x86/ia32/syscall: don't do RESTORE_EXTRA_REGS prematurely

x86/ia32/syscall: don't do RESTORE_EXTRA_REGS prematurely

With the recent spectre mitigation changes we save the full pt_regs on
the stack and zero the extra regs. This means that for the sysenter
(and cstar) calling conventions, the pt_regs state for %ebp contains
the user %esp instead of the 6th argument.

For the straight syscall (non-tracing) path we load the real %ebp from
the user-stack and all is well. In the tracing/seccomp path, however, we
do RESTORE_EXTRA_REGS before the syscall, thus clobbering the 6th
argument (which gets replaced with the old %ebp value.)

The fix is to RESTORE_EXTRA_REGS only if we are done with syscall
handling. A side benefit is that this mitigation now also extends to
the tracing path.

Orabug: 27461990
CVE: CVE-2017-5715

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>