CVE request - kernel: futex: Handle user space corruption gracefully

[Eugene Teo <eugeneteo () kernel sg>]

Description of the issue: "If the owner of a PI futex dies we fix up the 
pi_state and set pi_state->owner to NULL. When a malicious or just 
sloppy programmed user space application sets the futex value to 0 e.g. 
by calling pthread_mutex_init(), then the futex can be acquired again. A 
new waiter manages to enqueue itself on the pi_state w/o damage, but on 
unlock the kernel dereferences pi_state->owner and oopses.

Prevent this by checking pi_state->owner in the unlock path. If 
pi_state->owner is not current we know that user space manipulated the 
futex value. Ignore the mess and return -EINVAL.

This catches the above case and also the case where a task hijacks the 
futex by setting the tid value and then tries to unlock it."

Upstream commit:
http://git.kernel.org/linus/51246bfd189064079c54421507236fd2723b18f3

Note that pi-futex was introduced in:
http://git.kernel.org/linus/c87e2837be82df479a6bae9f155c43516d2feebc

Reference:
https://bugzilla.redhat.com/show_bug.cgi?id=563091

Thanks, Eugene