Re: mmotm 2019-08-27-20-39 uploaded (objtool: xen)

[Sedat Dilek <sedat.dilek@gmail.com>]

On Fri, Aug 30, 2019 at 1:38 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Thu, Aug 29, 2019 at 10:24:45AM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 28, 2019 at 03:01:34PM -0500, Josh Poimboeuf wrote:
> > > On Wed, Aug 28, 2019 at 10:56:25AM -0700, Randy Dunlap wrote:
> > > > >> drivers/xen/gntdev.o: warning: objtool: gntdev_copy()+0x229: call to __ubsan_handle_out_of_bounds() with UACCESS enabled
> > > > >
> > > > > Easy one :-)
> > > > >
> > > > > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > > > > index 0c8e17f946cd..6a935ab93149 100644
> > > > > --- a/tools/objtool/check.c
> > > > > +++ b/tools/objtool/check.c
> > > > > @@ -483,6 +483,7 @@ static const char *uaccess_safe_builtin[] = {
> > > > >         "ubsan_type_mismatch_common",
> > > > >         "__ubsan_handle_type_mismatch",
> > > > >         "__ubsan_handle_type_mismatch_v1",
> > > > > +       "__ubsan_handle_out_of_bounds",
> > > > >         /* misc */
> > > > >         "csum_partial_copy_generic",
> > > > >         "__memcpy_mcsafe",
> > > > >
> > > >
> > > >
> > > > then I get this one:
> > > >
> > > > lib/ubsan.o: warning: objtool: __ubsan_handle_out_of_bounds()+0x5d: call to ubsan_prologue() with UACCESS enabled
> > >
> > > And of course I jinxed it by calling it easy.
> > >
> > > Peter, how do you want to handle this?
> > >
> > > Should we just disable UACCESS checking in lib/ubsan.c?
> >
> > No, that is actually unsafe and could break things (as would you patch
> > above).
>
> Oops.  -EFIXINGTOOMANYOBJTOOLISSUESATONCE
>
> > I'm thinking the below patch ought to cure things:
> >
> > ---
> > Subject: x86/uaccess: Don't leak the AC flags into __get_user() argument evalidation
>
> s/evalidation/evaluation
>
> > Identical to __put_user(); the __get_user() argument evalution will too
> > leak UBSAN crud into the __uaccess_begin() / __uaccess_end() region.
> > While uncommon this was observed to happen for:
> >
> >   drivers/xen/gntdev.c: if (__get_user(old_status, batch->status[i]))
> >
> > where UBSAN added array bound checking.
> >
> > This complements commit:
> >
> >   6ae865615fc4 ("x86/uaccess: Dont leak the AC flag into __put_user() argument evaluation")
> >
> > Reported-by: Randy Dunlap <rdunlap@infradead.org>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: luto@kernel.org
> > ---
> >  arch/x86/include/asm/uaccess.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> > index 9c4435307ff8..35c225ede0e4 100644
> > --- a/arch/x86/include/asm/uaccess.h
> > +++ b/arch/x86/include/asm/uaccess.h
> > @@ -444,8 +444,10 @@ __pu_label:                                                      \
> >  ({                                                                   \
> >       int __gu_err;                                                   \
> >       __inttype(*(ptr)) __gu_val;                                     \
> > +     __typeof__(ptr) __gu_ptr = (ptr);                               \
> > +     __typeof__(size) __gu_size = (size);                            \
> >       __uaccess_begin_nospec();                                       \
> > -     __get_user_size(__gu_val, (ptr), (size), __gu_err, -EFAULT);    \
> > +     __get_user_size(__gu_val, __gu_ptr, __gu_size, __gu_err, -EFAULT);      \
> >       __uaccess_end();                                                \
> >       (x) = (__force __typeof__(*(ptr)))__gu_val;                     \
> >       __builtin_expect(__gu_err, 0);                                  \
>
> Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
>

Tested-by Sedat Dilek <sedat.dilek@gmail.com>

- Sedat -