[PATCH] add umask parameter to procfs

[PATCH] add umask parameter to procfs

[Rene Scharfe]

Add proc.umask kernel parameter.  It can be used to restrict permissions
on the numerical directories in the root of a proc filesystem, i.e. the
directories containing process specific information.

E.g. add proc.umask=077 to your kernel command line and all users except
root can only see their own process details (like command line
parameters) with ps or top.  It can be useful to add a bit of privacy to
multi-user servers.

The patch has been inspired by a similar feature in GrSecurity.

It could have also been implemented as a mount option to procfs, but at
a higher cost and no apparent benefit -- changes to this umask are not
supposed to happen very often.  Actually, the previous incarnation of
this patch was implemented as a half-assed mount option, but I didn't
know then how easy it is to add a kernel parameter.

Comments about usefulness or implementation are very welcome.

Thanks,
Rene


--- linux-2.6.11-rc4/fs/proc/base.c~	2005-02-11 21:16:13.000000000 +0100
+++ linux-2.6.11-rc4/fs/proc/base.c	2005-02-11 23:38:17.000000000 +0100
@@ -32,8 +32,14 @@
 #include <linux/mount.h>
 #include <linux/security.h>
 #include <linux/ptrace.h>
+#include <linux/module.h>
+#include <linux/moduleparam.h>
 #include "internal.h"
 
+static umode_t umask = 0;
+module_param(umask, ushort, 0);
+MODULE_PARM_DESC(umask, "umask for the numerical directories in /proc");
+
 /*
  * For hysterical raisins we keep the same inumbers as in the old procfs.
  * Feel free to change the macro below - just keep the range distinct from
@@ -1369,7 +1375,7 @@ static struct dentry *proc_pident_lookup
 		goto out;
 
 	ei = PROC_I(inode);
-	inode->i_mode = p->mode;
+	inode->i_mode = p->mode & ~(umask & S_IRWXUGO);
 	/*
 	 * Yes, it does not scale. And it should not. Don't add
 	 * new entries into /proc/<tgid>/ without very good reasons.
@@ -1699,7 +1705,7 @@ struct dentry *proc_pid_lookup(struct in
 		put_task_struct(task);
 		goto out;
 	}
-	inode->i_mode = S_IFDIR|S_IRUGO|S_IXUGO;
+	inode->i_mode = S_IFDIR | ((S_IRUGO|S_IXUGO) & ~umask);
 	inode->i_op = &proc_tgid_base_inode_operations;
 	inode->i_fop = &proc_tgid_base_operations;
 	inode->i_nlink = 3;
@@ -1754,7 +1760,7 @@ static struct dentry *proc_task_lookup(s
 
 	if (!inode)
 		goto out_drop_task;
-	inode->i_mode = S_IFDIR|S_IRUGO|S_IXUGO;
+	inode->i_mode = S_IFDIR | ((S_IRUGO|S_IXUGO) & ~umask);
 	inode->i_op = &proc_tid_base_inode_operations;
 	inode->i_fop = &proc_tid_base_operations;
 	inode->i_nlink = 3;

Re: [PATCH] add umask parameter to procfs

[Andrew Morton]

Rene Scharfe <rene.scharfe@lsrfire.ath.cx> wrote:
>
> Add proc.umask kernel parameter.  It can be used to restrict permissions
> on the numerical directories in the root of a proc filesystem, i.e. the
> directories containing process specific information.
> 
> E.g. add proc.umask=077 to your kernel command line and all users except
> root can only see their own process details (like command line
> parameters) with ps or top.  It can be useful to add a bit of privacy to
> multi-user servers.
> 
> The patch has been inspired by a similar feature in GrSecurity.
> 
> It could have also been implemented as a mount option to procfs, but at
> a higher cost and no apparent benefit -- changes to this umask are not
> supposed to happen very often.  Actually, the previous incarnation of
> this patch was implemented as a half-assed mount option, but I didn't
> know then how easy it is to add a kernel parameter.

The feature seems fairly obscure, although very simple.  Is anyone actually
likely to use this?

>  
> +static umode_t umask = 0;

a) I think the above should be called proc_umask.

b) You shouldn't initialise it.

c) When adding a kernel parameter you should update
   Documentation/kernel-parameters.txt

Re: [PATCH] add umask parameter to procfs

[Herbert Poetzl]

On Thu, Feb 17, 2005 at 03:41:19PM -0800, Andrew Morton wrote:
> Rene Scharfe <rene.scharfe@lsrfire.ath.cx> wrote:
> >
> > Add proc.umask kernel parameter.  It can be used to restrict permissions
> > on the numerical directories in the root of a proc filesystem, i.e. the
> > directories containing process specific information.
> > 
> > E.g. add proc.umask=077 to your kernel command line and all users except
> > root can only see their own process details (like command line
> > parameters) with ps or top.  It can be useful to add a bit of privacy to
> > multi-user servers.
> > 
> > The patch has been inspired by a similar feature in GrSecurity.
> > 
> > It could have also been implemented as a mount option to procfs, but at
> > a higher cost and no apparent benefit -- changes to this umask are not
> > supposed to happen very often.  Actually, the previous incarnation of
> > this patch was implemented as a half-assed mount option, but I didn't
> > know then how easy it is to add a kernel parameter.
> 
> The feature seems fairly obscure, although very simple.  
> Is anyone actually likely to use this?

what about parents (and especially the init process)
some tools like pstree (or ps in certain cases) depend
on their visibility/accessability ...

was this tested except for the trivial case where
just plain everything is visible?

what if you want to change it afterwards (when tools
did break)?

best,
Herbert

> > +static umode_t umask = 0;
> 
> a) I think the above should be called proc_umask.
> 
> b) You shouldn't initialise it.
> 
> c) When adding a kernel parameter you should update
>    Documentation/kernel-parameters.txt
> -
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Re: [PATCH] add umask parameter to procfs

[Debian User]

On Thu, Feb 17, 2005 at 03:41:19PM -0800, Andrew Morton wrote:
> The feature seems fairly obscure, although very simple.  Is anyone actually
> likely to use this?

Yes, it's an obscure obscurity feature. :)  There certainly seems to be
some interest in it: both GrSecurity and OpenWall contain something
similar.  They set the mask at compile time, though.  But being able to
change it at boot time makes it much easier for mainstream distros to
include it in their kernels.

Personally I felt the need for the feature when I was a student sharing
a server with hundreds others.  Sure, it's not very useful on machines
offering no shell access at all.

> > +static umode_t umask = 0;
> 
> a) I think the above should be called proc_umask.
> 
> b) You shouldn't initialise it.
> 
> c) When adding a kernel parameter you should update
>    Documentation/kernel-parameters.txt

Like below?

Hrm, maybe the option should be named pidumask instead of simply umask?


diff -urp linux-2.6.11-rc4/Documentation/kernel-parameters.txt linux-2.6.11-rc4+/Documentation/kernel-parameters.txt
--- linux-2.6.11-rc4/Documentation/kernel-parameters.txt	2005-02-11 21:16:00.000000000 +0100
+++ linux-2.6.11-rc4+/Documentation/kernel-parameters.txt	2005-02-12 01:37:42.000000000 +0100
@@ -1037,16 +1037,19 @@ running once the system is up.
 			[ISAPNP] Exclude memory regions for the autoconfiguration
 			Ranges are in pairs (memory base and size).
 
+	processor.max_cstate=   [HW, ACPI]
+			Limit processor to maximum C-state
+			max_cstate=9 overrides any DMI blacklist limit.
+
+	proc.umask=	[KNL] Restrict permissions of process specific
+			entries in /proc (i.e. the numerical directories).
+
 	profile=	[KNL] Enable kernel profiling via /proc/profile
 			{ schedule | <number> }
 			(param: schedule - profile schedule points}
 			(param: profile step/bucket size as a power of 2 for
 				statistical time based profiling)
 
-	processor.max_cstate=   [HW, ACPI]
-			Limit processor to maximum C-state
-			max_cstate=9 overrides any DMI blacklist limit.
-
 	prompt_ramdisk=	[RAM] List of RAM disks to prompt for floppy disk
 			before loading.
 			See Documentation/ramdisk.txt.
diff -urp linux-2.6.11-rc4/fs/proc/base.c linux-2.6.11-rc4+/fs/proc/base.c
--- linux-2.6.11-rc4/fs/proc/base.c	2005-02-11 21:16:13.000000000 +0100
+++ linux-2.6.11-rc4+/fs/proc/base.c	2005-02-12 01:33:14.000000000 +0100
@@ -32,8 +32,14 @@
 #include <linux/mount.h>
 #include <linux/security.h>
 #include <linux/ptrace.h>
+#include <linux/module.h>
+#include <linux/moduleparam.h>
 #include "internal.h"
 
+static umode_t proc_umask;
+module_param_named(umask, proc_umask, ushort, 0);
+MODULE_PARM_DESC(umask, "umask for the numerical directories in /proc");
+
 /*
  * For hysterical raisins we keep the same inumbers as in the old procfs.
  * Feel free to change the macro below - just keep the range distinct from
@@ -1369,7 +1375,7 @@ static struct dentry *proc_pident_lookup
 		goto out;
 
 	ei = PROC_I(inode);
-	inode->i_mode = p->mode;
+	inode->i_mode = p->mode & ~(proc_umask & S_IALLUGO);
 	/*
 	 * Yes, it does not scale. And it should not. Don't add
 	 * new entries into /proc/<tgid>/ without very good reasons.
@@ -1699,7 +1705,7 @@ struct dentry *proc_pid_lookup(struct in
 		put_task_struct(task);
 		goto out;
 	}
-	inode->i_mode = S_IFDIR|S_IRUGO|S_IXUGO;
+	inode->i_mode = S_IFDIR | ((S_IRUGO|S_IXUGO) & ~proc_umask);
 	inode->i_op = &proc_tgid_base_inode_operations;
 	inode->i_fop = &proc_tgid_base_operations;
 	inode->i_nlink = 3;
@@ -1754,7 +1760,7 @@ static struct dentry *proc_task_lookup(s
 
 	if (!inode)
 		goto out_drop_task;
-	inode->i_mode = S_IFDIR|S_IRUGO|S_IXUGO;
+	inode->i_mode = S_IFDIR | ((S_IRUGO|S_IXUGO) & ~proc_umask);
 	inode->i_op = &proc_tid_base_inode_operations;
 	inode->i_fop = &proc_tid_base_operations;
 	inode->i_nlink = 3;

Re: [PATCH] add umask parameter to procfs

[Bodo Eggert]

Herbert Poetzl <herbert@13thfloor.at> wrote:
> On Thu, Feb 17, 2005 at 03:41:19PM -0800, Andrew Morton wrote:
>> Rene Scharfe <rene.scharfe@lsrfire.ath.cx> wrote:

>> > Add proc.umask kernel parameter.  It can be used to restrict permissions
>> > on the numerical directories in the root of a proc filesystem, i.e. the
>> > directories containing process specific information.
>> > 
>> > E.g. add proc.umask=077 to your kernel command line and all users except
>> > root can only see their own process details (like command line
>> > parameters) with ps or top.  It can be useful to add a bit of privacy to
>> > multi-user servers.

>> The feature seems fairly obscure, although very simple.
>> Is anyone actually likely to use this?

I'm using a openwall-patched kernel on some of my boxes with a similar
feature. I did not yet test this patch, but I like it. It's cheap, and
it fixes a potential security leak.

> what about parents (and especially the init process)
> some tools like pstree (or ps in certain cases) depend
> on their visibility/accessability ...

pstree will break if /proc/1 isn't readable, unless you specify a readable
starting pid. Since this is not the default case, this is IMO ok. (It should
be easy to rewrite it to trace the ppid-chains like "ps auxf" already does
correctly, or rather implement it in ps where it belongs).

None of my other tools seemed to stop working in an unintended way, but I
don't usurally spend my time watching processes.

Sample output of "ps auxf" on a openwall-patched system:
USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
7eggert  22504  5.0  3.5  2800 1608 pts/4    S    04:14   0:00 -bash
7eggert  22522  0.0  1.7  2856  792 pts/4    R    04:14   0:00  \_ ps auxf
7eggert  22483  2.3  3.4  2800 1588 pts/2    S    04:14   0:00 -bash

> what if you want to change it afterwards (when tools
> did break)?

Change the kernel command line back and reboot (or reload the module).

Re: [PATCH] add umask parameter to procfs

[Herbert Poetzl]

On Fri, Feb 18, 2005 at 04:22:49AM +0100, Bodo Eggert wrote:
> Herbert Poetzl <herbert@13thfloor.at> wrote:
> > On Thu, Feb 17, 2005 at 03:41:19PM -0800, Andrew Morton wrote:
> >> Rene Scharfe <rene.scharfe@lsrfire.ath.cx> wrote:
> 
> >> > Add proc.umask kernel parameter.  It can be used to restrict permissions
> >> > on the numerical directories in the root of a proc filesystem, i.e. the
> >> > directories containing process specific information.
> >> > 
> >> > E.g. add proc.umask=077 to your kernel command line and all users except
> >> > root can only see their own process details (like command line
> >> > parameters) with ps or top.  It can be useful to add a bit of privacy to
> >> > multi-user servers.
> 
> >> The feature seems fairly obscure, although very simple.
> >> Is anyone actually likely to use this?
> 
> I'm using a openwall-patched kernel on some of my boxes with a similar
> feature. I did not yet test this patch, but I like it. It's cheap, and
> it fixes a potential security leak.

don't get me wrong, I'm all for something like this,
as a matter of fact we are using something more specialized
for linux-vserver to hide processes between contexts ...

> > what about parents (and especially the init process)
> > some tools like pstree (or ps in certain cases) depend
> > on their visibility/accessability ...
> 
> pstree will break if /proc/1 isn't readable, unless you specify a readable
> starting pid. Since this is not the default case, this is IMO ok. (It should
> be easy to rewrite it to trace the ppid-chains like "ps auxf" already does
> correctly, or rather implement it in ps where it belongs).

hmm, so what about debugger and similar not able to find the
parent process or something like that?

I'd say this needs some more investigation what tools and
applications will break once it is enabled ...

> None of my other tools seemed to stop working in an unintended way, but I
> don't usurally spend my time watching processes.
> 
> Sample output of "ps auxf" on a openwall-patched system:
> USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
> 7eggert  22504  5.0  3.5  2800 1608 pts/4    S    04:14   0:00 -bash
> 7eggert  22522  0.0  1.7  2856  792 pts/4    R    04:14   0:00  \_ ps auxf
> 7eggert  22483  2.3  3.4  2800 1588 pts/2    S    04:14   0:00 -bash
> 
> > what if you want to change it afterwards (when tools
> > did break)?
> 
> Change the kernel command line back and reboot (or reload the module).

hmm, reload the procfs module? not very likely ;)

best,
Herbert

Re: [PATCH] add umask parameter to procfs

[Rene Scharfe]

On Fri, Feb 18, 2005 at 04:56:01AM +0100, Herbert Poetzl wrote:
> hmm, so what about debugger and similar not able to find the
> parent process or something like that?

You can walk the parentage chain up until you reach your login shell.
So you can look up info about the parent of every one of your processes
except for your login shell and any zombies.

> I'd say this needs some more investigation what tools and
> applications will break once it is enabled ...

Sure, that can't be bad.  I didn't really do anything so far that
warrants being called testing (compiles, runs, doesn't crash on boot --
send patch! ;).

But I also didn't invent this feature: it has been in OpenWall and
grsecurity for a long time now, so a form of restricted /proc has
received at least *some* testing.

Rene