Re: [PATCH] Restrict write access to dmesg_restrict

Richard Weinberger
2011-03-14T20:02:46Z

Am Montag 14 März 2011, 20:49:55 schrieb Dan Rosenberg:
> 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:

Thanks for the info!
I did not notice commit ce6ada3 (security: Define CAP_SYSLOG).
But as you said, writing to dmesg_restrict should still require CAP_SYS_ADMIN.

> 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;