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Re: Alternative libio vtable hardening approach

On 05/31/2016 09:23 PM, Kees Cook wrote:
Well, this is certainly better than not having it, and the on/off
switch isn't in the FILE structure, so I would think this should be of
a similar protection level. Though, it'd be nice if a process could
opt-out for its entire lifetime. Right now, any call to _IO_file_init
disables the protection.

I don't quite see how to do this. The machine code sequence to set the flag has to be in the process image to enable backwards compatibility when needed. It doesn't really matter if this code is in an IFUNC handler or in a library subroutine. And even if we have some precondition check (say, the IFUNC handler checks that we are in and relocation processing is running), execution could start after it.

We could have another flag, this time in read-only memory, which is some sort of tunable which can be tweaked by the system administrator. (Maybe we could even query the SELinux policy engine to get the flag.) The fallback path could check this flag as well.

In general, I want to avoid policy-based solutions because I'm told that in too many relevant scenarios, only the âanything goesâ policy counts.

Do you think it's import to address this from the start, or can this wait until we have a better story for tunables?

(Also, is the call to _IO_new_file_init in _IO_new_popen wrong? Should
that be the internal one instead?)

Yes, these parts aren't completely done yet.

I also need to do something about static linking. The increase in executable size appears to be rather moderate, but there are linknamespace issues. I'll try to use weak function symbols for some of the methods (particularly the obstack ones), maybe this will help on both fronts.


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