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Subject: Re: [libssh2] Crash while Key Exchange at libssh2 session startup time!!

Re: [libssh2] Crash while Key Exchange at libssh2 session startup time!!

From: Chris Nystrom <cnystrom_at_newio.org>
Date: Tue, 7 Nov 2006 17:22:12 -0600

On 11/7/06, Satish Mittal <satish.mittal_at_gmail.com> wrote:
> Dear All,
>
> I am using libssh2-0.14 on windows. From my application, when i call
> libssh2_session_startup(), my application is randomly (but very frequently)
> crashing, reporting 'out of virtual memory'.
>
> The problem is during the DH key exchange phase, when the call to
> libssh2_packet_read() is made,
>
> long buf_len, payload_len; //line 876
> unsigned long packet_length;
> unsigned long padding_length;
>
> if (should_block) {
> buf_len = libssh2_blocking_read(session, buf, 5);
> } else {
> buf_len = recv(session->socket_fd, buf, 1,
> LIBSSH2_SOCKET_RECV_FLAGS(session));
> if (buf_len <= 0) {
> return 0;
> }
> buf_len += libssh2_blocking_read(session, buf, 5 - buf_len);
> //line 887

How about replacing the above line with something like:

                  tmpvar = libssh2_blocking_read(session, buf, 5 - buf_len);

                  if (tmpvar < 0) {
                      /* something bad happened */
                      return(-1);
                  }

                  buf_len += tmpvar;

It does no good for functions to provide error codes if you do not
check for them. :)

> }
> if (buf_len < 5) {
> /* Something bad happened */
> return -1; //line 891
> }
> packet_length = libssh2_ntohu32(buf); //line893
> padding_length = buf[4];
> #ifdef LIBSSH2_DEBUG_TRANSPORT
> _libssh2_debug(session, LIBSSH2_DBG_TRANS, "Processing plaintext packet
> %lu bytes long (with %lu bytes padding)", packet_length, padding_length);
> #endif
>
> payload_len = packet_length - padding_length - 1; /*
> padding_length(1) */ //line 899
> payload = LIBSSH2_ALLOC(session, payload_len);
>
> Here payload_len is coming out to be a huge number (junk value on stack)
> since packet_length in line 893 is junk. This is in turn because in line
> 887, the libssh2_blocking_read() returns -1. But since buf_len is declared
> as an unsigned long, so -1 gets converted to a huge 32bit value, which then
> eventually crashes the application.
>
> I have tried editing line 876 to declare buf_len and payload_len as long,
> instead of unsigned long. This then in turn returns -1 in line 891 and fails
> the libssh2_session_startup() call.

Chris

-- 
E-Mail: Chris Nystrom <cnystrom_at_newio.org>
http://www.newio.org/
AIM: nystromchris
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Received on 2006-11-08

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