linux-hardening - Re: [PATCH 2/4] elf: Move note processing after l

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Date:   Tue, 3 Nov 2020 15:04:12 +0000
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     "H.J. Lu" <hjl.tools@...il.com>
Cc:     Florian Weimer <fweimer@...hat.com>,
        GNU C Library <libc-alpha@...rceware.org>,
        Jeremy Linton <jeremy.linton@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Brown <broonie@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Salvatore Mesoraca <s.mesoraca16@...il.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Topi Miettinen <toiwoton@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 2/4] elf: Move note processing after l_phdr is updated
 [BZ #26831]

The 11/03/2020 04:36, H.J. Lu wrote:
> On Tue, Nov 3, 2020 at 2:38 AM Florian Weimer <fweimer@...hat.com> wrote:
> > * Szabolcs Nagy:
> >
> > > Program headers are processed in two pass: after the first pass
> > > load segments are mmapped so in the second pass target specific
> > > note processing logic can access the notes.
> > >
> > > The second pass is moved later so various link_map fields are
> > > set up that may be useful for note processing such as l_phdr.
> > > ---
> > >  elf/dl-load.c | 30 +++++++++++++++---------------
> > >  1 file changed, 15 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/elf/dl-load.c b/elf/dl-load.c
> > > index ceaab7f18e..673cf960a0 100644
> > > --- a/elf/dl-load.c
> > > +++ b/elf/dl-load.c
> > > @@ -1259,21 +1259,6 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
> > >                                 maplength, has_holes, loader);
> > >      if (__glibc_unlikely (errstring != NULL))
> > >        goto call_lose;
> > > -
> > > -    /* Process program headers again after load segments are mapped in
> > > -       case processing requires accessing those segments.  Scan program
> > > -       headers backward so that PT_NOTE can be skipped if PT_GNU_PROPERTY
> > > -       exits.  */
> > > -    for (ph = &phdr[l->l_phnum]; ph != phdr; --ph)
> > > -      switch (ph[-1].p_type)
> > > -     {
> > > -     case PT_NOTE:
> > > -       _dl_process_pt_note (l, fd, &ph[-1]);
> > > -       break;
> > > -     case PT_GNU_PROPERTY:
> > > -       _dl_process_pt_gnu_property (l, fd, &ph[-1]);
> > > -       break;
> > > -     }
> > >    }
> > >
> > >    if (l->l_ld == 0)
> > > @@ -1481,6 +1466,21 @@ cannot enable executable stack as shared object requires");
> > >      /* Assign the next available module ID.  */
> > >      l->l_tls_modid = _dl_next_tls_modid ();
> > >
> > > +  /* Process program headers again after load segments are mapped in
> > > +     case processing requires accessing those segments.  Scan program
> > > +     headers backward so that PT_NOTE can be skipped if PT_GNU_PROPERTY
> > > +     exits.  */
> > > +  for (ph = &l->l_phdr[l->l_phnum]; ph != l->l_phdr; --ph)
> > > +    switch (ph[-1].p_type)
> > > +      {
> > > +      case PT_NOTE:
> > > +     _dl_process_pt_note (l, fd, &ph[-1]);
> > > +     break;
> > > +      case PT_GNU_PROPERTY:
> > > +     _dl_process_pt_gnu_property (l, fd, &ph[-1]);
> > > +     break;
> > > +      }
> > > +
> > >  #ifdef DL_AFTER_LOAD
> > >    DL_AFTER_LOAD (l);
> > >  #endif
> >
> > Is this still compatible with the CET requirements?
> >
> > I hope it is because the CET magic happens in _dl_open_check, so after
> > the the code in elf/dl-load.c has run.

i believe the note processing and later cet magic
are not affected by this code move.

but i did not test this with cet.

> 
> _dl_process_pt_note and _dl_process_pt_gnu_property may call
> _dl_signal_error.  Are we prepared to clean more things up when it
> happens?  I am investigating:

yeah, this is difficult to reason about.

it seems to me that after _dl_map_object returns there
may be _dl_map_object_deps which can fail in a way that
all of dlopen has to be rolled back, so if i move things
around in _dl_map_object that should not introduce new
issues.

but it is not clear to me how robust the dlopen code is
against arbitrary failure in dl_open_worker.

> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=26825
> 
> I don't think cleanup of _dl_process_pt_gnu_property failure is done
> properly.
> 
> -- 
> H.J.

--