Re: [RFC PATCH v3 19/24] x86/cet/shstk: Introduce WRUSS instruction

[Yu-cheng Yu]

On Fri, 2018-08-31 at 15:16 -0700, Andy Lutomirski wrote:
> On Fri, Aug 31, 2018 at 2:49 PM, Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > 
> > 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@xxxxxxx
> > > > > om
> > > > > > 
> > > > > > 
> > > > > > wrote:
> > > > > > 
> > > > > > 
> > > > > > WRUSS is a new kernel-mode instruction but writes directly
> > > > > > to user shadow stack memory.ÂÂThis 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.ÂÂIn 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)
> > > > > > +{
> > > > > > +ÂÂÂÂÂÂÂint err = 0;
> > > > > > +
> > > > > > +ÂÂÂÂÂÂÂasm volatile("1: wrussq %1, (%0)\n"
> > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ"2:\n"
> > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ_ASM_EXTABLE_HANDLE(1b, 2b,
> > > > > > ex_handler_wruss)
> > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ:
> > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ: "r" (addr), "r" (val));
> > > > > > +
> > > > > > +ÂÂÂÂÂÂÂreturn 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,
> > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂstruct pt_regs *regs, int
> > > > > > trapnr)
> > > > > > +{
> > > > > > +ÂÂÂÂÂÂÂregs->ip = ex_fixup_addr(fixup);
> > > > > > +ÂÂÂÂÂÂÂregs->ax = -1;
> > > > > > +ÂÂÂÂÂÂÂreturn 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.ÂÂHow about using
> > > > that?ÂÂYou
> > > > won't even need a special exception handler.
> > Maybe something like this?ÂÂIt looks simple now.
> > 
> > static inline int write_user_shstk_64(unsigned long addr, unsigned
> > long val)
> > {
> > ÂÂÂÂÂÂÂÂasm_volatile_goto("wrussq %1, (%0)\n"
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ"jmp %l[ok]\n"
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ".section .fixup,\"ax\"n"
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ"jmp %l[fail]\n"
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ".previous\n"
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ:: "r" (addr), "r" (val)
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ:: ok, fail);
> > ok:
> > ÂÂÂÂÂÂÂÂreturn 0;
> > fail:
> > ÂÂÂÂÂÂÂÂreturn -1;
> > }
> > 
> I think you can get rid of 'jmp %l[ok]' and the ok label and just fall
> through.ÂÂAnd you don't need an explicit jmp to fail -- just set the
> _EX_HANDLER entry to land on the fail label.

Thanks! ÂThis now looks simple and much better.

Yu-cheng



+static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
+{
+	asm_volatile_goto("1: wrussq %1, (%0)\n"
+			ÂÂ_ASM_EXTABLE(1b, %l[fail])
+			ÂÂ:: "r" (addr), "r" (val)
+			ÂÂ:: fail);
+	return 0;
+fail:
+	return -1;
+}