linux-kernel - Re: [PATCH v10 06/11] seccomp: add SECCOMP_RET

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Date:	Tue, 21 Feb 2012 14:41:28 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Will Drewry <wad@...omium.org>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	netdev@...r.kernel.org, x86@...nel.org, arnd@...db.de,
	davem@...emloft.net, hpa@...or.com, mingo@...hat.com,
	oleg@...hat.com, peterz@...radead.org, rdunlap@...otime.net,
	mcgrathr@...omium.org, tglx@...utronix.de, luto@....edu,
	eparis@...hat.com, serge.hallyn@...onical.com, djm@...drot.org,
	scarybeasts@...il.com, indan@....nu, pmoore@...hat.com,
	akpm@...ux-foundation.org, corbet@....net, eric.dumazet@...il.com,
	markus@...omium.org
Subject: Re: [PATCH v10 06/11] seccomp: add SECCOMP_RET_ERRNO

On Tue, Feb 21, 2012 at 11:30:30AM -0600, Will Drewry wrote:
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 0043b7e..23f1844 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -136,22 +136,18 @@ static void *bpf_load(const void *nr, int off, unsigned int size, void *buf)
>  static u32 seccomp_run_filters(int syscall)
>  {
>  	struct seccomp_filter *f;
> -	u32 ret = SECCOMP_RET_KILL;
>  	static const struct bpf_load_fn fns = {
>  		bpf_load,
>  		sizeof(struct seccomp_data),
>  	};
> +	u32 ret = SECCOMP_RET_ALLOW;
>  	const void *sc_ptr = (const void *)(uintptr_t)syscall;
> -
>  	/*
>  	 * All filters are evaluated in order of youngest to oldest. The lowest
>  	 * BPF return value always takes priority.
>  	 */
> -	for (f = current->seccomp.filter; f; f = f->prev) {
> -		ret = bpf_run_filter(sc_ptr, f->insns, &fns);
> -		if (ret != SECCOMP_RET_ALLOW)
> -			break;
> -	}
> +	for (f = current->seccomp.filter; f; f = f->prev)
> +		ret = min_t(u32, ret, bpf_run_filter(sc_ptr, f->insns, &fns));
>  	return ret;
>  }

I'd like to see this fail closed in the (theoretically impossible, but
why risk it) case of there being no filters at all. Could do something
like this:

	u32 ret = current->seccomp.filter ? SECCOMP_RET_ALLOW : SECCOMP_RET_KILL;

Or, just this, to catch the misbehavior:

	if (unlikely(current->seccomp.filter == NULL))
		return SECCOMP_RET_KILL;

-Kees

-- 
Kees Cook
ChromeOS Security
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