[Qemu-devel] [patch] make '-smb $HOME' work

[John Coiner <jcoiner@stanfordalumni.org>]

The most common use case for the '-smb' option may be '-smb $HOME'.

There is a problem with this case:

Windows attempts to connect as user "nobody". Smbd allows the connection 
-- unfortunately, it also maps the "nobody" accesses to the host's 
"nobody" account, so all write accesses fail.

How are people using '-smb'? Am I the only person that runs into this? 
One lame workaround is to point '-smb' at an area on /tmp that 
everybody, including "nobody", has access to.

The problem happens with a Windows 2000 guest, and maybe other NT 
derivatives.

This patch sets up smbd to only allow "guest" access from Windows, and 
no other access. (I suspect and hope that smbd can coax any version of 
Windows into doing a "guest" access, by rejecting everything else. This 
is only tested with Win2K.) When smbd receives a guest access, it maps 
that onto the account of the same user who is running qemu.

This fixes the common, personal use, '-smb $HOME' case. For more 
complicated cases, for example if you don't trust the guest, you may 
want to craft your own 'smb.conf' rather than relying on '-smb'. From a 
security standpoint, the patched '-smb' has no authentication to break, 
and it constrains smb access to a single user on the host. So while the 
gates are wide open to whatever directory you share, you at least know 
what you're getting.

-- John



--- qemu-0.7.2-dmapatch/vl.c    2005-09-04 13:11:31.000000000 -0400
+++ qemu-0.7.2-broken/vl.c      2005-10-08 14:41:55.000000000 -0400
@@ -29,6 +29,8 @@
  #include <time.h>
  #include <errno.h>
  #include <sys/time.h>
+#include <sys/types.h>
+#include <pwd.h>

  #ifndef _WIN32
  #include <sys/times.h>
@@ -1605,15 +1607,17 @@
              "log file=%s/log.smbd\n"
              "smb passwd file=%s/smbpasswd\n"
              "security = share\n"
+           "guest account=%s\n"
              "[qemu]\n"
              "path=%s\n"
              "read only=no\n"
-            "guest ok=yes\n",
+            "guest only=yes\n",
              smb_dir,
              smb_dir,
              smb_dir,
              smb_dir,
              smb_dir,
+           getpwuid( geteuid( ) )->pw_name,
              exported_dir
              );
      fclose(f);