On Thu, 2018-08-30 at 09:22 -0700, Yu-cheng Yu wrote:
> On Thu, 2018-08-30 at 08:55 -0700, Andy Lutomirski wrote:
> >
> > On Thu, Aug 30, 2018 at 8:39 AM, Jann Horn <jannh@xxxxxxxxxx>
> > wrote:
> > >
> > >
> > > On Thu, Aug 30, 2018 at 4:44 PM Yu-cheng Yu <yu-cheng.yu@intel.c
> > > om
> > > >
> > > > wrote:
> > > >
> > > >
> > > > WRUSS is a new kernel-mode instruction but writes directly
> > > > to user shadow stack memory. This is used to construct
> > > > a return address on the shadow stack for the signal
> > > > handler.
> > > >
> > > > This instruction can fault if the user shadow stack is
> > > > invalid shadow stack memory. In that case, the kernel does
> > > > fixup.
> > > >
> > > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > > [...]
> > > >
> > > >
> > > > +static inline int write_user_shstk_64(unsigned long addr,
> > > > unsigned long val)
> > > > +{
> > > > + int err = 0;
> > > > +
> > > > + asm volatile("1: wrussq %1, (%0)\n"
> > > > + "2:\n"
> > > > + _ASM_EXTABLE_HANDLE(1b, 2b,
> > > > ex_handler_wruss)
> > > > + :
> > > > + : "r" (addr), "r" (val));
> > > > +
> > > > + return err;
> > > > +}
> > > What's up with "err"? You set it to zero, and then you return
> > > it,
> > > but
> > > nothing can ever set it to non-zero, right?
> > >
> > > >
> > > >
> > > > +__visible bool ex_handler_wruss(const struct
> > > > exception_table_entry *fixup,
> > > > + struct pt_regs *regs, int
> > > > trapnr)
> > > > +{
> > > > + regs->ip = ex_fixup_addr(fixup);
> > > > + regs->ax = -1;
> > > > + return true;
> > > > +}
> > > And here you just write into regs->ax, but your "asm volatile"
> > > doesn't
> > > reserve that register. This looks wrong to me.
> > >
> > > I think you probably want to add something like an explicit
> > > `"+&a"(err)` output to the asm statements.
> > We require asm goto support these days. How about using
> > that? You
> > won't even need a special exception handler.
Maybe something like this? It looks simple now.
static inline int write_user_shstk_64(unsigned long addr, unsigned
long val)
{
asm_volatile_goto("wrussq %1, (%0)\n"
"jmp %l[ok]\n"
".section .fixup,\"ax\"n"
"jmp %l[fail]\n"
".previous\n"
:: "r" (addr), "r" (val)
:: ok, fail);
ok:
return 0;
fail:
return -1;
}
