CVE request: kernel: shm: fix shmctl(SHM_INFO) lockup with !CONFIG_SHMEM

[Eugene Teo <eugeneteo () kernel sg>]

According to the upstream commit
a68e61e8ff2d46327a37b69056998b47745db6fa, shm_get_stat() assumes that
the inode is a "struct shmem_inode_info", which is incorrect for
!CONFIG_SHMEM (see fs/ramfs/inode.c: ramfs_get_inode() vs.
mm/shmem.c: shmem_get_inode()).

This bad assumption can cause shmctl(SHM_INFO) to lockup when
shm_get_stat() tries to spin_lock(&info->lock).  Users of
!CONFIG_SHMEM may encounter this lockup simply by invoking the 'ipcs'
command.

Reported by Jiri Olsa back in February 2008:
http://lkml.org/lkml/2008/2/29/74

Thanks, Eugene