fix auth_key cleanup in SMB2_sess_setup() for possible crash

The ses->auth_key.len field should be zeroed out during error paths, along with the 'response' field.

Rationale:

It is possible with a specially crafted SMB2 server to cause the setup to free the key but keep the session. When the session is recovered (after a connection drop, for example), the following condition turn out to be true:

     ses->auth_key.len != 0  &&  ses->auth_key.response == NULL

This will cause the following memcpy() in setup_ntlmv2_rsp() to GPF, because tiblob == NULL and tilen != 0 (these are the old auth_key values):

     memcpy(ses->auth_key.response + baselen, tiblob, tilen);

As seen here (Fedora 20 kernel build 3.16.3-200.fc20.x86_64):

[985673.540019] BUG: unable to handle kernel NULL pointer dereference at           (null)
[985673.540049] IP: [<ffffffff8136b7a6>] memcpy+0x6/0x110
[...]
[985673.540957]  [<ffffffffa09d28e5>] ? setup_ntlmv2_rsp+0x235/0x9d0 [cifs]
[985673.540980]  [<ffffffffa09ccc8a>] ? cifs_small_buf_get+0x1a/0x30 [cifs]
[985673.541003]  [<ffffffffa09dfe05>] ? small_smb2_init+0x285/0x510 [cifs]
[985673.541025]  [<ffffffffa09d5331>] build_ntlmssp_auth_blob+0x91/0x290 [cifs]
[985673.541047]  [<ffffffffa09e06a0>] SMB2_sess_setup+0x1f0/0x590 [cifs]

[...]

Commit applies to 3.18-rc1 and various preceding stable versions.

---
 fs/cifs/smb2pdu.c | 3 +++
 1 file changed, 3 insertions(+)

index 8f1672bb82d5..e0304f258533 100644
@@ -551,6 +551,7 @@ SMB2_sess_setup(const unsigned int xid, struct cifs_ses *ses,
 	 */
 	kfree(ses->auth_key.response);
 	ses->auth_key.response = NULL;
+	ses->auth_key.len = 0;
 
 	/*
 	 * If memory allocation is successful, caller of this function

@@ -713,6 +714,7 @@ ssetup_exit:
 			rc = server->ops->generate_signingkey(ses);
 			kfree(ses->auth_key.response);
 			ses->auth_key.response = NULL;
+			ses->auth_key.len = 0;
 			if (rc) {
 				cifs_dbg(FYI,
 					"SMB3 session key generation failed\n");

@@ -737,6 +739,7 @@ keygen_exit:
 	if (!server->sign) {
 		kfree(ses->auth_key.response);
 		ses->auth_key.response = NULL;
+		ses->auth_key.len = 0;
 	}
 	kfree(ses->ntlmssp);
 

1.9.3