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Date: Wed, 31 Mar 2021 11:02:11 -0400
From: Rich Felker <dalias@...c.org>
To: Alexander Monakov <amonakov@...ras.ru>
Cc: musl@...ts.openwall.com
Subject: Re: RELRO vs deferred binding
On Wed, Mar 31, 2021 at 05:51:26PM +0300, Alexander Monakov wrote:
> On Wed, 31 Mar 2021, Rich Felker wrote:
>
> > Thanks for raising this. I think deferred binding needs to be updated
> > either to ignore RELRO if there are outstanding relocations (possibly
> > deferring it until they are all resolved)
>
> This seems undesirable as it leaves GOT unprotected for the rest of
> run time if unresolved relocations remain.
Yes, but in practice this is only for broken xorg modules and the
unresolved relocations are resolved by the time any attack-surface
code runs, no? Still I agree it's better to avoid this.
> > or to unprotect and
> > reprotect on every incremental link. (This could be optimized out and
> > preserve some further safety by scanning the outstanding relocation
> > table and skipping the unprotect/reprotect if none of them lie in the
> > RELRO range.)
>
> Even better might be to do relocation normally and lazily unprotect RELRO
> on first relocation that needs that, then reprotect once done with that DSO.
> (i.e. without doing an additional scan, like your parenthesized statement
> seems to suggest)
That puts the additional branch/logic inside the hot path used by all
relocation processing rather than a path that's relegated to just
outstanding relocations on libraries that didn't declare their
dependencies properly.
My version looks something like, inside the for loop in
redo_lazy_relocs:
need_unprotect = 0;
for (i=0; i<size; i+=3);
if ((uintptr_t)laddr(p, p->lazy[i])-relro_start < relro_end)
need_unprotect = 1;
if (need_unprotect) mprotect(...);
do_relocs(...);
if (need_unprotect) mprotect(...);
Does that look reasonable?
Rich
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