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

Lukas Slebodnik lslebodn at redhat.com
Tue Nov 3 10:46:42 UTC 2015


On (02/11/15 13:51), Sumit Bose wrote:
>On Mon, Nov 02, 2015 at 10:30:51AM +0100, Lukas Slebodnik wrote:
>> 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.
>
>good point, new version attached.
>
>bye,
>Sumit

>From 9561733a6ce32b5787acc5277e2d45026c5126db 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
>
>While handling well-known SIDs a debug statement tries to access memory that is
>already freed. This can be seen with the following output from valgrind.
>
>==17600== Invalid read of size 4
>==17600==    at 0x805ACC6: nss_cmd_getbysid (nsssrv_cmd.c:5458)
>==17600==    by 0x805AF41: nss_cmd_getnamebysid (nsssrv_cmd.c:5509)
>==17600==    by 0x80662F4: sss_cmd_execute (responder_cmd.c:161)
>==17600==    by 0x8067015: client_cmd_execute (responder_common.c:249)
>==17600==    by 0x80671F5: client_recv (responder_common.c:283)
>==17600==    by 0x806741C: client_fd_handler (responder_common.c:335)
>==17600==    by 0x45F5112: epoll_event_loop (tevent_epoll.c:728)
>==17600==    by 0x45F5112: epoll_event_loop_once (tevent_epoll.c:926)
>==17600==    by 0x45F32EE: std_event_loop_once (tevent_standard.c:114)
>==17600==    by 0x45EF3BF: _tevent_loop_once (tevent.c:530)
>==17600==    by 0x45EF5AB: tevent_common_loop_wait (tevent.c:634)
>==17600==    by 0x45F326E: std_event_loop_wait (tevent_standard.c:140)
>==17600==    by 0x45EF647: _tevent_loop_wait (tevent.c:653)
>==17600==  Address 0x4b248a0 is 72 bytes inside a block of size 88 free'd
>==17600==    at 0x402C26D: free (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
>==17600==    by 0x45FEC9E: _talloc_free_internal (talloc.c:1057)
>==17600==    by 0x45FEC9E: _talloc_free (talloc.c:1581)
>==17600==    by 0x8066085: sss_cmd_done (responder_cmd.c:93)
>==17600==    by 0x805A9B0: nss_check_well_known_sid (nsssrv_cmd.c:5382)
>==17600==    by 0x805AC86: nss_cmd_getbysid (nsssrv_cmd.c:5455)
>==17600==    by 0x805AF41: nss_cmd_getnamebysid (nsssrv_cmd.c:5509)
>==17600==    by 0x80662F4: sss_cmd_execute (responder_cmd.c:161)
>==17600==    by 0x8067015: client_cmd_execute (responder_common.c:249)
>==17600==    by 0x80671F5: client_recv (responder_common.c:283)
>==17600==    by 0x806741C: client_fd_handler (responder_common.c:335)
>==17600==    by 0x45F5112: epoll_event_loop (tevent_epoll.c:728)
>==17600==    by 0x45F5112: epoll_event_loop_once (tevent_epoll.c:926)
>==17600==    by 0x45F32EE: std_event_loop_once (tevent_standard.c:114)
>==17600==
>
>The patch contains a change to the unit tests which frees the memory in
>the wrapper for sss_cmd_done() too. This allows to detect this kind of
>issue in the unit tests as well.
>---
> 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;
Previously we used "goto done" for EOK.
I tested with pysss_nss_idmap.getnamebysid('S-1-5-32-545') and
I could not see any difference between "goto done" and "return EOK".
So I'm fine with both version or did I miss something?

But it might be simpler to read the code if we transform nested if statements
to swith case. Otherwise LGTM.

LS


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