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

Lukas Slebodnik lslebodn at redhat.com
Wed Nov 4 08:36:13 UTC 2015


On (03/11/15 12:38), Sumit Bose wrote:
>On Tue, Nov 03, 2015 at 11:46:42AM +0100, Lukas Slebodnik wrote:
>> 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?
>
>No, I just think it is safer to immediately return here. nss_cmd_done()
>just returns in the ret==EOK case as well, but since it expects cmdctx
>as an argument I think it should be only called as long as cmdctx is
>still valid.
>
>> 
>> But it might be simpler to read the code if we transform nested if statements
>> to swith case. Otherwise LGTM.
>
>good point, new version attached. I hope you don't mind the
>'return-break' sequence. I see the break here as syntactical sugar to
>underline that the case block is closed and since it is already used in
>the code
I fine with this version.

>I hope static analysers won't complain about unreachable code
>here.
No complains :-)

>From 2ceeaa87de7c2125def64cb5430f9acffbcccb72 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.
>
ACK

http://sssd-ci.duckdns.org/logs/job/31/93/summary.html

LS


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