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

On Fri, Dec 23, 2016 at 5:37 PM, Ricardo Neri
<[email protected]> 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 <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: H. Peter Anvin <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Cc: Brian Gerst <[email protected]>
> Cc: Chen Yucong <[email protected]>
> Cc: Chris Metcalf <[email protected]>
> Cc: Dave Hansen <[email protected]>
> Cc: Fenghua Yu <[email protected]>
> Cc: Huang Rui <[email protected]>
> Cc: Jiri Slaby <[email protected]>
> Cc: Jonathan Corbet <[email protected]>
> Cc: Michael S. Tsirkin <[email protected]>
> Cc: Paul Gortmaker <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Ravi V. Shankar <[email protected]>
> Cc: Shuah Khan <[email protected]>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Tony Luck <[email protected]>
> Cc: Paolo Bonzini <[email protected]>
> Cc: Liang Z. Li <[email protected]>
> Cc: Alexandre Julliard <[email protected]>
> Cc: Stas Sergeev <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Signed-off-by: Ricardo Neri <[email protected]>
> ---
>  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.

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.