tevent signal - access after free?

[Lukas Slebodnik]

On (02/06/15 12:25), Stefan (metze) Metzmacher wrote:
>Hi Pavel,
>
>thanks for the report. I've created
>https://bugzilla.samba.org/show_bug.cgi?id=11308
>
>Does the following patch fixes the problem?
>
Yes

Do you plan to write unit test as well?


>From d022768519a54663f47cdf931e7df88cf1425dc9 Mon Sep 17 00:00:00 2001
>From: Stefan Metzmacher <metze at samba.org>
>Date: Tue, 2 Jun 2015 12:18:22 +0200
>Subject: [PATCH] tevent: fix access after free in tevent_common_check_signal()
>MIME-Version: 1.0
>Content-Type: text/plain; charset=UTF-8
>Content-Transfer-Encoding: 8bit
>
>This was reported by Pavel Březina <pbrezina at redhat.com>:
>
>    We found a crash in SSSD when a tevent signal is freed in its handler, tevent
>    than crashes when it access siginfo.
>
>    sig_info is freed in signal destructor:
>
>    > #ifdef SA_SIGINFO
>    >         if (se->sa_flags & SA_SIGINFO) {
>    >             if (sig_state->sig_info[se->signum]) {
>    >                 talloc_free(sig_state->sig_info[se->signum]);
>    >                 sig_state->sig_info[se->signum] = NULL;
>    >             }
>    >         }
>    > #endif
>
>    (gdb) bt
>    #0  0x00007f5d4d86cc74 in tevent_signal_destructor (se=0x7f5d5370f920) at
>    ../tevent_signal.c:213
>    #1  0x00007f5d4d65f233 in _talloc_free_internal () from /lib64/libtalloc.so.2
>    #2  0x00007f5d4d6593a3 in _talloc_free () from /lib64/libtalloc.so.2
>    #3  0x00007f5d4342f3d4 in proxy_child_init_done (subreq=0x7f5d5370f600) at
>    src/providers/proxy/proxy_auth.c:436
>    #4  0x00007f5d4d86b0c2 in _tevent_req_error (req=req at entry=0x7f5d5370f600,
>    error=error at entry=5, location=location at entry=0x7f5d43433010
>    "src/providers/proxy/proxy_auth.c:356")
>        at ../tevent_req.c:167
>    #5  0x00007f5d4342ef5e in pc_init_sig_handler (ev=<optimized out>,
>    sige=<optimized out>, signum=<optimized out>, count=<optimized out>,
>    __siginfo=<optimized out>, pvt=<optimized out>)
>        at src/providers/proxy/proxy_auth.c:356
>    #6  0x00007f5d4d86d48c in tevent_common_check_signal (ev=0x7f5d536de670) at
>    ../tevent_signal.c:428
>    #7  0x00007f5d4d86f28c in epoll_event_loop (tvalp=0x7fff7b568490,
>    epoll_ev=0x7f5d536de8b0) at ../tevent_epoll.c:647
>    #8  epoll_event_loop_once (ev=<optimized out>, location=<optimized out>) at
>    ../tevent_epoll.c:926
>    #9  0x00007f5d4d86d7d7 in std_event_loop_once (ev=0x7f5d536de670,
>    location=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent_standard.c:114
>    #10 0x00007f5d4d869fbd in _tevent_loop_once (ev=ev at entry=0x7f5d536de670,
>    location=location at entry=0x7f5d50faedc3 "src/util/server.c:668") at
>    ../tevent.c:530
>    #11 0x00007f5d4d86a15b in tevent_common_loop_wait (ev=0x7f5d536de670,
>    location=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent.c:634
>    #12 0x00007f5d4d86d777 in std_event_loop_wait (ev=0x7f5d536de670,
>    location=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent_standard.c:140
>    #13 0x00007f5d50f96863 in server_loop (main_ctx=0x7f5d536dfac0) at
>    src/util/server.c:668
>    #14 0x00007f5d5180aa42 in main (argc=8, argv=<optimized out>) at
>    src/providers/data_provider_be.c:2909
>
>    But then it is accessed again in tevent_common_check_signal:
>
>    > #ifdef SA_SIGINFO
>    >         if (clear_processed_siginfo) {
>    >             uint32_t j;
>    >             for (j=0;j<count;j++) {
>    >                 uint32_t ofs = (counter.seen + j)
>    >                     % TEVENT_SA_INFO_QUEUE_COUNT;
>    >                 memset((void*)&sig_state->sig_info[i][ofs],
>    >                     '\0',
>    >                     sizeof(siginfo_t));
>    >             }
>    >         }
>    > #endif
>
>    (gdb) bt
>    #0  0x00007fd7ba400505 in memset (__len=<optimized out>, __ch=<optimized out>,
>    __dest=<optimized out>) at /usr/include/bits/string3.h:84
>    #1  tevent_common_check_signal (ev=0x7fd7bfddf670) at ../tevent_signal.c:459
>    #2  0x00007fd7ba40228c in epoll_event_loop (tvalp=0x7fff85536430,
>    epoll_ev=0x7fd7bfddf8b0) at ../tevent_epoll.c:647
>    #3  epoll_event_loop_once (ev=<optimized out>, location=<optimized out>) at
>    ../tevent_epoll.c:926
>    #4  0x00007fd7ba4007d7 in std_event_loop_once (ev=0x7fd7bfddf670,
>    location=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent_standard.c:114
>    #5  0x00007fd7ba3fcfbd in _tevent_loop_once (ev=ev at entry=0x7fd7bfddf670,
>    location=location at entry=0x7fd7bdb417c3 "src/util/server.c:668") at
>    ../tevent.c:530
>    #6  0x00007fd7ba3fd15b in tevent_common_loop_wait (ev=0x7fd7bfddf670,
>    location=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent.c:634
>    #7  0x00007fd7ba400777 in std_event_loop_wait (ev=0x7fd7bfddf670,
>    location=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent_standard.c:140
>    #8  0x00007fd7bdb29343 in server_loop (main_ctx=0x7fd7bfde0ac0) at
>    src/util/server.c:668
>    #9  0x00007fd7be39ca42 in main (argc=8, argv=<optimized out>) at
>    src/providers/data_provider_be.c:2909
>
>BUG: https://bugzilla.samba.org/show_bug.cgi?id=11308
>
>Signed-off-by: Stefan Metzmacher <metze at samba.org>
>---
> lib/tevent/tevent_signal.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/lib/tevent/tevent_signal.c b/lib/tevent/tevent_signal.c
>index b54da2e..924dc05 100644
>--- a/lib/tevent/tevent_signal.c
>+++ b/lib/tevent/tevent_signal.c
>@@ -457,7 +457,7 @@ int tevent_common_check_signal(struct tevent_context *ev)
> 		}
> 
> #ifdef SA_SIGINFO
>-		if (clear_processed_siginfo) {
>+		if (clear_processed_siginfo && sig_state->sig_info[i] != NULL) {
> 			uint32_t j;
> 			for (j=0;j<count;j++) {
> 				uint32_t ofs = (counter.seen + j)
>-- 
>1.9.1
>
ACK

LS