[bug report] cifs: multichannel: move channel selection above transport layer

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Aurelien Aptel,

The patch feeaec621c09: "cifs: multichannel: move channel selection
above transport layer" from Apr 24, 2020, leads to the following
static checker warning:

	fs/cifs/smb2pdu.c:149 smb2_hdr_assemble()
	error: we previously assumed 'tcon->ses' could be null (see line 133)

fs/cifs/smb2pdu.c
   120          shdr->ProcessId = cpu_to_le32((__u16)current->tgid);
   121  
   122          if (!tcon)
   123                  goto out;
   124  
   125          /* GLOBAL_CAP_LARGE_MTU will only be set if dialect > SMB2.02 */
   126          /* See sections 2.2.4 and 3.2.4.1.5 of MS-SMB2 */
   127          if (server && (server->capabilities & SMB2_GLOBAL_CAP_LARGE_MTU))
   128                  shdr->CreditCharge = cpu_to_le16(1);
   129          /* else CreditCharge MBZ */
   130  
   131          shdr->TreeId = tcon->tid;
   132          /* Uid is not converted */
   133          if (tcon->ses)
                    ^^^^^^^^^
Check for NULL.

   134                  shdr->SessionId = tcon->ses->Suid;
   135  
   136          /*
   137           * If we would set SMB2_FLAGS_DFS_OPERATIONS on open we also would have
   138           * to pass the path on the Open SMB prefixed by \\server\share.
   139           * Not sure when we would need to do the augmented path (if ever) and
   140           * setting this flag breaks the SMB2 open operation since it is
   141           * illegal to send an empty path name (without \\server\share prefix)
   142           * when the DFS flag is set in the SMB open header. We could
   143           * consider setting the flag on all operations other than open
   144           * but it is safer to net set it for now.
   145           */
   146  /*      if (tcon->share_flags & SHI1005_FLAGS_DFS)
   147                  shdr->Flags |= SMB2_FLAGS_DFS_OPERATIONS; */
   148  
   149          if (server && server->sign && !smb3_encryption_required(tcon))
                                                                        ^^^^
Unchecked dereference inside the function.  There used to be a bunch
of checks for NULL "tcon->ses" but some of them were replaces with a
check for "server" instead.

   150                  shdr->Flags |= SMB2_FLAGS_SIGNED;
   151  out:
   152          return;
   153  }

regards,
dan carpenter