Re: [Qemu-devel] [PATCH] target/i386: fix interrupt CPL error when

[Paolo Bonzini <pbonzini@redhat.com>]

----- Original Message -----
> From: "Wu Xiang" <willx8@gmail.com>
> To: "Paolo Bonzini" <pbonzini@redhat.com>, qemu-devel@nongnu.org
> Cc: "Eduardo Habkost" <ehabkost@redhat.com>, "RichardHenderson" <rth@twiddle.net>
> Sent: Sunday, June 25, 2017 1:13:24 PM
> Subject: Re: [Qemu-devel][PATCH] target/i386: fix interrupt CPL error when
> 
> In-Reply-To: <641e1f35-813a-4244-0194-ffce08130533@redhat.com>
> 
> On Fri, Jun 23, 2017 at 01:19:35PM +0200, Paolo Bonzini wrote:
> > 
> > 
> > On 21/06/2017 16:21, Wu Xiang wrote:
> > > In do_interrupt64(), when interrupt stack table(ist) is enabled
> > > and the the target code segment is conforming(e2 & DESC_C_MASK), the
> > > old implementation always set new CPL to 0, and SS.RPL to 0.
> > > 
> > > This is incorrect for when CPL3 code access a CPL0 conforming code
> > > segment, the CPL should remain unchanged. Otherwise higher privileged
> > > code can be compromised.
> > > 
> > > The patch fix this for always set dpl = cpl when the target code segment
> > > is conforming, and modify the last parameter `flags`, which contains
> > > correct new CPL, in cpu_x86_load_seg_cache().
> > > 
> > > Signed-off-by: Wu Xiang <willx8@gmail.com>
> > > ---
> > >  target/i386/seg_helper.c | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > The patch looks good, but I'm thinking of a cleanup on top that simplifies
> > the handling of conforming code segments:
> > 
> > diff --git a/target/i386/seg_helper.c b/target/i386/seg_helper.c
> > index 9af69c23e0..600a4d7586 100644
> > --- a/target/i386/seg_helper.c
> > +++ b/target/i386/seg_helper.c
> > @@ -692,7 +692,10 @@ static void do_interrupt_protected(CPUX86State *env,
> > int intno, int is_int,
> >      if (!(e2 & DESC_P_MASK)) {
> >          raise_exception_err(env, EXCP0B_NOSEG, selector & 0xfffc);
> >      }
> > -    if (!(e2 & DESC_C_MASK) && dpl < cpl) {
> > +    if (e2 & DESC_C_MASK) {
> > +        dpl = cpl;
> > +    }
> > +    if (dpl < cpl) {
> >          /* to inner privilege */
> >          get_ss_esp_from_tss(env, &ss, &esp, dpl, 0);
> >          if ((ss & 0xfffc) == 0) {
> > @@ -719,7 +722,7 @@ static void do_interrupt_protected(CPUX86State *env,
> > int intno, int is_int,
> >          new_stack = 1;
> >          sp_mask = get_sp_mask(ss_e2);
> >          ssp = get_seg_base(ss_e1, ss_e2);
> > -    } else if ((e2 & DESC_C_MASK) || dpl == cpl) {
> > +    } else  {
> >          /* to same privilege */
> >          if (vm86) {
> >              raise_exception_err(env, EXCP0D_GPF, selector & 0xfffc);
> > @@ -728,13 +731,6 @@ static void do_interrupt_protected(CPUX86State *env,
> > int intno, int is_int,
> >          sp_mask = get_sp_mask(env->segs[R_SS].flags);
> >          ssp = env->segs[R_SS].base;
> >          esp = env->regs[R_ESP];
> > -        dpl = cpl;
> > -    } else {
> > -        raise_exception_err(env, EXCP0D_GPF, selector & 0xfffc);
> > -        new_stack = 0; /* avoid warning */
> > -        sp_mask = 0; /* avoid warning */
> > -        ssp = 0; /* avoid warning */
> > -        esp = 0; /* avoid warning */
> >      }
> >  
> >      shift = type >> 3;
> > @@ -919,25 +915,21 @@ static void do_interrupt64(CPUX86State *env, int
> > intno, int is_int,
> >      if (!(e2 & DESC_L_MASK) || (e2 & DESC_B_MASK)) {
> >          raise_exception_err(env, EXCP0D_GPF, selector & 0xfffc);
> >      }
> > -    if ((!(e2 & DESC_C_MASK) && dpl < cpl) || ist != 0) {
> > +    if (e2 & DESC_C_MASK) {
> > +        dpl = cpl;
> > +    }
> > +    if (dpl < cpl || ist != 0) {
> >          /* to inner privilege */
> >          new_stack = 1;
> >          esp = get_rsp_from_tss(env, ist != 0 ? ist + 3 : dpl);
> >          ss = 0;
> > -    } else if ((e2 & DESC_C_MASK) || dpl == cpl) {
> > +    } else {
> >          /* to same privilege */
> >          if (env->eflags & VM_MASK) {
> >              raise_exception_err(env, EXCP0D_GPF, selector & 0xfffc);
> >          }
> >          new_stack = 0;
> >          esp = env->regs[R_ESP];
> > -    } else {
> > -        raise_exception_err(env, EXCP0D_GPF, selector & 0xfffc);
> > -        new_stack = 0; /* avoid warning */
> > -        esp = 0; /* avoid warning */
> > -    }
> > -    if (e2 & DESC_C_MASK) {
> > -        dpl = cpl;
> >      }
> >      esp &= ~0xfLL; /* align stack */
> >  
> > 
> > Because dpl == cpl after the new "if", it's now unnecessary to check
> > the C bit when testing dpl < cpl.  Furthermore, dpl > cpl is checked
> > slightly above this code, so the final "else" is unreachable.
> > 
> > What do you think?
> > 
> > Paolo
> 
> The patch seems good and clean.
> 
> However I am not quite familiar with the code review process. Now that
> you have come up with the new patch, do I have to re-send a v2?

No, your patch is okay.  Mine comes after yours, I'll send it out
tomorrow.

Paolo