CVE-2019-13648: Linux kernel: powerpc: kernel crash in TM handling triggerable by any local user

[Michael Neuling]

The Linux kernel for powerpc since v3.9 has a bug in the TM handling  where any
unprivileged local user may crash the operating system.

This bug affects machines using 64-bit CPUs where Transactional Memory (TM) is
not present or has been disabled (see below for more details on affected CPUs).

To trigger the bug a process constructs a signal context which still has the MSR
TS bits set. That process then passes this signal context to the sigreturn()
system call. When returning back to userspace, the kernel then crashes with a
bad TM transition (TM Bad Thing) or by executing TM code on a non-TM system.

All 64bit machines where TM is not present are affected. This includes PowerPC
970 (G5), PA6T, POWER5/6/7 VMs under KVM or LPARs under PowerVM and POWER9 bare
metal. 

Additionally systems with TM hardware but where TM is disabled in software (via
ppc_tm=off kernel cmdline) are also affected. This includes POWER8/9 VMs under
KVM or LPARs under PowerVM and POWER8 bare metal.

The bug was introduced in commit:
  2b0a576d15e0 ("powerpc: Add new transactional memory state to the signal context")

Which was originally merged in v3.9. 

The upstream fix is here:
  https://git.kernel.org/torvalds/c/f16d80b75a096c52354c6e0a574993f3b0dfbdfe

The fix can be verified by running `sigfuz -m` from the kernel selftests:
 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/powerpc/signal/sigfuz.c?h=v5.2

cheers
Mikey