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Date: Thu, 11 Feb 2016 03:41:05 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: list of security features in musl
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
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