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From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 27 Apr 2021 17:11:11 -0700
Message-ID: <CALCETrWzL=jgnWd+6YuBo02GG8vTvsG22sXGaUQCc37vwQ6HdA@mail.gmail.com>
Subject: Re: pt_regs->ax == -ENOSYS
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Will Drewry <wad@chromium.org>
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On Tue, Apr 27, 2021 at 5:05 PM H. Peter Anvin <hpa@zytor.com> wrote:
>
> On 4/27/21 4:23 PM, Andy Lutomirski wrote:
> >
> > I much prefer the model of saying that the bits that make sense for
> > the syscall type (all 64 for 64-bit SYSCALL and the low 32 for
> > everything else) are all valid.  This way there are no weird reserved
> > bits, no weird ptrace() interactions, etc.  I'm a tiny bit concerned
> > that this would result in a backwards compatibility issue, but not
> > very.  This would involve changing syscall_get_nr(), but that doesn't
> > seem so bad.  The biggest problem is that seccomp hardcoded syscall
> > nrs to 32 bit.
> >
> > An alternative would be to declare that we always truncate to 32 bits,
> > except that 64-bit SYSCALL with high bits set is an error and results
> > in ENOSYS. The ptrace interaction there is potentially nasty.
> >
> > Basically, all choices here kind of suck, and I haven't done a real
> > analysis of all the issues...
> >
>
> OK, I really don't understand this. The *current* way of doing it causes
> a bunch of ugly corner conditions, including in ptrace, which this would
> get rid of. It isn't any different than passing any other argument which
> is an int -- in fact we have this whole machinery to deal with that subcase.
>

Let's suppose we decide to truncate the syscall nr.  What would the
actual semantics be?  Would ptrace see the truncated value in orig_ax?
 How about syscall user dispatch?  What happens if ptrace writes a
value with high bits set to orig_ax?  Do we truncate it again?  Or do
we say that ptrace *can't* write too large a value?

For better for worse, RAX is 64 bits, orig_ax is a 64-bit field, and
it currently has nonsensical semantics.  Redefining orig_ax as a
32-bit field is surely possible, but doing so cleanly is not
necessarily any easier than any other approach.  If it weren't for
seccomp, I would say that the obviously correct answer is to just
treat it everywhere as a 64-bit number.

--Andy