Subject: [PATCH v2 0/6] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831]

From: Szabolcs Nagy <[email-masked]>
Date: Fri, 27 Nov 2020 13:19:16 +0000

This is v2 of
https://sourceware.org/pipermail/libc-alpha/2020-November/119305.html

To enable BTI support, re-mmap executable segments instead of
mprotecting them in case mprotect is seccomp filtered.

I would like linux to change to map the main exe with PROT_BTI when
that is marked as BTI compatible. From the linux side i heard the
following concerns about this:
- it's an ABI change so requires some ABI bump. (this is fine with
  me, i think glibc does not care about backward compat as nothing
  can reasonably rely on the current behaviour, but if we have a
  new bit in auxv or similar then we can save one mprotect call.)
- in case we discover compatibility issues with user binaries it's
  better if userspace can easily disable BTI (e.g. removing the
  mprotect based on some env var, but if kernel adds PROT_BTI and
  mprotect is filtered then we have no reliable way to remove that
  from executables. this problem already exists for static linked
  exes, although admittedly those are less of a compat concern.)
- ideally PROT_BTI would be added via a new syscall that does not
  interfere with PROT_EXEC filtering. (this does not conflict with
  the current patches: even with a new syscall we need a fallback.)
- solve it in systemd (e.g. turn off the filter, use better filter):
  i would prefer not to have aarch64 (or BTI) specific policy in
  user code. and there was no satisfying way to do this portably.

Other concerns about the approach:
- mmap is more expensive than mprotect: in my measurements using
  mmap instead of mprotect is 3-8x slower (and after mmap pages
  have to be faulted in again), but e.g. the exec time of a program
  with 4 deps only increases by < 8% due to the 4 new mmaps. (the
  kernel side resource usage may increase too, i didnt look at that.)
- _dl_signal_error is not valid from the _dl_process_gnu_property
  hook. The v2 set addresses this problem: i could either propagate
  the errors up until they can be handled or solve it in the aarch64
  backend by first recording failures and then dealing with them in
  _dl_open_check. I choose the latter, but did some refactorings in
  _dl_map_object_from_fd that makes the former possible too.

v2:
- [1/6]: New patch that fixes a missed BTI bug found during v2.
- [2-3/6]: New, _dl_map_object_from_fd failure handling improvements,
  these are independent of the rest of the series.
- [4/6]: Move the note handling to a different place (after l_phdr
  setup, but before fd is closed).
- [5/6]: Rebased.
- [6/6]: First record errors and only report them later. (this fixes
  various failure handling issues.)

Szabolcs Nagy (6):
  aarch64: Fix missing BTI protection from dependencies [BZ #26926]
  elf: lose is closely tied to _dl_map_object_from_fd
  elf: Fix failure handling in _dl_map_object_from_fd
  elf: Move note processing after l_phdr is updated
  elf: Pass the fd to note processing
  aarch64: Use mmap to add PROT_BTI instead of mprotect [BZ #26831]

 elf/dl-load.c              | 110 ++++++++++++++++++++-----------------
 elf/rtld.c                 |   4 +-
 sysdeps/aarch64/dl-bti.c   |  74 ++++++++++++++++++-------
 sysdeps/aarch64/dl-prop.h  |  14 +++--
 sysdeps/aarch64/linkmap.h  |   2 +-
 sysdeps/generic/dl-prop.h  |   6 +-
 sysdeps/generic/ldsodefs.h |   5 +-
 sysdeps/x86/dl-prop.h      |   6 +-
 8 files changed, 135 insertions(+), 86 deletions(-)

-- 
2.17.1

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