Security-What can be done in kernel to disable forever executable memory modificaton

Security-What can be done in kernel to disable forever executable memory modificaton

[Lev Olshvang]

Hi All,



The fact that the text segment could be modified is bad news from the security standpoint.
For example, in order to set a breakpoint GDB should map a text segment with MAP_PRIVATE flag which allows kernel to ignore the dirty bit that MMU  sets on this page.

Somewhere in the middle of this mapping,  perhaps in mprotect,  permission bits of  page's PTE entry are modified as well from their original RO+X   to RWX
I am not sure whether it is actually happening, perhaps instead new pages are allocated, sort of COW (copy on write).

And here I am getting to the point :

Is there any way to disable the change of permission bits of PTE? Is it possible in the hardware (ARM) or should kernel be patched?

Regards to  All,

Happy new year.

Lev. 


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Re: Security-What can be done in kernel to disable forever executable memory modificaton

[valdis.kletnieks]

On Sat, 12 Jan 2019 16:19:00 +0300, Lev Olshvang said:

> The fact that the text segment could be modified is bad news from the
> security standpoint.

We've known that for at least a decade now. Maybe longer. And we
already had this discussion once, about a week ago.

> I am not sure whether it is actually happening, perhaps instead new pages are
> allocated, sort of COW (copy on write).

In which case, you should probably stop and verify if it's happening.

> And here I am getting to the point :

> Is there any way to disable the change of permission bits of PTE? Is it
> possible in the hardware (ARM) or should kernel be patched?

Are you sure you want to disable *all* changes of a PTE?
Hint: Figure out how shared libraries are loaded before you go any further.

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