Re: [PATCH] [RFC] Make it easier to harden /proc/

[Kees Cook <kees.cook@canonical.com>]

Hi Richard,

On Wed, Mar 16, 2011 at 08:31:47PM +0100, Richard Weinberger wrote:
> When containers like LXC are used a unprivileged and jailed
> root user can still write to critical files in /proc/.
> E.g: /proc/sys/kernel/{sysrq, panic, panic_on_oops, ... }
> 
> This new restricted attribute makes it possible to protect such
> files. When restricted is set to true root needs CAP_SYS_ADMIN
> to into the file.

I was thinking about this too. I'd prefer more fine-grained control
in this area, since some sysctl entries aren't strictly controlled by
CAP_SYS_ADMIN (e.g. mmap_min_addr is already checking CAP_SYS_RAWIO).

How about this instead?

Signed-off-by: Kees Cook <kees.cook@canonical.com>
---
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 8eb2522..5c5cfab 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -149,6 +149,10 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
 	if (sysctl_perm(head->root, table, write ? MAY_WRITE : MAY_READ))
 		goto out;
 
+	if (write && !cap_isclear(table->write_caps) &&
+            !cap_issubset(table->write_caps, current_cred()->cap_permitted))
+		goto out;
+
 	/* if that can happen at all, it should be -EINVAL, not -EISDIR */
 	error = -EINVAL;
 	if (!table->proc_handler)
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 11684d9..4e05493 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -1018,6 +1018,7 @@ struct ctl_table
 	void *data;
 	int maxlen;
 	mode_t mode;
+	kernel_cap_t write_caps;	/* Capabilities required to write */
 	struct ctl_table *child;
 	struct ctl_table *parent;	/* Automatically set */
 	proc_handler *proc_handler;	/* Callback for text formatting */


-- 
Kees Cook
Ubuntu Security Team