[PATCH 1/2] packet: Do not decrypt zero length rest of buffer

[Alan Dunn <amdunn@xxxxxxxxx>]

If we receive a packet of length exactly blocksize, then
packet_decrypt gets called on a buffer of size 0.  The check at the
beginning of packet_decrypt indicates that the function should be
called on buffers of at least one blocksize, though the check allows
through zero length.  As is packet_decrypt can return -1 when len is 0
because malloc can return NULL in this case: according to the ISO C
standard, malloc is free to return NULL or a pointer that can be freed
when size == 0, and uclibc by default will return NULL here (in
"non-glibc-compatible" mode).  The net result is that when using
uclibc connections with libssh can anomalously fail.

Alternatively, packet_decrypt (and probably packet_encrypt for
consistency) could be made to always succeed on len == 0 without
depending on the behavior of malloc.

Thanks to Josh Berlin for bringing conneciton failures with uclibc to
my attention.

Signed-off-by: Alan Dunn <amdunn@xxxxxxxxx>
---
 src/packet.c  |   16 ++++++++++------
 src/packet1.c |    5 ++++-
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/src/packet.c b/src/packet.c
index 87b9100..7f75b44 100644
--- a/src/packet.c
+++ b/src/packet.c
@@ -251,12 +251,16 @@ int ssh_packet_socket_callback(const void *data, size_t receivedlen, void *user)
                  * Decrypt the rest of the packet (blocksize bytes already
                  * have been decrypted)
                  */
-                rc = packet_decrypt(session,
-                                    ((uint8_t*)buffer_get_rest(session->in_buffer) + blocksize),
-                                    buffer_get_rest_len(session->in_buffer) - blocksize);
-                if (rc < 0) {
-                    ssh_set_error(session, SSH_FATAL, "Decrypt error");
-                    goto error;
+
+                /* The following check avoids decrypting zero bytes */
+                if (buffer_get_rest_len(session->in_buffer) != blocksize) {
+                    rc = packet_decrypt(session,
+                                        ((uint8_t*)buffer_get_rest(session->in_buffer) + blocksize),
+                                        buffer_get_rest_len(session->in_buffer) - blocksize);
+                    if (rc < 0) {
+                        ssh_set_error(session, SSH_FATAL, "Decrypt error");
+                        goto error;
+                    }
                 }
 
                 /* copy the last part from the incoming buffer */
diff --git a/src/packet1.c b/src/packet1.c
index ec72f16..4728b4b 100644
--- a/src/packet1.c
+++ b/src/packet1.c
@@ -168,7 +168,8 @@ int ssh_packet_socket_callback1(const void *data, size_t receivedlen, void *user
          * We decrypt everything, missing the lenght part (which was
          * previously read, unencrypted, and is not part of the buffer
          */
-        if (packet_decrypt(session,
+        if (ssh_buffer_get_len(session->in_buffer) > 0 &&
+            packet_decrypt(session,
               ssh_buffer_get_begin(session->in_buffer),
               ssh_buffer_get_len(session->in_buffer)) < 0) {
           ssh_set_error(session, SSH_FATAL, "Packet decrypt error");
@@ -300,6 +301,8 @@ int packet_send1(ssh_session session) {
       ssh_buffer_get_len(session->out_buffer));
 #endif
 
+  /* session->out_buffer should have more than sizeof(uint32_t) bytes
+     in it as required for packet_encrypt */
   packet_encrypt(session, (unsigned char *)ssh_buffer_get_begin(session->out_buffer) + sizeof(uint32_t),
       ssh_buffer_get_len(session->out_buffer) - sizeof(uint32_t));
 
-- 
1.7.9.5