Re: [Xen-devel] [PATCH] x86/vmx: Fix injection of #DB traps following XSA-165

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/12/2015 01:36, Tian, Kevin wrote:
> > From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> > Sent: Friday, December 25, 2015 2:23 AM
> >
> > Most debug exceptions are traps rather than faults.  Blindly re-injecting them
> > as hardware exception causes the instruction pointer in the exception frame
> > to point at the target instruct, rather than after it.  This in turn breaks
> > debuggers in the guests.
> >
> > Introduce a helper which copies VM_EXIT_INTR_INTO to VM_ENTRY_INTR_INFO, and
> > use it to mirror the intercepted interrupt back to the guest.  As part of
> > doing so, introduce vmx_intr_info_t with a bitfield representation of an
> > INTR_INFO field.
> >
> > Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > ---
> > CC: Jan Beulich <JBeulich@xxxxxxxx>
> > CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> > CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> > ---
> >   xen/arch/x86/hvm/vmx/vmx.c        | 33
> > ++++++++++++++++++++++++++++++---
> >   xen/include/asm-x86/hvm/vmx/vmx.h | 12 ++++++++++++
> >   2 files changed, 42 insertions(+), 3 deletions(-)
> >
> > diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> > index b918b8a..aacf07a 100644
> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > @@ -2877,6 +2877,34 @@ static int vmx_handle_eoi_write(void)
> >       return 0;
> >   }
> >
> > +/*
> > + * Propagate VM_EXIT_INTR_INFO to VM_ENTRY_INTR_INFO.  Used to mirror an
> > + * intercepted exception back to the guest as if Xen hadn't intercepted it.
> > + *
> > + * It is the callers responsibility to ensure that this function is only used
> > + * in the context of an appropriate vmexit.
> > + */
> > +static void vmx_propagate_intr(void)
> > +{
> > +    vmx_intr_info_t intr;
> > +    unsigned long tmp;
> > +
> > +    __vmread(VM_EXIT_INTR_INFO, &intr.raw);
> > +
> > +    ASSERT(intr.valid);
> > +
> > +    __vmwrite(VM_ENTRY_INTR_INFO, intr.raw);
> > +
> > +    if ( intr.ec_valid )
> > +    {
> > +        __vmread(VM_EXIT_INTR_ERROR_CODE, &tmp);
> > +        __vmwrite(VM_ENTRY_EXCEPTION_ERROR_CODE, tmp);
> > +    }
> > +
> > +    __vmread(VM_EXIT_INSTRUCTION_LEN, &tmp);
> > +    __vmwrite(VM_ENTRY_INSTRUCTION_LEN, tmp);
> > +}
> > +
> >   static void vmx_idtv_reinject(unsigned long idtv_info)
> >   {
> >
> > @@ -3137,7 +3165,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
> >               HVMTRACE_1D(TRAP_DEBUG, exit_qualification);
> >               write_debugreg(6, exit_qualification | DR_STATUS_RESERVED_ONE);
> >               if ( !v->domain->debugger_attached )
> > -                hvm_inject_hw_exception(vector, HVM_DELIVER_NO_ERROR_CODE);
> > +                vmx_propagate_intr();
> >               else
> >                   domain_pause_for_debugger();
> >               break;
> > @@ -3206,8 +3234,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
> >               break;
> >           case TRAP_alignment_check:
> >               HVMTRACE_1D(TRAP, vector);
> > -            __vmread(VM_EXIT_INTR_ERROR_CODE, &ecode);
> > -            hvm_inject_hw_exception(vector, ecode);
> > +            vmx_propagate_intr();
> >               break;
> >           case TRAP_nmi:
> >               if ( MASK_EXTR(intr_info, INTR_INFO_INTR_TYPE_MASK) !=
> > diff --git a/xen/include/asm-x86/hvm/vmx/vmx.h
> > b/xen/include/asm-x86/hvm/vmx/vmx.h
> > index 1719965..ad2018f 100644
> > --- a/xen/include/asm-x86/hvm/vmx/vmx.h
> > +++ b/xen/include/asm-x86/hvm/vmx/vmx.h
> > @@ -224,6 +224,18 @@ static inline void pi_clear_sn(struct pi_desc *pi_desc)
> >   #define INTR_INFO_VALID_MASK            0x80000000      /* 31 */
> >   #define INTR_INFO_RESVD_BITS_MASK       0x7ffff000
> >
> > +typedef union vmx_intr_info {
> > +    unsigned long raw;
> > +    struct {
> > +        uint8_t vector;
> > +        uint32_t type: 3,
> > +                 ec_valid: 1,
> > +                 nmi_unblocked: 1,
> > +                 rsvd: 18,
> > +                 valid: 1;
> > +    };
> > +} vmx_intr_info_t;
> > +
> Is there a value to separate vector from bitfield definition? Although
> it works, spec defines all fields in one 32bits format...
Not specifically - I can certainly replace it with a single bitfield list.

> btw seems there's no other users of this new structure. To be consistent
> I'd suggest either staying same with other place reading intr_info or
> split into a new patch to change all references together.
I will split into two patches.  A bugfix first, and a cleanup second.  
The former will then be easier to backport.
~Andrew

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