bug#47614: [security] Chunked store references in .zo files in Racket 8

[Léo Le Bouter]

On Tue, 2021-04-06 at 17:27 -0400, Mark H Weaver wrote:
> Hi Léo,
> 
> Léo Le Bouter <lle-bout@zaclys.net> writes:
> 
> > I think that probably replacing arbitrary paths in built binaries
> > is a
> > risky and maybe unreliable engineering choice and that mechanisms
> > inside kernels should be preferred to give processes a different
> > view
> > of the file system (retaining the path but changing the contents of
> > the
> > folder).
> 
> I've had thoughts along these lines myself, but I don't think it can
> work properly.  The fundamental problem is that in general, each
> process
> includes shared objects from many different Guix packages.  There
> would
> need to be a mechanism to determine, when looking up a file, which
> Guix
> package that file lookup was originating from (or whether it was
> coming
> from a file name provided by the user), in order to determine which
> "view of the file system" to use for purposes of that
> lookup.  There's
> no way to determine this reliably.

Is it really that big a deal if it's impossible to access the ungrafted
/gnu/store item? If really required we could document a way to disable
it temporarily maybe? Do we need a specific view for each and every
package? I am thinking that overriding the view to the store item
that's a result of a package with a replacement field globally would be
sufficient.

> > OTOH, what would be wrong with replacing hashes directly without
> > expecting them to be next to anything else?
> 
> Personally, I would find that limitation acceptable, and that's
> fairly
> close to what our grafter originally did (although my fast grafting
> code
> always assumed that a "-" would follow the hash).  However, we've
> since
> become accustomed to being able to have replacements with different
> version numbers.  That's a nice feature.
> 

Version numbers, agree, I didnt realize that replacing the program name
and version was also required there. However I am thinking we could
fake (or alias, with a symlink) the version in the store item name on
purpose so that it remains the same while pointing to something with a
newer version, it would actually be better that way because we wouldnt
have to think about retaining identical version string length during
grafts.

> Anyway, I doubt that imposing such a limitation would adequately
> solve
> the problem here of chunked references in Racket 8, because I suspect
> that Racket 8 could split store references at arbitrary points in the
> string.  I doubt that we can safely assume that the hash component of
> store references will be stored contiguously in *.zo files.

Indeed, is the format for the string references in .zo files documented
anywhere? Is there hope you think we can recognize and automatically
rewrite these strings?

Thanks,
Léo

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