use-after-free in xc_cpupool_getinfo() under memory pressure

ISSUE DESCRIPTION

If xc_cpumap_alloc() fails then xc_cpupool_getinfo() will free and incorrectly return the then-free pointer to the result structure.

IMPACT

An attacker may be able to cause a multi-threaded toolstack using this function to race against itself leading to heap corruption and a potential DoS.
Depending on the malloc implementation, privilege escalation cannot be ruled out.

VULNERABLE SYSTEMS

The flaw is present in Xen 4.1 onwards. Only multithreaded toolstacks are vulnerable. Only systems where management functions (such as domain creation) are exposed to untrusted users are vulnerable.
xl is not multithreaded, so is not vulnerable. However, multithreaded toolstacks using libxl as a library are vulnerable. xend is vulnerable.