CVE-2008-7256: Null Pointer Dereference

Description of problem: knfsd crashes if you are using it to export shmemfs objects and run strict overcommit. In this situation the current->mm based modifier to the overcommit goes through a NULL pointer. We could simply check for NULL and skip the modifier but we've caught other real bugs in the past from mm being NULL here - cases where we did need a valid mm set up (eg the exec bug in 2005). To preserve the checks and get the logic we want shuffle the checking around and add a new helper to the vm security wrappers Also fix a current->mm reference in nommu that should use the passed mm

Upstream commit: nfsd: fix vm overcommit crash http://git.kernel.org/linus/731572d39fcd3498702eda4600db4c43d51e0b26 nfsd: fix vm overcommit crash fix #2 http://git.kernel.org/linus/1b79cd04fab80be61dcd2732e2423aafde9a4c1c

Reference: [PATCH] knfsd: add nfs-export support to tmpfs http://git.kernel.org/linus/91828a405ae454a9503c41a7744f6ff877a80714