Re: Changing stack protector initialization

[Carlos O'Donell <carlos at redhat dot com>]

On 10/27/2016 10:00 AM, Florian Weimer wrote:
> I need a few more pseudo-random bits (32 instead of 16 on 64-bit
> architectures).  I talked to some cryptography people and they told
> me to expand the 16-byte secret by hashing it with SHA-256.
> 
> This key expansion has to happen both in ld.so (for the stack
> protector and pointer guard) and libc.so (for the new stuff).  My
> first attempt failed because doing the initialization in ld.so
> triggers duplication of the new guard variables from libc.so in
> ld.so, and the libc.so variables are never initialized.  (This is
> very confusing to GDB, which does not tell you that you have two
> variables with the same name at different addresses.)

Could you explain this in a bit more detail?

> I think the best approach is to duplicate the initialization code and
> run it twice, once based on the _dl_random value, and once using
> getauxval (AT_RANDOM).  The other alternative would be to put the
> values computed by rtld into rtld_global or some similar place and
> use those values to initialize the libc.so variables, but this wastes
> 16 bytes in the data segment per process.  (I need to make a copy so
> that the variable access does not go through the GOT.)

I don't like the idea of multiple initialization, it leads to difficult
to answer ordering questions. It is much easier conceptually to initialize
once and then copy the result.
 
> Is there another option to implement this?

Yes, initialize once, and copy the result as required?

-- 
Cheers,
Carlos.