On Sat, Jun 13, 2015 at 2:23 PM, Andy Lutomirski <l...@amacapital.net> wrote: > On Sat, Jun 13, 2015 at 12:03 AM, Ingo Molnar <mi...@kernel.org> wrote: >> >> * Brian Gerst <brge...@gmail.com> wrote: >> >>> On Fri, Jun 12, 2015 at 4:36 AM, Ingo Molnar <mi...@kernel.org> wrote: >>> > >>> > * H. Peter Anvin <h...@zytor.com> wrote: >>> > >>> >> %es is used implicitly by string instructions. >>> > >>> > Ok, so we are probably better off reloading ES as well early, right >>> > when we return from the firmware, just in case something does >>> > a copy before we hit the ES restore in restore_processor_state(), >>> > which is a generic C function? >>> > >>> > Something like the patch below? >>> > >>> > I also added FS/GS/SS reloading to make it complete. If this (or a variant >>> > thereof, it's still totally untested) works then we can remove the segment >>> > save/restore layer in __save/restore_processor_state(). >>> > >>> > Thanks, >>> > >>> > Ingo >>> > >>> > ===========> >>> > arch/x86/kernel/acpi/wakeup_32.S | 13 +++++++++++++ >>> > 1 file changed, 13 insertions(+) >>> > >>> > diff --git a/arch/x86/kernel/acpi/wakeup_32.S >>> > b/arch/x86/kernel/acpi/wakeup_32.S >>> > index 665c6b7d2ea9..1376a7fc21b7 100644 >>> > --- a/arch/x86/kernel/acpi/wakeup_32.S >>> > +++ b/arch/x86/kernel/acpi/wakeup_32.S >>> > @@ -61,6 +61,19 @@ ENTRY(wakeup_pmode_return) >>> > >>> > >>> > restore_registers: >>> > + /* >>> > + * In case the BIOS corrupted our segment descriptors, >>> > + * reload them to clear out any shadow descriptor >>> > + * state: >>> > + */ >>> > + movl $__USER_DS, %eax >>> > + movl %eax, %ds >>> > + movl %eax, %es >>> > + movl %eax, %fs >>> > + movl %eax, %gs >>> > + movl $__KERNEL_DS, %eax >>> > + movl %eax, %ss >>> > + >>> > movl saved_context_ebp, %ebp >>> > movl saved_context_ebx, %ebx >>> > movl saved_context_esi, %esi >>> >>> If you follow the convoluted flow of the calls in this file, >>> wakeup_pmode_return >>> is the first thing called from the trampoline on resume, and that loads the >>> data >>> segments with __KERNEL_DS. [...] >> >> So if wakeup_pmode_return is really the first thing called then the whole >> premise >> of shadow descriptor corruption goes out the window: we reload all relevant >> segment registers. > > True, but it still leaves the fact that we're loading __KERNEL_DS > instead of __USER_DS, right? So we end up in the kernel in some > context (I have no clue what context) with __KERNEL_DS loaded. It's > very easy for us to inadvertently fix it: we could return to userspace > by any means whatsoever except SYSEXIT, or we could even return back > to some preempted kernel context. > > I still think we should replace __KERNEL_DS with __USER_DS in > wakeup_pmode_return and see if the problem goes away.
I'm pretty sure that's what the problem is. If you look at the sysexit path, it never reloads ds/es. It assumes they are still __USER_DS set at sysenter. The iret path does restore all the user segments. -- Brian Gerst -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
