[PATCH] Read only syscall tables for x86_64 and i386

[PATCH] Read only syscall tables for x86_64 and i386

[Christoph Lameter]

Place x86_64 and i386 syscall table into the read only section.

Remove the syscall tables from the data section and place them into the 
readonly section (like IA64).

Signed-off-by: Christoph Lameter <christoph@scalex86.org>

Index: linux-2.6.12-mm2/arch/i386/kernel/entry.S
===================================================================
--- linux-2.6.12-mm2.orig/arch/i386/kernel/entry.S	2005-06-28 17:46:31.000000000 +0000
+++ linux-2.6.12-mm2/arch/i386/kernel/entry.S	2005-06-28 17:47:11.000000000 +0000
@@ -680,6 +680,7 @@ ENTRY(spurious_interrupt_bug)
 	pushl $do_spurious_interrupt_bug
 	jmp error_code
 
+.section .rodata,"a"
 #include "syscall_table.S"
 
 syscall_table_size=(.-sys_call_table)
Index: linux-2.6.12-mm2/arch/i386/kernel/syscall_table.S
===================================================================
--- linux-2.6.12-mm2.orig/arch/i386/kernel/syscall_table.S	2005-06-28 17:46:31.000000000 +0000
+++ linux-2.6.12-mm2/arch/i386/kernel/syscall_table.S	2005-06-28 17:47:11.000000000 +0000
@@ -1,4 +1,3 @@
-.data
 ENTRY(sys_call_table)
 	.long sys_restart_syscall	/* 0 - old "setup()" system call, used for restarting */
 	.long sys_exit
Index: linux-2.6.12-mm2/arch/x86_64/kernel/syscall.c
===================================================================
--- linux-2.6.12-mm2.orig/arch/x86_64/kernel/syscall.c	2005-06-17 19:48:29.000000000 +0000
+++ linux-2.6.12-mm2/arch/x86_64/kernel/syscall.c	2005-06-28 18:22:41.000000000 +0000
@@ -19,7 +19,7 @@ typedef void (*sys_call_ptr_t)(void); 
 
 extern void sys_ni_syscall(void);
 
-sys_call_ptr_t sys_call_table[__NR_syscall_max+1] __cacheline_aligned = { 
+const sys_call_ptr_t sys_call_table[__NR_syscall_max+1] = { 
 	/* Smells like a like a compiler bug -- it doesn't work when the & below is removed. */ 
 	[0 ... __NR_syscall_max] = &sys_ni_syscall,
 #include <asm-x86_64/unistd.h>

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Arjan van de Ven]

On Tue, 2005-06-28 at 11:47 -0700, Christoph Lameter wrote:
> Place x86_64 and i386 syscall table into the read only section.
> 
> Remove the syscall tables from the data section and place them into the 
> readonly section (like IA64).

I like it.. however I think the 32 bit compat syscall table on x86-64
deserves the same treatment....

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Christoph Lameter]

On Tue, 28 Jun 2005, Arjan van de Ven wrote:

> I like it.. however I think the 32 bit compat syscall table on x86-64
> deserves the same treatment....

Ok.

---

Place x86_64 and i386 syscall table into the read only section.

Remove the syscall tables from the data section and place them into the 
readonly section (like IA64). Includes the ia32 syscall table on x86_64.

Note that AFS seems to be modifying the syscall table. Is that legit?

Signed-off-by: Christoph Lameter <christoph@scalex86.org>

Index: linux-2.6.12-mm2/arch/i386/kernel/entry.S
===================================================================
--- linux-2.6.12-mm2.orig/arch/i386/kernel/entry.S	2005-06-28 18:34:11.000000000 +0000
+++ linux-2.6.12-mm2/arch/i386/kernel/entry.S	2005-06-28 19:06:42.000000000 +0000
@@ -680,6 +680,7 @@ ENTRY(spurious_interrupt_bug)
 	pushl $do_spurious_interrupt_bug
 	jmp error_code
 
+.section .rodata,"a"
 #include "syscall_table.S"
 
 syscall_table_size=(.-sys_call_table)
Index: linux-2.6.12-mm2/arch/i386/kernel/syscall_table.S
===================================================================
--- linux-2.6.12-mm2.orig/arch/i386/kernel/syscall_table.S	2005-06-28 18:34:11.000000000 +0000
+++ linux-2.6.12-mm2/arch/i386/kernel/syscall_table.S	2005-06-28 19:06:42.000000000 +0000
@@ -1,4 +1,3 @@
-.data
 ENTRY(sys_call_table)
 	.long sys_restart_syscall	/* 0 - old "setup()" system call, used for restarting */
 	.long sys_exit
Index: linux-2.6.12-mm2/arch/x86_64/kernel/syscall.c
===================================================================
--- linux-2.6.12-mm2.orig/arch/x86_64/kernel/syscall.c	2005-06-28 18:34:11.000000000 +0000
+++ linux-2.6.12-mm2/arch/x86_64/kernel/syscall.c	2005-06-28 19:06:42.000000000 +0000
@@ -19,7 +19,7 @@ typedef void (*sys_call_ptr_t)(void); 
 
 extern void sys_ni_syscall(void);
 
-sys_call_ptr_t sys_call_table[__NR_syscall_max+1] __cacheline_aligned = { 
+const sys_call_ptr_t sys_call_table[__NR_syscall_max+1] = { 
 	/* Smells like a like a compiler bug -- it doesn't work when the & below is removed. */ 
 	[0 ... __NR_syscall_max] = &sys_ni_syscall,
 #include <asm-x86_64/unistd.h>
Index: linux-2.6.12-mm2/arch/x86_64/ia32/ia32entry.S
===================================================================
--- linux-2.6.12-mm2.orig/arch/x86_64/ia32/ia32entry.S	2005-06-28 17:46:31.000000000 +0000
+++ linux-2.6.12-mm2/arch/x86_64/ia32/ia32entry.S	2005-06-28 19:13:20.000000000 +0000
@@ -298,7 +298,7 @@ ENTRY(ia32_ptregs_common)
 	jmp  ia32_sysret	/* misbalances the return cache */
 	CFI_ENDPROC
 
-	.data
+	.section .rodata,"a"
 	.align 8
 	.globl ia32_sys_call_table
 ia32_sys_call_table:

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Christoph Hellwig]

On Tue, Jun 28, 2005 at 12:26:43PM -0700, Christoph Lameter wrote:
> Note that AFS seems to be modifying the syscall table. Is that legit?

No.

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Jeremy Maitin-Shepard]

Christoph Lameter <christoph@lameter.com> writes:

> On Tue, 28 Jun 2005, Jeremy Maitin-Shepard wrote:
>> As I mentioned previously when this patch was first posted to the list,
>> AFS writes to the syscall table.  It does this even for Linux 2.6.
>> Apparently, the rodata section is not actually mapped read-only, so this
>> patch will probably not break AFS; nonetheless, it seems it would still
>> be better to keep the syscall table in a section that is supposed to be
>> writable.

> Maybe this needs to be fixed?

It would probably be better implemented with a more generic mechanism,
but I don't believe anyone is working on that now, so it looks like AFS
will continue to use a special syscall.

-- 
Jeremy Maitin-Shepard

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Christoph Lameter]

On Tue, 28 Jun 2005, Jeremy Maitin-Shepard wrote:

> It would probably be better implemented with a more generic mechanism,
> but I don't believe anyone is working on that now, so it looks like AFS
> will continue to use a special syscall.

We could put an #ifdef CONFIG_AFS into the syscall table definition?
That makes it explicit.

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Jeremy Maitin-Shepard]

Christoph Lameter <christoph@lameter.com> writes:

> On Tue, 28 Jun 2005, Jeremy Maitin-Shepard wrote:
>> It would probably be better implemented with a more generic mechanism,
>> but I don't believe anyone is working on that now, so it looks like AFS
>> will continue to use a special syscall.

> We could put an #ifdef CONFIG_AFS into the syscall table definition?
> That makes it explicit.

I haven't looked much at the AFS support in the mainline kernel,
but I believe it is read-only support, and doesn't support
authentication.  It may well have no need for a system call.

I was actually referring to the OpenAFS implementation, which is built
separately from the kernel as a module; thus, the #ifdef CONFIG_AFS
would not work.  An additional configuration option could be added, but
I'm not sure that is a good idea.

-- 
Jeremy Maitin-Shepard

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Christoph Hellwig]

On Tue, Jun 28, 2005 at 12:31:33PM -0700, Christoph Lameter wrote:
> On Tue, 28 Jun 2005, Jeremy Maitin-Shepard wrote:
> 
> > It would probably be better implemented with a more generic mechanism,
> > but I don't believe anyone is working on that now, so it looks like AFS
> > will continue to use a special syscall.
> 
> We could put an #ifdef CONFIG_AFS into the syscall table definition?
> That makes it explicit.

No.  AFS is utterly wrong, and the sooner we make it fail to work the better.

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Jeremy Maitin-Shepard]

Christoph Hellwig <hch@infradead.org> writes:

> On Tue, Jun 28, 2005 at 12:31:33PM -0700, Christoph Lameter wrote:
>> On Tue, 28 Jun 2005, Jeremy Maitin-Shepard wrote:
>> 
>> > It would probably be better implemented with a more generic mechanism,
>> > but I don't believe anyone is working on that now, so it looks like AFS
>> > will continue to use a special syscall.
>> 
>> We could put an #ifdef CONFIG_AFS into the syscall table definition?
>> That makes it explicit.

> No.  AFS is utterly wrong, and the sooner we make it fail to work the
> better.

Heh, well that is nice, but breaking it will only mean that I and every
other AFS user will have to revert the patch that breaks it;
furthermore, many distributions that provide binary kernels will
probably also have to revert the patch because many of their users will
want to use AFS.

-- 
Jeremy Maitin-Shepard

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Arjan van de Ven]

On Tue, 2005-06-28 at 15:52 -0400, Jeremy Maitin-Shepard wrote:
> Christoph Hellwig <hch@infradead.org> writes:
> 
> > On Tue, Jun 28, 2005 at 12:31:33PM -0700, Christoph Lameter wrote:
> >> On Tue, 28 Jun 2005, Jeremy Maitin-Shepard wrote:
> >> 
> >> > It would probably be better implemented with a more generic mechanism,
> >> > but I don't believe anyone is working on that now, so it looks like AFS
> >> > will continue to use a special syscall.
> >> 
> >> We could put an #ifdef CONFIG_AFS into the syscall table definition?
> >> That makes it explicit.
> 
> > No.  AFS is utterly wrong, and the sooner we make it fail to work the
> > better.
> 
> Heh, well that is nice, but breaking it will only mean that I and every
> other AFS user will have to revert the patch that breaks it;
> furthermore, many distributions that provide binary kernels will
> probably also have to revert the patch because many of their users will
> want to use AFS.

AFS isn't even using it... after all it's not even exported.

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Jeremy Maitin-Shepard]

Arjan van de Ven <arjan@infradead.org> writes:

> AFS isn't even using it... after all it's not even exported.

Even if it is not exported, the OpenAFS kernel module can locate the
system call table using various methods.  It most certainly does write
to the system call table, setting entry 137, which is reserved for the
afs system call, to the correct function address.

-- 
Jeremy Maitin-Shepard

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Arjan van de Ven]

On Tue, 2005-06-28 at 15:27 -0400, Jeremy Maitin-Shepard wrote:
> Christoph Lameter <christoph@lameter.com> writes:
> 
> > On Tue, 28 Jun 2005, Jeremy Maitin-Shepard wrote:
> >> As I mentioned previously when this patch was first posted to the list,
> >> AFS writes to the syscall table.  It does this even for Linux 2.6.
> >> Apparently, the rodata section is not actually mapped read-only, so this
> >> patch will probably not break AFS; nonetheless, it seems it would still
> >> be better to keep the syscall table in a section that is supposed to be
> >> writable.
> 
> > Maybe this needs to be fixed?
> 
> It would probably be better implemented with a more generic mechanism,
> but I don't believe anyone is working on that now, so it looks like AFS
> will continue to use a special syscall.

the kernel afs doesnt' seem to need a special syscall....

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Jeremy Maitin-Shepard]

Arjan van de Ven <arjan@infradead.org> writes:

> the kernel afs doesnt' seem to need a special syscall....

As I mentioned in a previous message, I believe the in-kernel AFS
supports neither writing nor authentication, making it hardly a viable
replacement of the out-of-kernel OpenAFS for most AFS users.  I believe
OpenAFS requires the system call in order to support authentication.

-- 
Jeremy Maitin-Shepard

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Andi Kleen]

Christoph Lameter <christoph@lameter.com> writes:

> Place x86_64 and i386 syscall table into the read only section.
> 
> Remove the syscall tables from the data section and place them into the 
> readonly section (like IA64).

It's unfortunately useless because all the kernel is mapped in the
same 2 or 4MB page has to be writable because it overlaps with real
direct mapped memory.

On x86-64 there is a separate kernel mapping which could be made
read only. But that would be useless again because the memory
is aliased in the real direct mapping which has the same
overlapping problem.

The only way to write protect the kernel would be to pad
it to 2MB (or 4MB on i386/non PAE) which would be a big waste
of memory or use significantly more TLB entries in normal
operation.

Both is probably not worth the modest safety increase you
get from such a change.

-Andi

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Christoph Lameter]

On Tue, 28 Jun 2005, Andi Kleen wrote:

> It's unfortunately useless because all the kernel is mapped in the
> same 2 or 4MB page has to be writable because it overlaps with real
> direct mapped memory.

The question is: Are syscall tables are supposed to be 
writable? If no then this patch should go in. If yes then forget about it.
On IA64 they are readonly and so I thought they should also be readonly
on i386 and x86_64.

The ability to protect a readonly section may be another issue.

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Arnd Bergmann]

On Dinsdag 28 Juni 2005 21:41, Christoph Lameter wrote:

> The ability to protect a readonly section may be another issue.

Exactly. Mapping the readonly section readonly adds a nice way to
check that constant data is handled correctly by all of the code.
Otherwise, there might be some surprises if gcc performs
constant folding and we incorrectly rely on one copy to be writable.

A read-only text segment also raises the bar for authors of rootkits
or other evil hacks that patch the running kernel code.

Right now, s390 (and I believe arm, maybe others as well) is already
able to map in the readonly sections of the kernel from ROM, in order
to have more available RAM for other purposes.

	Arnd <><

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Andi Kleen]

On Tue, Jun 28, 2005 at 12:41:59PM -0700, Christoph Lameter wrote:
> On Tue, 28 Jun 2005, Andi Kleen wrote:
> 
> > It's unfortunately useless because all the kernel is mapped in the
> > same 2 or 4MB page has to be writable because it overlaps with real
> > direct mapped memory.
> 
> The question is: Are syscall tables are supposed to be 
> writable? If no then this patch should go in. If yes then forget about it.

I think it would make sense in theory to write protect them
together with the kernel code and the modules
(just to make root kit writing slightly harder)

It is just that it is not practical on i386/x86-64 right now
without undue performance impact for the main kernel. TLB pressure is 
unfortunately quite performance critical and we cannot goof off on this.

Write protecting the modules would be possible right now because 
they're vmalloced, but might be a problem later if we move them to the 
direct mapping again (that was a beneficial 2.4 optimization), so I am not sure
it would be a good idea.

BTW the kernel actually needs to write to code once
to apply alternative(), but it would't be a problem to use
a temporary mapping for this.

> On IA64 they are readonly and so I thought they should also be readonly
> on i386 and x86_64.
> 
> The ability to protect a readonly section may be another issue.

Well, it's the overriding issue here. Just pretending it's readonly
when it isn't doesn't seem useful.

-Andi

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Christoph Lameter]

On Wed, 29 Jun 2005, Andi Kleen wrote:

> On Tue, Jun 28, 2005 at 12:41:59PM -0700, Christoph Lameter wrote:
> > On Tue, 28 Jun 2005, Andi Kleen wrote:
> > 
> > > It's unfortunately useless because all the kernel is mapped in the
> > > same 2 or 4MB page has to be writable because it overlaps with real
> > > direct mapped memory.
> > 
> > The question is: Are syscall tables are supposed to be 
> > writable? If no then this patch should go in. If yes then forget about it.
> 
> I think it would make sense in theory to write protect them
> together with the kernel code and the modules
> (just to make root kit writing slightly harder)

Seems that you are evading the question that I asked. Are syscall tables 
supposed to be writable?

> BTW the kernel actually needs to write to code once
> to apply alternative(), but it would't be a problem to use
> a temporary mapping for this.

What does this have to do with the syscall table???

> > The ability to protect a readonly section may be another issue.
> 
> Well, it's the overriding issue here. Just pretending it's readonly
> when it isn't doesn't seem useful.

This is all are off-topic talking about a different issue. And we are 
already "pretending" that lots of other stuff in the readonly section is 
readonly.

The issue is correct placement of variables. Read only variables are 
placed in a different section and the syscall tables are read only and 
need to be place in the correct section.

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Andi Kleen]

On Fri, Jul 01, 2005 at 01:10:12PM -0700, Christoph Lameter wrote:
> > I think it would make sense in theory to write protect them
> > together with the kernel code and the modules
> > (just to make root kit writing slightly harder)
> 
> Seems that you are evading the question that I asked. Are syscall tables 
> supposed to be writable?

I did answer it. But again: yes I think it makes sense in theory
to make them read only.

Just we cannot do it right now on i386/x86-64 due to the reasons I lined out
in my previous mail.


> 
> > BTW the kernel actually needs to write to code once
> > to apply alternative(), but it would't be a problem to use
> > a temporary mapping for this.
> 
> What does this have to do with the syscall table???


It is directly related to writable .text.

> 
> > > The ability to protect a readonly section may be another issue.
> > 
> > Well, it's the overriding issue here. Just pretending it's readonly
> > when it isn't doesn't seem useful.
> 
> This is all are off-topic talking about a different issue. And we are 
> already "pretending" that lots of other stuff in the readonly section is 
> readonly.

Putting it into a "ro section" when it isn't actually read only is completely 
useless and does not do anything useful. So unless you figure out
a way to make a true ro section without performance penalty I wouldn't bother. 

If you really want it for cosmetic reasons you can still do it,
but it is about at the same usefullness level as shuffling white space in 
the source around.

-Andi

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Richard B. Johnson]

On Fri, 1 Jul 2005, Andi Kleen wrote:

> On Fri, Jul 01, 2005 at 01:10:12PM -0700, Christoph Lameter wrote:
>>> I think it would make sense in theory to write protect them
>>> together with the kernel code and the modules
>>> (just to make root kit writing slightly harder)
>>
>> Seems that you are evading the question that I asked. Are syscall tables
>> supposed to be writable?
>
> I did answer it. But again: yes I think it makes sense in theory
> to make them read only.
>
> Just we cannot do it right now on i386/x86-64 due to the reasons I lined out
> in my previous mail.
>
>
>>
>>> BTW the kernel actually needs to write to code once
>>> to apply alternative(), but it would't be a problem to use
>>> a temporary mapping for this.
>>
>> What does this have to do with the syscall table???
>
>
> It is directly related to writable .text.
>
>>
>>>> The ability to protect a readonly section may be another issue.
>>>
>>> Well, it's the overriding issue here. Just pretending it's readonly
>>> when it isn't doesn't seem useful.
>>
>> This is all are off-topic talking about a different issue. And we are
>> already "pretending" that lots of other stuff in the readonly section is
>> readonly.
>
> Putting it into a "ro section" when it isn't actually read only is completely
> useless and does not do anything useful. So unless you figure out
> a way to make a true ro section without performance penalty I wouldn't bother.
>
> If you really want it for cosmetic reasons you can still do it,
> but it is about at the same usefullness level as shuffling white space in
> the source around.
>
> -Andi

The fact that the syscall table is R/W can be used to an advantage
for security. Yes!

After all modules are loaded, you (startup) loads a module that
makes the module-loader stuff return -ENOSYS. Then, nobody can
load any new modules. The running kernel is (more) secure.

You just need to make sure that all modules that you will need
are loaded before you do this. Also, you don't need to disable
auto module unloading because, nothing can be unloaded as
well.


Script started on Fri 01 Jul 2005 04:43:17 PM EDT
[root@chaos driver]# insmod LastDev.ko
[root@chaos driver]# insmod LastDev.ko
insmod: error inserting 'LastDev.ko': -1 Function not implemented
[root@chaos driver]# insmod LastDev.ko
insmod: error inserting 'LastDev.ko': -1 Function not implemented
[root@chaos driver]# insmod LastDev.ko
insmod: error inserting 'LastDev.ko': -1 Function not implemented
[root@chaos driver]# exit
Script done on Fri 01 Jul 2005 04:44:29 PM EDT


Cheers,
Dick Johnson
Penguin : Linux version 2.6.12 on an i686 machine (5537.79 BogoMips).
  Notice : All mail here is now cached for review by Dictator Bush.
                  98.36% of all statistics are fiction.

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Alan Cox]

On Gwe, 2005-07-01 at 21:47, Richard B. Johnson wrote:
> After all modules are loaded, you (startup) loads a module that
> makes the module-loader stuff return -ENOSYS. Then, nobody can
> load any new modules. The running kernel is (more) secure.

Just use an SELinux policy like everyone else 8). You need to block more
otherwise I can load a module by hand through /dev/mem etc

Alan
--
        " If knowledge does not have owners, then intellectual property
                is a trap set by neo-liberalism." -- Hugo Chavez

Re: [PATCH] Read only syscall tables for x86_64 and i386

[Richard B. Johnson]

On Fri, 1 Jul 2005, Christoph Lameter wrote:

> On Wed, 29 Jun 2005, Andi Kleen wrote:
>
>> On Tue, Jun 28, 2005 at 12:41:59PM -0700, Christoph Lameter wrote:
>>> On Tue, 28 Jun 2005, Andi Kleen wrote:
>>>
>>>> It's unfortunately useless because all the kernel is mapped in the
>>>> same 2 or 4MB page has to be writable because it overlaps with real
>>>> direct mapped memory.
>>>
>>> The question is: Are syscall tables are supposed to be
>>> writable? If no then this patch should go in. If yes then forget about it.
>>
>> I think it would make sense in theory to write protect them
>> together with the kernel code and the modules
>> (just to make root kit writing slightly harder)
>
> Seems that you are evading the question that I asked. Are syscall tables
> supposed to be writable?
>
>> BTW the kernel actually needs to write to code once
>> to apply alternative(), but it would't be a problem to use
>> a temporary mapping for this.
>
> What does this have to do with the syscall table???
>
>>> The ability to protect a readonly section may be another issue.
>>
>> Well, it's the overriding issue here. Just pretending it's readonly
>> when it isn't doesn't seem useful.
>
> This is all are off-topic talking about a different issue. And we are
> already "pretending" that lots of other stuff in the readonly section is
> readonly.
>
> The issue is correct placement of variables. Read only variables are
> placed in a different section and the syscall tables are read only and
> need to be place in the correct section.

I modified my sycall table to put it in ".section .rodata". It appears
as though read-only is not enforced in the kernel. I don't know
why because, at least with ix86, both data and code can be made read-
only.

You just write to it with a segment descriptor that allows R/W, then
use another for R/O.  It is true that kernel code can create any
segment descriptor it wants, dynamically, thus a properly-written
program can ultimately write to what was once R/O data. However,
I think it should default to R/O which should cut down on the
number of cheap hacks that can damage it.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.12 on an i686 machine (5537.79 BogoMips).
  Notice : All mail here is now cached for review by Dictator Bush.
                  98.36% of all statistics are fiction.