linux-kernel - Re: ptrace: seccomp: Return value when the call was already invalid

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Message-ID: <202007030851.D11F1EFA@keescook>
Date:   Fri, 3 Jul 2020 08:52:05 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Will Deacon <will@...nel.org>
Cc:     Keno Fischer <keno@...iacomputing.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>
Subject: Re: ptrace: seccomp: Return value when the call was already invalid

On Fri, Jul 03, 2020 at 04:44:27PM +0100, Will Deacon wrote:
> On Fri, Jul 03, 2020 at 08:17:19AM -0700, Kees Cook wrote:
> > On Fri, Jul 03, 2020 at 09:39:14AM +0100, Will Deacon wrote:
> > > diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
> > > index 5f5b868292f5..a13661f44818 100644
> > > --- a/arch/arm64/kernel/syscall.c
> > > +++ b/arch/arm64/kernel/syscall.c
> > > @@ -121,12 +121,10 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
> > >  	user_exit();
> > >  
> > >  	if (has_syscall_work(flags)) {
> > > -		/* set default errno for user-issued syscall(-1) */
> > > -		if (scno == NO_SYSCALL)
> > > -			regs->regs[0] = -ENOSYS;
> > > -		scno = syscall_trace_enter(regs);
> > > -		if (scno == NO_SYSCALL)
> > > +		if (syscall_trace_enter(regs))
> > >  			goto trace_exit;
> > > +
> > > +		scno = regs->syscallno;
> > >  	}
> > >  
> > >  	invoke_syscall(regs, scno, sc_nr, syscall_table);
> > 
> > What effect do either of these patches have on the existing seccomp
> > selftests: tools/testing/selftests/seccomp/seccomp_bpf ?
> 
> Tests! Thanks, I'll have a look.

Thanks!

(And either way, that this behavioral difference went unnoticed means we
need to add a test to the selftests for this patch.)

-- 
Kees Cook