Re: [PATCH] xen: hypercall: fix out-of-bounds memcpy

[Boris Ostrovsky]

On 02/03/2018 10:12 AM, Arnd Bergmann wrote:
> On Sat, Feb 3, 2018 at 12:33 AM, Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 02/02/2018 10:32 AM, Arnd Bergmann wrote:
> > > The legacy hypercall handlers were originally added with
> > > a comment explaining that "copying the argument structures in
> > > HYPERVISOR_event_channel_op() and HYPERVISOR_physdev_op() into the local
> > > variable is sufficiently safe" and only made sure to not write
> > > past the end of the argument structure, the checks in linux/string.h
> > > disagree with that, when link-time optimizations are used:
> > >
> > > In function 'memcpy',
> > >      inlined from 'pirq_query_unmask' at drivers/xen/fallback.c:53:2,
> > >      inlined from '__startup_pirq' at drivers/xen/events/events_base.c:529:2,
> > >      inlined from 'restore_pirqs' at drivers/xen/events/events_base.c:1439:3,
> > >      inlined from 'xen_irq_resume' at drivers/xen/events/events_base.c:1581:2:
> > > include/linux/string.h:350:3: error: call to '__read_overflow2' declared with attribute error: detected read beyond size of object passed as 2nd parameter
> > >     __read_overflow2();
> > >     ^
> > > make[3]: *** [ccLujFNx.ltrans15.ltrans.o] Error 1
> > > make[3]: Target 'all' not remade because of errors.
> > > lto-wrapper: fatal error: make returned 2 exit status
> > > compilation terminated.
> > > ld: error: lto-wrapper failed
> > >
> > > This changes the functions so that each argument is accessed with
> > > exactly the correct length based on the command code.
> > >
> > > Fixes: cf47a83fb06e ("xen/hypercall: fix hypercall fallback code for very old hypervisors")
> > > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > > ---
> > >   drivers/xen/fallback.c | 94 ++++++++++++++++++++++++++++----------------------
> > >   1 file changed, 53 insertions(+), 41 deletions(-)
>
> > >        default:
> > > -             WARN_ON(rc != -ENOSYS);
> > > -             break;
> > > +             return -ENOSYS;
> > >        }
> > >
> > > +     memcpy(&op.u, arg, len);
> > > +     rc = _hypercall1(int, event_channel_op_compat, &op);
> > > +     memcpy(arg, &op.u, len);
> >
> >
> > We don't copy back for all commands, only those that are COPY_BACK.
>
> Not sure what you mean. Is it harmful to copy back the data for the others
> in any way? Otherwise I wouldn't micro-optimize this.


I should have checked the original commit for fallback.c --- the code 
that it replaced was doing copybacks for all hypercalls and selective 
copybacks is an optimization introduced in that commit.

-boris