Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Andy Lutomirski]

On Thu, Aug 27, 2020 at 12:38 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
>
> On Thu, Aug 27, 2020 at 11:56 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >
> >
> >
> > > On Aug 27, 2020, at 11:13 AM, Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
> > >
> > > On 8/27/2020 6:36 AM, Florian Weimer wrote:
> > >> * H. J. Lu:
> > >>>> On Thu, Aug 27, 2020 at 6:19 AM Florian Weimer <fweimer@xxxxxxxxxx> wrote:
> > >>>>>
> > >>>>> * Dave Martin:
> > >>>>>
> > >>>>>> You're right that this has implications: for i386, libc probably pulls
> > >>>>>> more arguments off the stack than are really there in some situations.
> > >>>>>> This isn't a new problem though.  There are already generic prctls with
> > >>>>>> fewer than 4 args that are used on x86.
> > >>>>>
> > >>>>> As originally posted, glibc prctl would have to know that it has to pull
> > >>>>> an u64 argument off the argument list for ARCH_X86_CET_DISABLE.  But
> > >>>>> then the u64 argument is a problem for arch_prctl as well.
> > >>>>>
> > >>>
> > >>> Argument of ARCH_X86_CET_DISABLE is int and passed in register.
> > >> The commit message and the C source say otherwise, I think (not sure
> > >> about the C source, not a kernel hacker).
> > >
> > > H.J. Lu suggested that we fix x86 arch_prctl() to take four arguments, and then keep MMAP_SHSTK as an arch_prctl().  Because now the map flags and size are all in registers, this also solves problems being pointed out earlier.  Without a wrapper, the shadow stack mmap call (from user space) will be:
> > >
> > > syscall(_NR_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, size, MAP_32BIT).
> >
> > I admit I don’t see a show stopping technical reason we can’t add arguments to an existing syscall, but I’m pretty sure it’s unprecedented, and it doesn’t seem like a good idea.
>
> prctl prototype is:
>
> extern int prctl (int __option, ...)
>
> and implemented in kernel as:
>
>       int prctl(int option, unsigned long arg2, unsigned long arg3,
>                  unsigned long arg4, unsigned long arg5);
>
> Not all prctl operations take all 5 arguments.   It also applies
> to arch_prctl.  It is quite normal for different operations of
> arch_prctl to take different numbers of arguments.

If by "quite normal" you mean "does not happen", then I agree.

In any event, I will not have anything to do with a patch that changes
an existing syscall signature unless Linus personally acks it.  So if
you want to email him and linux-abi, be my guest.