A flaw was found in the way the KVM hypervisor on the Power8 processor stores the r1 register state in the 'HSTATEHOSTR1' field on the Linux kernel stack. This flaw occurs while handling hypercalls in Transactional Memory (TM) suspend mode in the kvmppcsavetm and kvmppcrestoretm routines, leading to host stack corruption. R1 register holds a stack frame address and its corruption leads the kernel into panic state. A guest user can use this flaw to crash the host kernel, resulting in a denial of service.

A flaw was found in the Linux kernels handling of signal frame processing where any user process can craft a signal frame and then do a sigreturn so that the kernel will take an exception (interrupt), and use the r1 value from the signal frame as the kernel stack pointer. As part of the exception entry the content of the signal frame is written to the kernel stack, allowing an attacker to overwrite arbitrary locations with arbitrary values. The exception handling does produce an oops, and a panic if paniconoops=1, but only after kernel memory has been over written.

This flaw only affects PowerPC systems running on Power8 or later processors.

References:

http://seclists.org/oss-sec/2017/q4/51

This issue was introduced by commit:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5d176f751ee3

An upstream fix:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=265e60a170d0a0ecfc2d20490134ed2c48dd45ab