump. Do you prefer the valgrind message or
>the first steps of a stack trace?
>
We can see when memory was released from valgrind message.
A stack trace just shows when it crashed.

LS


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