linux-kernel - Re: [PATCH 1/2] sysctl: add a new generic strategy to make permanent changes on negative values

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Date:	Mon, 3 Aug 2015 11:33:30 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Willy Tarreau <w@....eu>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Jan Beulich <jbeulich@...e.com>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH 1/2] sysctl: add a new generic strategy to make permanent
 changes on negative values

On Mon, Aug 3, 2015 at 11:23 AM, Willy Tarreau <w@....eu> wrote:
> The new function is proc_dointvec_minmax_negperm(), it refuses to change
> the value if the current one is already negative. This will be used to
> lock down some settings such as sensitive system calls.
>
> Signed-off-by: Willy Tarreau <w@....eu>
> ---
>  kernel/sysctl.c | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 19b62b5..86c95a8 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -185,6 +185,9 @@ static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
>                                 void __user *buffer, size_t *lenp, loff_t *ppos);
>  #endif
>
> +static int proc_dointvec_minmax_negperm(struct ctl_table *table, int write,
> +               void __user *buffer, size_t *lenp, loff_t *ppos);
> +
>  static int proc_dointvec_minmax_coredump(struct ctl_table *table, int write,
>                 void __user *buffer, size_t *lenp, loff_t *ppos);
>  #ifdef CONFIG_COREDUMP
> @@ -2249,6 +2252,33 @@ static void validate_coredump_safety(void)
>  #endif
>  }
>
> +/* Like minmax except that it refuses any change if the value was already
> + * negative. It silently ignores overrides with the same negative value.
> + */
> +static int do_proc_dointvec_negperm_conv(bool *negp, unsigned long *lvalp,
> +                                        int *valp,
> +                                        int write, void *data)
> +{
> +       if (write && *valp < 0 && (!*negp || *valp != (int)*lvalp))

I could easily have failed to follow the bizarre negative sign
convention, but shouldn't that be "*valp != -(int)*lvalp" or similar?

--Andy
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