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Date: Fri, 28 Aug 2020 12:21:13 +0200 (CEST)
From: Miroslav Benes <>
To: Kristen Carlson Accardi <>
cc: Josh Poimboeuf <>, Kees Cook <>,,,,,,,,,, Hongjiu Lu <>,
Subject: Re: [PATCH v4 00/10] Function Granular KASLR

Leaving Josh's proposals here for reference...

> > I'm not sure how LTO does it, but a few more (half-brained) ideas
> > that
> > could work:
> > 
> > 1) Add a field in kallsyms to keep track of a symbol's original
> > offset
> >    before randomization/re-sorting.  Livepatch could use that field
> > to
> >    determine the original sympos.
> > 
> > 2) In fgkaslr code, go through all the sections and mark the ones
> > which
> >    have duplicates (i.e. same name).  Then when shuffling the
> > sections,
> >    skip a shuffle if it involves a duplicate section.  That way all
> > the
> >    duplicates would retain their original sympos.
> > 
> > 3) Livepatch could uniquely identify symbols by some feature other
> > than
> >    sympos.  For example:
> > 
> >    Symbol/function size - obviously this would only work if
> > duplicately
> >    named symbols have different sizes.
> > 
> >    Checksum - as part of a separate feature we're also looking at
> > giving
> >    each function its own checksum, calculated based on its
> > instruction
> >    opcodes.  Though calculating checksums at runtime could be
> >    complicated by IP-relative addressing.
> > 
> > I'm thinking #1 or #2 wouldn't be too bad.  #3 might be harder.
> > 
> Hi there! I was trying to find a super easy way to address this, so I
> thought the best thing would be if there were a compiler or linker
> switch to just eliminate any duplicate symbols at compile time for
> vmlinux. I filed this question on the binutils bugzilla looking to see
> if there were existing flags that might do this, but H.J. Lu went ahead
> and created a new one "-z unique", that seems to do what we would need
> it to do. 
> When I use this option, it renames any duplicate symbols with an
> extension - for example duplicatefunc.1 or duplicatefunc.2. You could
> either match on the full unique name of the specific binary you are
> trying to patch, or you match the base name and use the extension to
> determine original position. Do you think this solution would work?

Yes, I think so (thanks, Joe, for testing!).

It looks cleaner to me than the options above, but it may just be a matter 
of taste. Anyway, I'd go with full name matching, because -z unique-symbol 
would allow us to remove sympos altogether, which is appealing.

> If
> so, I can modify livepatch to refuse to patch on duplicated symbols if
> CONFIG_FG_KASLR and when this option is merged into the tool chain I
> can add it to KBUILD_LDFLAGS when CONFIG_FG_KASLR and livepatching
> should work in all cases. 


Josh, Petr, would this work for you too?


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