touch-packages team mailing list archive

[Launchpad Bug Tracker <1412553@xxxxxxxxxxxxxxxxxx>]

You have been subscribed to a public bug:

RELRO (RELocation Read-Only) is security feature provided by the linker
and implemented in conjunction with glibc to relocations sections that
are used to resolve dynamically loaded functions read-only.

The GLIBC side looks at the segments placement set by the linker and if
they met some criteria regarding alignment, the are mprotect to be read-
only.  More specifically, on GLIBC code:

elf/dl-reloc.c:

324 void internal_function
325 _dl_protect_relro (struct link_map *l)
326 {
327   ElfW(Addr) start = ((l->l_addr + l->l_relro_addr)
328                       & ~(GLRO(dl_pagesize) - 1));
329   ElfW(Addr) end = ((l->l_addr + l->l_relro_addr + l->l_relro_size)
330                     & ~(GLRO(dl_pagesize) - 1));
331 
332   if (start != end
333       && __mprotect ((void *) start, end - start, PROT_READ) < 0)
334     {
335       static const char errstring[] = N_("\
336 cannot apply additional memory protection after relocation");
337       _dl_signal_error (errno, l->l_name, NULL, errstring);
338     }
339 }

The problem is, if linker does not set the alignment correctly, 'start'
and 'end' will be equal and thus not protected. And this is happening on
Ubuntu 14.04 due the fact its uses default binutils elf{32,64}-ppc.c
ELF_COMMONPAGESIZE to align it to 4k instead of 64k.

There is a recent patch on binutils-dev maillist [1] to change the
default for 64k and Fedora rawhide already sets it [2] in its
binutils.spec spec:

# On ppc64 and aarch64, we might use 64KiB pages
sed -i -e '/#define.*ELF_COMMONPAGESIZE/s/0x1000$/0x10000/' bfd/elf*ppc.c
sed -i -e '/#define.*ELF_COMMONPAGESIZE/s/0x1000$/0x10000/' bfd/elf*aarch64.c

Ubuntu for powerpc64le should do the same.

[1] https://sourceware.org/ml/binutils/2014-12/msg00165.html
[2] http://dl.fedoraproject.org/pub/fedora/linux/development/rawhide/source/SRPMS/b/binutils-2.24-29.fc22.src.rpm

I tested and building a binutils with the ELF_COMMONPAGESIZE set to 64k
instead of default 4k and rebuilding GLIBC I see its relocation sections
being correctly mprotected.

** Affects: binutils (Ubuntu)
     Importance: Undecided
     Assignee: Taco Screen team (taco-screen-team)
         Status: Confirmed


** Tags: architecture-ppc64le bugnameltc-119848 severity-high targetmilestone-inin14042
-- 
RELRO not working on Ubuntu 14.04
https://bugs.launchpad.net/bugs/1412553
You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to binutils in Ubuntu.