Re: list of security features in musl

[Rich Felker <dalias@libc.org>]

On Thu, Feb 11, 2016 at 08:56:13AM +0100, Natanael Copa wrote:
> Hi,
> 
> Once in a while I get question about what security features there are
> in musl. Are there such list some where?

Some are briefly mentioned in the libc comparison:

http://www.etalabs.net/compare_libcs.html

but it's not very complete in this area. The things I would really
call security _features_ in musl are:

- Stack protector, with failure handling that rapidly terminates the
  process rather than continuing along error-reporting code paths
  which can themselves provide an attack surface.

- Double-free protection (to the extent possible), with the same rapid
  termination.

- Moderate level heap overflow protection - checking for clobbered
  heap chunk footers on realloc and free, also with rapid termination.

- Ability to build libc itself with stack protector enabled, to catch
  libc-internal stack smashing.

- Password hashing with bcrypt.

- Ability to use PIE together with static linking (load static-linked
  program at randomized address).

- Blocking all LD_* for suid/sgid binaries, not just partially
  restricting what they can do.

- Translatable text in libc is purely literal strings, no format
  strings, so buggy or malicious translations are not a format string
  attack vector.

In addition, the following design concepts/practices contribute to
security:

- Simplicity/reviewability of code ("The main security feature is that
  you can read the code" - nsz).

- Non-use of arbitrary-size VLA/alloca, minimal dynamic allocation.

- Attention to subtle race condition and async-signal safety issues,
  as demonstrated by the extensive list of such bugs found in glibc by
  musl developers.

- Aim to avoid/remove all undefined behavior even when it's not yet
  dangerous with current compiler technology.

- Safe, fully-standards-conforming handling of UTF-8.

- Producing consistent results even under transient errors (failure to
  obtain a file/resource does not cause silent fallback to defaults
  that may be wrong).

- No late/lazy allocation that would require aborting the program if
  it fails.

There are probably more interesting points for security, but I think I
covered the most important ones. If I missed any, reply with
additions.

Rich