• Linus Torvalds's avatar
    i387: move AMD K7/K8 fpu fxsave/fxrstor workaround from save to restore · 4903062b
    Linus Torvalds authored
    The AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception is
    pending.  In order to not leak FIP state from one process to another, we
    need to do a floating point load after the fxsave of the old process,
    and before the fxrstor of the new FPU state.  That resets the state to
    the (uninteresting) kernel load, rather than some potentially sensitive
    user information.
    
    We used to do this directly after the FPU state save, but that is
    actually very inconvenient, since it
    
     (a) corrupts what is potentially perfectly good FPU state that we might
         want to lazy avoid restoring later and
    
     (b) on x86-64 it resulted in a very annoying ordering constraint, where
         "__unlazy_fpu()" in the task switch needs to be delayed until after
         the DS segment has been reloaded just to get the new DS value.
    
    Coupling it to the fxrstor instead of the fxsave automatically avoids
    both of these issues, and also ensures that we only do it when actually
    necessary (the FP state after a save may never actually get used).  It's
    simply a much more natural place for the leaked state cleanup.
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    4903062b
process_64.c 15.4 KB