[PATCH] Restrict write access to dmesg_restrict

[Richard Weinberger]

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@xxxxxx>
---
 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

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