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

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Date:	Thu, 17 Mar 2011 09:51:41 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Richard Weinberger <richard@....at>,
	Kees Cook <kees.cook@...onical.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	serge@...lyn.com, eparis@...hat.com, jmorris@...ei.org,
	eugeneteo@...nel.org, drosenberg@...curity.com
Subject: Re: [PATCH] [RFC] Make it easier to harden /proc/

Alexey Dobriyan <adobriyan@...il.com> writes:

> On Wed, Mar 16, 2011 at 09:08:16PM +0100, Richard Weinberger wrote:
>> Kees,
>> 
>> Am Mittwoch 16 März 2011, 20:55:49 schrieb Kees Cook:
>> > 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?
>> 
>> Good Idea.
>> May we should also consider a per-directory restriction.
>> Every file in /proc/sys/{kernel/, vm/, fs/, dev/} needs a protection.
>> It would be much easier to set the protection on the parent directory
>> instead of protecting file by file...
>
> Of course, not.
>
> You should _enable_ them one by one, not the other way around.
>
> 	"default deny"

Right.

Since the primary problem here is containers we can use the
user_namespace to add the default deny policy.

Something like the trivial patch below should make /proc/sys safe,
and the technique applies in general.

Richard is that a good enough example to get you started?

Eric

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 0f1bd83..a172a9d 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1674,10 +1674,12 @@ void register_sysctl_root(struct ctl_table_root *root)
 
 static int test_perm(int mode, int op)
 {
-	if (!current_euid())
-		mode >>= 6;
-	else if (in_egroup_p(0))
-		mode >>= 3;
+	if (current_user_ns() == &init_user_ns) {
+		if (!current_euid())
+			mode >>= 6;
+		else if (in_egroup_p(0))
+			mode >>= 3;
+	}
 	if ((op & ~mode & (MAY_READ|MAY_WRITE|MAY_EXEC)) == 0)
 		return 0;
 	return -EACCES;


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