Re: [patch V3 01/13] entry: Provide generic syscall entry functionality

[Kees Cook]

On Thu, Jul 16, 2020 at 08:22:09PM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> On syscall entry certain work needs to be done:
> 
>    - Establish state (lockdep, context tracking, tracing)
>    - Conditional work (ptrace, seccomp, audit...)
> 
> This code is needlessly duplicated and  different in all
> architectures.
> 
> Provide a generic version based on the x86 implementation which has all the
> RCU and instrumentation bits right.

Ahh! You're reading my mind! I was just thinking about this while
reviewing the proposed syscall redirection series[1], and pondering the
lack of x86 TIF flags, and that nearly everything in the series (and for
seccomp and other things) didn't need to be arch-specific. And now that
series absolutely needs to be rebased and it'll magically work for every
arch that switches to the generic entry code. :)

Notes below...

[1] https://lore.kernel.org/lkml/20200716193141.4068476-2-krisman@xxxxxxxxxxxxx/

> +/*
> + * Define dummy _TIF work flags if not defined by the architecture or for
> + * disabled functionality.
> + */

When I was thinking about this last week I was pondering having a split
between the arch-agnositc TIF flags and the arch-specific TIF flags, and
that each arch could have a single "there is agnostic work to be done"
TIF in their thread_info, and the agnostic flags could live in
task_struct or something. Anyway, I'll keep reading...

> +/**
> + * syscall_enter_from_user_mode - Check and handle work before invoking
> + *				 a syscall
> + * @regs:	Pointer to currents pt_regs
> + * @syscall:	The syscall number
> + *
> + * Invoked from architecture specific syscall entry code with interrupts
> + * disabled. The calling code has to be non-instrumentable. When the
> + * function returns all state is correct and the subsequent functions can be
> + * instrumented.
> + *
> + * Returns: The original or a modified syscall number
> + *
> + * If the returned syscall number is -1 then the syscall should be
> + * skipped. In this case the caller may invoke syscall_set_error() or
> + * syscall_set_return_value() first.  If neither of those are called and -1
> + * is returned, then the syscall will fail with ENOSYS.

There's been some recent confusion over "has the syscall changed,
or did seccomp request it be skipped?" that was explored in arm64[2]
(though I see Will and Keno in CC already). There might need to be a
clearer way to distinguish between "wild userspace issued a -1 syscall"
and "seccomp or ptrace asked for the syscall to be skipped". The
difference is mostly about when ENOSYS gets set, with respect to calls
to syscall_set_return_value(), but if the syscall gets changed, the arch
may need to recheck the value and consider ENOSYS, etc. IIUC, what Will
ended up with[3] was having syscall_trace_enter() return the syscall return
value instead of the new syscall.

[2] https://lore.kernel.org/lkml/20200704125027.GB21185@willie-the-truck/
[3] https://lore.kernel.org/lkml/20200703083914.GA18516@willie-the-truck/

> +static long syscall_trace_enter(struct pt_regs *regs, long syscall,
> +				unsigned long ti_work)
> +{
> +	long ret = 0;
> +
> +	/* Handle ptrace */
> +	if (ti_work & (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_EMU)) {
> +		ret = arch_syscall_enter_tracehook(regs);
> +		if (ret || (ti_work & _TIF_SYSCALL_EMU))
> +			return -1L;
> +	}
> +
> +	/* Do seccomp after ptrace, to catch any tracer changes. */
> +	if (ti_work & _TIF_SECCOMP) {
> +		ret = arch_syscall_enter_seccomp(regs);
> +		if (ret == -1L)
> +			return ret;
> +	}
> +
> +	if (unlikely(ti_work & _TIF_SYSCALL_TRACEPOINT))
> +		trace_sys_enter(regs, syscall);
> +
> +	arch_syscall_enter_audit(regs);
> +
> +	return ret ? : syscall;
> +}

Modulo the notes about -1 vs syscall number above, this looks correct to
me for ptrace and seccomp.

-- 
Kees Cook