Re: [PATCH] Restrict write access to dmesg_restrict

Dan Rosenberg
2011-03-14T19:50:01Z

On Mon, 2011-03-14 at 20:35 +0100, Richard Weinberger wrote:
> When dmesg_restrict is set to 1 CAP_SYS_ADMIN is needed
> to read the kernel ring buffer.
> But a root user without CAP_SYS_ADMIN is able to reset
> dmesg_restrict to 0.

A minor correction, CAP_SYSLOG is needed to read the kernel syslog. But
I think requiring CAP_SYS_ADMIN is appropriate to modify the value of
the sysctl, so assuming the commit message reflects this:

Acked-by: Dan Rosenberg <drosenberg@vsecurity.com>

>
> This is an issue when e.g. LXC (Linux Containers) are used
> and complete user space is running without CAP_SYS_ADMIN.
> A unprivileged and jailed root user can bypass the
> dmesg_restrict protection.
>
> With this patch writing to dmesg_restrict is only allowed
> when root has CAP_SYS_ADMIN.
>
> Signed-off-by: Richard Weinberger <richard@nod.at>
> ---
> kernel/sysctl.c | 18 +++++++++++++++++-
> 1 files changed, 17 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 4eed0af..f90c8f6 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -169,6 +169,11 @@ static int proc_taint(struct ctl_table *table, int write,
> void __user *buffer, size_t *lenp, loff_t *ppos);
> #endif
>
> +#ifdef CONFIG_PRINTK
> +static int proc_dmesg_restrict(struct ctl_table *table, int write,
> + void __user *buffer, size_t *lenp, loff_t *ppos);
> +#endif
> +
> #ifdef CONFIG_MAGIC_SYSRQ
> /* Note: sysrq code uses it's own private copy */
> static int __sysrq_enabled = SYSRQ_DEFAULT_ENABLE;
> @@ -704,7 +709,7 @@ static struct ctl_table kern_table[] = {
> .data = &dmesg_restrict,
> .maxlen = sizeof(int),
> .mode = 0644,
> - .proc_handler = proc_dointvec_minmax,
> + .proc_handler = proc_dmesg_restrict,
> .extra1 = &zero,
> .extra2 = &one,
> },
> @@ -2397,6 +2402,17 @@ static int proc_taint(struct ctl_table *table, int write,
> return err;
> }
>
> +#ifdef CONFIG_PRINTK
> +static int proc_dmesg_restrict(struct ctl_table *table, int write,
> + void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> + if (write && !capable(CAP_SYS_ADMIN))
> + return -EPERM;
> +
> + return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> +}
> +#endif
> +
> struct do_proc_dointvec_minmax_conv_param {
> int *min;
> int *max;
> --
> 1.6.6.1