ksmbd: fix mechToken leak when SPNEGO decode fails after token alloc

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: fix mechToken leak when SPNEGO decode fails after token alloc

The kernel ASN.1 BER decoder calls action callbacks incrementally as it

walks the input. When ksmbd_decode_negTokenInit() reaches the mechToken

[2] OCTET STRING element, ksmbd_neg_token_alloc() allocates

conn->mechToken immediately via kmemdup_nul(). If a later element in

the same blob is malformed, then the decoder will return nonzero after

the allocation is already live. This could happen if mechListMIC [3]

overrunse the enclosing SEQUENCE.

decode_negotiation_token() then sets conn->use_spnego = false because

both the negTokenInit and negTokenTarg grammars failed. The cleanup at

the bottom of smb2_sess_setup() is gated on use_spnego:

if (conn->use_spnego && conn->mechToken) {

kfree(conn->mechToken);

conn->mechToken = NULL;

}

so the kfree is skipped, causing the mechToken to never be freed.

This codepath is reachable pre-authentication, so untrusted clients can

cause slow memory leaks on a server without even being properly

authenticated.

Fix this up by not checking check for use_spnego, as it's not required,

so the memory will always be properly freed. At the same time, always

free the memory in ksmbd_conn_free() incase some other failure path

forgot to free it.