K 10
svn:author
V 3
jhb
K 8
svn:date
V 27
2020-01-31T19:00:48.242426Z
K 7
svn:log
V 634
Add stricter checks on user changes to SSTATUS.

Rather than trying to blacklist which bits userland can't write to via
sigreturn() or setcontext(), only permit changes to whitelisted bits.

- Permit arbitrary writes to bits in the user-writable USTATUS
  register that shadows SSTATUS.

- Ignore changes in write-only bits maintained by the CPU.

- Ignore the user-supplied value of the FS field used to track
  floating point state and instead set it to a value matching the
  actions taken by set_fpcontext().

Discussed with:	mhorne
MFC after:	2 weeks
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D23338

END