Re: [v2 6/7] x86/traps: Fixup general protection faults caused by UMIP

[Ricardo Neri]

On Fri, 2016-12-23 at 18:11 -0800, Andy Lutomirski wrote:
> On Fri, Dec 23, 2016 at 5:37 PM, Ricardo Neri
> <ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
> > If the User-Mode Instruction Prevention CPU feature is available and
> > enabled, a general protection fault will be issued if the instructions
> > sgdt, sldt, sidt, str or smsw are executed from user-mode context
> > (CPL > 0). If the fault was caused by any of the instructions protected
> > by UMIP, fixup_umip_exceptino will emulate dummy results for these
> > instructions.
> >
> > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Brian Gerst <brgerst@xxxxxxxxx>
> > Cc: Chen Yucong <slaoub@xxxxxxxxx>
> > Cc: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> > Cc: Huang Rui <ray.huang@xxxxxxx>
> > Cc: Jiri Slaby <jslaby@xxxxxxx>
> > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
> > Cc: Shuah Khan <shuah@xxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Tony Luck <tony.luck@xxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Liang Z. Li <liang.z.li@xxxxxxxxx>
> > Cc: Alexandre Julliard <julliard@xxxxxxxxxx>
> > Cc: Stas Sergeev <stsp@xxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Cc: linux-msdos@xxxxxxxxxxxxxxx
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/traps.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> > index bf0c6d0..5044fb3 100644
> > --- a/arch/x86/kernel/traps.c
> > +++ b/arch/x86/kernel/traps.c
> > @@ -64,6 +64,7 @@
> >  #include <asm/trace/mpx.h>
> >  #include <asm/mpx.h>
> >  #include <asm/vm86.h>
> > +#include <asm/umip.h>
> >
> >  #ifdef CONFIG_X86_64
> >  #include <asm/x86_init.h>
> > @@ -491,6 +492,9 @@ do_general_protection(struct pt_regs *regs, long error_code)
> >         RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> >         cond_local_irq_enable(regs);
> >
> > +       if (user_mode(regs) && !fixup_umip_exception(regs))
> > +               return;
> > +
> 
> I would do fixup_umip_exception(regs) == 0 to make it more obvious
> what's going on.

Sure. I will make this change.
> 
> Also, since you're allowing this in v8086 mode, I think this should
> have an explicit test in
> tools/testing/selftests/x86/entry_from_vm86.c.  I *think* it will work
> fine, but the pt_regs handling in vm86 mode is quite scary and has
> been rewritten recently.

I will include a test for this.

Thanks and BR,
Ricardo