The BMC (IBM Power 9 AC922 OP910, OP920, OP930, and OP940) may be subject to a firmware downgrade attack which may affect its ability to operate its host. IBM X-Force ID: 221442.

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