linux-kernel - Re: [PATCH] riscv: reject invalid syscalls below -1

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Message-ID: <CAEn-LTojXuAbY31nReCsbE=7Q9PkuKexAttTc+35ZFn7SLG-_w@mail.gmail.com>
Date:   Wed, 18 Dec 2019 12:06:57 +0200
From:   David Abdurachmanov <david.abdurachmanov@...il.com>
To:     Andreas Schwab <schwab@...e.de>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        David Abdurachmanov <david.abdurachmanov@...ive.com>,
        Kees Cook <keescook@...omium.org>,
        Anup Patel <Anup.Patel@....com>,
        Vincent Chen <vincent.chen@...ive.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Bin Meng <bmeng.cn@...il.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] riscv: reject invalid syscalls below -1

On Wed, Dec 18, 2019 at 11:46 AM Andreas Schwab <schwab@...e.de> wrote:
>
> On Dez 18 2019, David Abdurachmanov wrote:
>
> > diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> > index a1349ca64669..e163b7b64c86 100644
> > --- a/arch/riscv/kernel/entry.S
> > +++ b/arch/riscv/kernel/entry.S
> > @@ -246,6 +246,7 @@ check_syscall_nr:
> >        */
> >       li t1, -1
> >       beq a7, t1, ret_from_syscall_rejected
> > +     blt a7, t1, 1f
>
> How about using bgeu instead in the preceding check?

The syscall number could be -1 if tracer rejected it.

We could do:

li t0, __NR_syscalls
[..]
// first check if syscall was rejected
li t1, -1
beq a7, t1, ret_from_syscall_rejected
// then check the bounds
bgeu a7, t0, 1f

>
>         /*
>          * Syscall number held in a7.
>          * If syscall number is above allowed value, redirect to ni_syscall.
>          */
>         bge a7, t0, 1f
>
> Andreas.
>
> --
> Andreas Schwab, SUSE Labs, schwab@...e.de
> GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
> "And now for something completely different."