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

[Sumit Bose]

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(a)redhat.com&gt;
>  >> >> >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(a)redhat.com&gt;
>  >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 hope static analysers won't complain about unreachable code
here.

bye,
Sumit
>  
>  LS
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