4.15-stable review patch. If anyone has any objections, please let me know.
------------------ From: Ingo Molnar <mi...@kernel.org> commit e48657573481a5dff7cfdc3d57005c80aa816500 upstream. Josh Poimboeuf noticed the following bug: "The paranoid exit code only restores the saved CR3 when it switches back to the user GS. However, even in the kernel GS case, it's possible that it needs to restore a user CR3, if for example, the paranoid exception occurred in the syscall exit path between SWITCH_TO_USER_CR3_STACK and SWAPGS." Josh also confirmed via targeted testing that it's possible to hit this bug. Fix the bug by also restoring CR3 in the paranoid_exit_no_swapgs branch. The reason we haven't seen this bug reported by users yet is probably because "paranoid" entry points are limited to the following cases: idtentry double_fault do_double_fault has_error_code=1 paranoid=2 idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK idtentry machine_check do_mce has_error_code=0 paranoid=1 Amongst those entry points only machine_check is one that will interrupt an IRQS-off critical section asynchronously - and machine check events are rare. The other main asynchronous entries are NMI entries, which can be very high-freq with perf profiling, but they are special: they don't use the 'idtentry' macro but are open coded and restore user CR3 unconditionally so don't have this bug. Reported-and-tested-by: Josh Poimboeuf <jpoim...@redhat.com> Reviewed-by: Andy Lutomirski <l...@kernel.org> Acked-by: Thomas Gleixner <t...@linutronix.de> Cc: Arjan van de Ven <ar...@linux.intel.com> Cc: Borislav Petkov <b...@alien8.de> Cc: Dan Williams <dan.j.willi...@intel.com> Cc: Dave Hansen <dave.han...@linux.intel.com> Cc: David Woodhouse <dw...@infradead.org> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Peter Zijlstra <pet...@infradead.org> Link: http://lkml.kernel.org/r/20180214073910.boevmg65upbk3...@gmail.com Signed-off-by: Ingo Molnar <mi...@kernel.org> Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org> --- arch/x86/entry/entry_64.S | 1 + 1 file changed, 1 insertion(+) --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1168,6 +1168,7 @@ ENTRY(paranoid_exit) jmp .Lparanoid_exit_restore .Lparanoid_exit_no_swapgs: TRACE_IRQS_IRETQ_DEBUG + RESTORE_CR3 scratch_reg=%rbx save_reg=%r14 .Lparanoid_exit_restore: jmp restore_regs_and_return_to_kernel END(paranoid_exit)
