Fix potential double-free() in Interactive Service (CVE-2018-9336)

Malformed input data on the service pipe towards the OpenVPN interactive service (normally used by the OpenVPN GUI to request openvpn instances from the service) can result in a double free() in the error handling code.

This usually only leads to a process crash (DoS by an unprivileged local account) but since it could possibly lead to memory corruption if happening while multiple other threads are active at the same time, CVE-2018-9336 has been assigned to acknowledge this risk.

Fix by ensuring that sud->directory is set to NULL in GetStartUpData() for all error cases (thus not being free()ed in FreeStartupData()).

Rewrite control flow to use explicit error label for error exit.

Discovered and reported by Jacob Baines <[email protected]...>.

CVE: 2018-9336

--
v2: reword commit message, no code changes

---
 src/openvpnserv/interactive.c | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

index fbc32f90..861f5e70 100644
@@ -453,7 +453,6 @@ static BOOL
 GetStartupData(HANDLE pipe, STARTUP_DATA *sud)
 {
     size_t size, len;
-    BOOL ret = FALSE;
     WCHAR *data = NULL;
     DWORD bytes, read;
 

@@ -462,7 +461,7 @@ GetStartupData(HANDLE pipe, STARTUP_DATA *sud)
     {
         MsgToEventLog(M_SYSERR, TEXT("PeekNamedPipeAsync failed"));
         ReturnLastError(pipe, L"PeekNamedPipeAsync");
-        goto out;
+        goto err;
     }
 
     size = bytes / sizeof(*data);

@@ -470,7 +469,7 @@ GetStartupData(HANDLE pipe, STARTUP_DATA *sud)
     {
         MsgToEventLog(M_SYSERR, TEXT("malformed startup data: 1 byte received"));
         ReturnError(pipe, ERROR_STARTUP_DATA, L"GetStartupData", 1, &exit_event);
-        goto out;
+        goto err;
     }
 
     data = malloc(bytes);

@@ -478,7 +477,7 @@ GetStartupData(HANDLE pipe, STARTUP_DATA *sud)
     {
         MsgToEventLog(M_SYSERR, TEXT("malloc failed"));
         ReturnLastError(pipe, L"malloc");
-        goto out;
+        goto err;
     }
 
     read = ReadPipeAsync(pipe, data, bytes, 1, &exit_event);

@@ -486,14 +485,14 @@ GetStartupData(HANDLE pipe, STARTUP_DATA *sud)
     {
         MsgToEventLog(M_SYSERR, TEXT("ReadPipeAsync failed"));
         ReturnLastError(pipe, L"ReadPipeAsync");
-        goto out;
+        goto err;
     }
 
     if (data[size - 1] != 0)
     {
         MsgToEventLog(M_ERR, TEXT("Startup data is not NULL terminated"));
         ReturnError(pipe, ERROR_STARTUP_DATA, L"GetStartupData", 1, &exit_event);
-        goto out;
+        goto err;
     }
 
     sud->directory = data;

@@ -503,7 +502,7 @@ GetStartupData(HANDLE pipe, STARTUP_DATA *sud)
     {
         MsgToEventLog(M_ERR, TEXT("Startup data ends at working directory"));
         ReturnError(pipe, ERROR_STARTUP_DATA, L"GetStartupData", 1, &exit_event);
-        goto out;
+        goto err;
     }
 
     sud->options = sud->directory + len;

@@ -513,16 +512,16 @@ GetStartupData(HANDLE pipe, STARTUP_DATA *sud)
     {
         MsgToEventLog(M_ERR, TEXT("Startup data ends at command line options"));
         ReturnError(pipe, ERROR_STARTUP_DATA, L"GetStartupData", 1, &exit_event);
-        goto out;
+        goto err;
     }
 
     sud->std_input = sud->options + len;
-    data = NULL; /* don't free data */
-    ret = TRUE;
+    return TRUE;
 
-out:
+err:
+    sud->directory = NULL;		/* caller must not free() */
     free(data);
-    return ret;
+    return FALSE;
 }
 
 

2.16.1

Just for completeness: all good so ACK again.

Selva

Your patch has been applied to the master and release/2.4 branch.

commit 1394192b210cb3c6624a7419bcf3ff966742e79b (master) commit da242af8d3750a231bfd687d0a92cf2004dae988 (release/2.4)

Author: Gert Doering
Date:   Sat Apr 14 09:26:17 2018 +0200

     Fix potential double-free() in Interactive Service (CVE-2018-9336)

     Signed-off-by: Gert Doering <[email protected]...>
     Acked-by: Selva Nair <[email protected]...>
     Message-Id: <[email protected]...>
     URL: https://www.mail-archive.com/search?l=mid&[email protected]...
     Signed-off-by: Gert Doering <[email protected]...>

--
kind regards,

Gert Doering

Hi,

[..]

Due to the sensitive nature of the patch, it was held under embargo rules
- that is "only sent to the security@ list, privately ACKed, commits not announced to the public list right away".

I do have an ACK for this from Selva, and have my usually "it went it with commit ID..." mail, but for whatever reason I cannot re-send my own mail to the -devel list, and I think SPF is stopping me from bouncing Selva's mail... which is a bit of an annoyance.

So I just forward the e-mails below, PGP sign all this (and the commits are PGP-signed as well), and leave verification of "is the commit the same as in the repo? is it what Selva ACKed?" to the interested reader.

-------------------------------------------------
From: Selva Nair <[email protected]...>
Date: Sat, 14 Apr 2018 12:44:57 -0400
Message-ID: <[email protected]...>
Subject: Re: [PATCH v2] Fix potential double-free() in Interactive Service (CVE-2018-9336)
To: Gert Doering <[email protected]...>
Cc: [email protected]..., [email protected]...

Just for completeness: all good so ACK again.

Selva

-------------------------------------------------
Date: Thu, 19 Apr 2018 17:24:49 +0200 (CEST)
Message-Id: <[email protected]...>
From: Gert Doering <[email protected]...>
To: Gert Doering <[email protected]...>
Cc: [email protected]
Subject: [PATCH applied] Re: Fix potential double-free() in Interactive Service (CVE-2018-9336)

Your patch has been applied to the master and release/2.4 branch.

commit 1394192b210cb3c6624a7419bcf3ff966742e79b (master) commit da242af8d3750a231bfd687d0a92cf2004dae988 (release/2.4)

Author: Gert Doering
Date:   Sat Apr 14 09:26:17 2018 +0200

     Fix potential double-free() in Interactive Service (CVE-2018-9336)

     Signed-off-by: Gert Doering <[email protected]...>
     Acked-by: Selva Nair <[email protected]...>
     Message-Id: <[email protected]...>
     URL: https://www.mail-archive.com/search?l=mid&[email protected]...
     Signed-off-by: Gert Doering <[email protected]...>

--
kind regards,

Gert Doering

"If was one thing all people took for granted, was conviction that if you 
 feed honest figures into a computer, honest figures come out. Never doubted 
 it myself till I met a computer with a sense of humor."
                             Robert A. Heinlein, The Moon is a Harsh Mistress

Gert Doering - Munich, Germany                             [email protected]...