The compat_alloc_user_space functions in include/asm/compat.h files in the
Linux kernel before 2.6.36-rc4-git2 on 64-bit platforms do not properly
allocate the userspace memory required for the 32-bit compatibility layer,
which allows local users to gain privileges by leveraging the ability of
the compat_mc_getsockopt function (aka the MCAST_MSFILTER getsockopt
support) to control a certain length value, related to a "stack pointer
underflow" issue, as exploited in the wild in September 2010.

Ben Hawkes discovered that the Linux kernel did not correctly validate
memory ranges on 64bit kernels when allocating memory on behalf of 32bit
system calls. On a 64bit system, a local attacker could perform malicious
multicast getsockopt calls to gain root privileges.

smb