[PATCH] Restrict write access to dmesg_restrict

Richard Weinberger
2011-03-14T19:43:50Z

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.

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