Why was Intel a no-show on No Execute?

Grumble wrote:

Yousuf Khan wrote:

One thing he got partially wrong was his statement about Intel
having no execute protection in the 16-bit segments. The feature
was still there in the 32-bit segments, Intel never got rid of
them. It was stupid OS designers who decided to ignore the
feature that caused this problem.

Are you calling them "stupid" because they opted for paging
instead of segmentation, in an effort to write a portable OS?

Do you think there should be an x86-specific Linux branch,
using segmentation instead of paging?


I don't think it would be so hard to put all the data in a
data segment, and the code in a code segment, without overlapping
them. It requires the CS: prefix on any loads from the code
segment. Self modifying code is out of style these days,
so that shouldn't be much of a problem.

Now, for things like JIT where code is constantly being
written while running some arrangement would need to be made.

-- glen

In comp.arch Grumble wrote:
Yousuf Khan wrote:

One thing he got partially wrong was his statement about Intel
having no execute protection in the 16-bit segments. The feature
was still there in the 32-bit segments, Intel never got rid of
them. It was stupid OS designers who decided to ignore the
feature that caused this problem.

Are you calling them "stupid" because they opted for paging
instead of segmentation, in an effort to write a portable OS?

Do you think there should be an x86-specific Linux branch,
using segmentation instead of paging?


There was one for quite a while for pre-386 modes/machines.

--
Sander

+++ Out of cheese error +++

In comp.sys.ibm.pc.hardware.chips glen herrmannsfeldt wrote:
I don't think it would be so hard to put all the data in a
data segment, and the code in a code segment, without overlapping
them. It requires the CS: prefix on any loads from the code
segment. Self modifying code is out of style these days,
so that shouldn't be much of a problem.

That _still_ won't help (never mind interpreted or JIT).

If an attacker can redirect execution by modifying the
return address on the stack, s/he doesn't need their own
executable code. Just point to data like "/bin/sh" and
return to an `exec` syscall.

-- Robert

In comp.sys.ibm.pc.hardware.chips Robert Redelmeier wrote:
In comp.sys.ibm.pc.hardware.chips glen herrmannsfeldt wrote:
I don't think it would be so hard to put all the data in a
data segment, and the code in a code segment, without overlapping
them. It requires the CS: prefix on any loads from the code
segment. Self modifying code is out of style these days,
so that shouldn't be much of a problem.

That _still_ won't help (never mind interpreted or JIT).

If an attacker can redirect execution by modifying the
return address on the stack, s/he doesn't need their own
executable code. Just point to data like "/bin/sh" and
return to an `exec` syscall.

Ah, but you make me think -- all current CPUs have an internal
hardware call/return stack to speed up branch [mis]prediction.

It would be relatively simple to check this hw stack against
the memory stack and generate a fault if return addresses
don't match.

This could be enabled by a bit in the MSR if the OS has support
to handle/log "return addr faults". Most pgms should never
generate a return fault, but a mechanism could be made to
except those few that do.

A slightly bigger problem is the hw stacks are of limited
depth (6?) and it might be possible to flood them out.
But variable stack entry pointers would become more effective.

-- Robert

It would be relatively simple to check this hw stack against
the memory stack and generate a fault if return addresses
don't match.

Lookup "call-with-current-continuation" to see why this is not a good idea.
Or maybe just think of how to implement exception handling.


Stefan

Grumble wrote:
Yousuf Khan wrote:

One thing he got partially wrong was his statement about Intel
having no execute protection in the 16-bit segments. The feature
was still there in the 32-bit segments, Intel never got rid of
them. It was stupid OS designers who decided to ignore the
feature that caused this problem.

Are you calling them "stupid" because they opted for paging
instead of segmentation, in an effort to write a portable OS?

No, for not opting to use both. There was no mutual exclusivity between
paging and segmentation. Both could be used and complement each other.

I think the original OS designers in their haste to port Unix to the new
32-bit Intel chip did a simple cross-compile, and then didn't bother to make
use of any of the Intel-specific features of their architecture. They just
left it at "good enough". Of course, using Intel features would've made them
non-portable, but a lot of stuff gets non-portable at the lowest levels of
the kernel anyways.

Do you think there should be an x86-specific Linux branch,
using segmentation instead of paging?

There already was. The original pre-1.0 Linux kernels were using segments
*and* paging. I think with addition of new people into the development team,
Linux's original purpose got changed from being the ultimate Intel OS (Unix
or otherwise), to being a free version of portable Unix.

Yousuf Khan

Robert Redelmeier wrote:
That _still_ won't help (never mind interpreted or JIT).

If an attacker can redirect execution by modifying the
return address on the stack, s/he doesn't need their own
executable code. Just point to data like "/bin/sh" and
return to an `exec` syscall.

How's an attacker to do that, when the the code, the stack and the heap
don't even share the same memory addresses?

Yousuf Khan

In comp.sys.ibm.pc.hardware.chips Yousuf Khan wrote:
How's an attacker to do that, when the the code, the stack and the heap
don't even share the same memory addresses?

Easy. Overwrite the stack with crafted input to an unrestricted
input call (getch() is a frequent culprit). This is the basic
buffer overflow.

In the location for the return address (where EBP is usually
pointing), put in a return address that points to a suitably
dangerous part of the existing code. Like an `exec` syscall.
Above this return address, put in data to make that syscall
nefarious.

-- Robert

In comp.sys.ibm.pc.hardware.chips Stefan Monnier wrote:
It would be relatively simple to check this hw stack against
the memory stack and generate a fault if return addresses
don't match.

Lookup "call-with-current-continuation" to see why this is not a good idea.
Or maybe just think of how to implement exception handling.

Exception handling is easy -- mismatch produces a MC interrupt.
The kernelspace ISR checks the MSRs which tell it that a return
addr mismatch occurred. Kenel decides what to do -- abort proc,
log, or proceed.

Sure it'll be slow, but how often are calls not paired with
returns? call jtable[eax*4] is the standard syntax for a
jump table, not `push eax/ret`

-- Robert

Sander Vesik wrote:
Do you think there should be an x86-specific Linux branch,
using segmentation instead of paging?


There was one for quite a while for pre-386 modes/machines.

That was Minix. Linux has always been for 386 and later machines only.

Yousuf Khan