K 10
svn:author
V 3
jhb
K 8
svn:date
V 27
2020-05-11T16:14:20.459354Z
K 7
svn:log
V 815
MFC 356840,357344: Add stricter checks on user changes to SSTATUS.

356840:
Check for invalid sstatus values in set_mcontext().

Previously, this check was only in sys_sigreturn() which meant that
user applications could write invalid values to the register via
setcontext() or swapcontext().

357344:
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().

END