x86/pti: Switch to kernel CR3 at early in entry_SYSCALL_compat()

x86/pti: Switch to kernel CR3 at early in entry_SYSCALL_compat()

The preparation for PTI which added CR3 switching to the entry code
misplaced the CR3 switch in entry_SYSCALL_compat().

With PTI enabled the entry code tries to access a per cpu variable after
switching to kernel GS. This fails because that variable is not mapped to
user space. This results in a double fault and in the worst case a kernel
crash.

Move the switch ahead of the access and clobber RSP which has been saved
already.

Fixes: 8a09317b895f ("x86/mm/pti: Prepare the x86/entry assembly code for entry/exit CR3 switching")
Reported-by: Lars Wendler <[email protected]>
Reported-by: Laura Abbott <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Borislav Betkov <[email protected]>
Cc: Andy Lutomirski <[email protected]>,
Cc: Dave Hansen <[email protected]>,
Cc: Peter Zijlstra <[email protected]>,
Cc: Greg KH <[email protected]>, ,
Cc: Boris Ostrovsky <[email protected]>,
Cc: Juergen Gross <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801031949200.1957@nanos