The Linux kernel before 3.15.4 on Intel processors does not properly restrict use of a non-canonical value for the saved RIP address in the case of a system call that does not use IRET, which allows local users to leverage a race condition and gain privileges, or cause a denial of service (double fault), via a crafted application that makes ptrace and fork system calls.
The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.