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wrapper: add a helper to generate numbers from a CSPRNG

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There are many situations in which having access to a cryptographically
secure pseudorandom number generator (CSPRNG) is helpful.  In the
future, we'll encounter one of these when dealing with temporary files.
To make this possible, let's add a function which reads from a system
CSPRNG and returns some bytes.

We know that all systems will have such an interface.  A CSPRNG is
required for a secure TLS or SSH implementation and a Git implementation
which provided neither would be of little practical use.  In addition,
POSIX is set to standardize getentropy(2) in the next version, so in the
(potentially distant) future we can rely on that.

For systems which lack one of the other interfaces, we provide the
ability to use OpenSSL's CSPRNG.  OpenSSL is highly portable and
functions on practically every known OS, and we know it will have access
to some source of cryptographically secure randomness.  We also provide
support for the arc4random in libbsd for folks who would prefer to use
that.

Because this is a security sensitive interface, we take some
precautions.  We either succeed by filling the buffer completely as we
requested, or we fail.  We don't return partial data because the caller
will almost never find that to be a useful behavior.

Specify a makefile knob which users can use to specify one or more
suitable CSPRNGs, and turn the multiple string options into a set of
defines, since we cannot match on strings in the preprocessor.  We allow
multiple options to make the job of handling this in autoconf easier.

The order of options is important here.  On systems with arc4random,
which is most of the BSDs, we use that, since, except on MirBSD and
macOS, it uses ChaCha20, which is extremely fast, and sits entirely in
userspace, avoiding a system call.  We then prefer getrandom over
getentropy, because the former has been available longer on Linux, and
then OpenSSL. Finally, if none of those are available, we use
/dev/urandom, because most Unix-like operating systems provide that API.
We prefer options that don't involve device files when possible because
those work in some restricted environments where device files may not be
available.

Set the configuration variables appropriately for Linux and the BSDs,
including macOS, as well as Windows and NonStop.  We specifically only
consider versions which receive publicly available security support
here.  For the same reason, we don't specify getrandom(2) on Linux,
because CentOS 7 doesn't support it in glibc (although its kernel does)
and we don't want to resort to making syscalls.

Finally, add a test helper to allow this to be tested by hand and in
tests.  We don't add any tests, since invoking the CSPRNG is not likely
to produce interesting, reproducible results.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
brian m. carlson
2022-01-17 21:56:16 +00:00
committed by Junio C Hamano
parent dcc0cd074f
commit 05cd988dce
9 changed files with 165 additions and 1 deletions
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8
config.mak.uname
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@@ -141,6 +141,7 @@ ifeq ($(uname_S),Darwin)
HAVE_BSD_SYSCTL = YesPlease
FREAD_READS_DIRECTORIES = UnfortunatelyYes
HAVE_NS_GET_EXECUTABLE_PATH = YesPlease
CSPRNG_METHOD = arc4random
# Workaround for `gettext` being keg-only and not even being linked via
# `brew link --force gettext`, should be obsolete as of
@@ -256,6 +257,7 @@ ifeq ($(uname_S),FreeBSD)
HAVE_PATHS_H = YesPlease
HAVE_BSD_SYSCTL = YesPlease
HAVE_BSD_KERN_PROC_SYSCTL = YesPlease
CSPRNG_METHOD = arc4random
PAGER_ENV = LESS=FRX LV=-c MORE=FRX
FREAD_READS_DIRECTORIES = UnfortunatelyYes
FILENO_IS_A_MACRO = UnfortunatelyYes
@@ -274,6 +276,7 @@ ifeq ($(uname_S),OpenBSD)
HAVE_PATHS_H = YesPlease
HAVE_BSD_SYSCTL = YesPlease
HAVE_BSD_KERN_PROC_SYSCTL = YesPlease
CSPRNG_METHOD = arc4random
PROCFS_EXECUTABLE_PATH = /proc/curproc/file
FREAD_READS_DIRECTORIES = UnfortunatelyYes
FILENO_IS_A_MACRO = UnfortunatelyYes
@@ -285,6 +288,7 @@ ifeq ($(uname_S),MirBSD)
NEEDS_LIBICONV = YesPlease
HAVE_PATHS_H = YesPlease
HAVE_BSD_SYSCTL = YesPlease
CSPRNG_METHOD = arc4random
endif
ifeq ($(uname_S),NetBSD)
ifeq ($(shell expr "$(uname_R)" : '[01]\.'),2)
@@ -296,6 +300,7 @@ ifeq ($(uname_S),NetBSD)
HAVE_PATHS_H = YesPlease
HAVE_BSD_SYSCTL = YesPlease
HAVE_BSD_KERN_PROC_SYSCTL = YesPlease
CSPRNG_METHOD = arc4random
PROCFS_EXECUTABLE_PATH = /proc/curproc/exe
endif
ifeq ($(uname_S),AIX)
@@ -425,6 +430,7 @@ ifeq ($(uname_S),Windows)
NO_STRTOUMAX = YesPlease
NO_MKDTEMP = YesPlease
NO_INTTYPES_H = YesPlease
CSPRNG_METHOD = rtlgenrandom
# VS2015 with UCRT claims that snprintf and friends are C99 compliant,
# so we don't need this:
#
@@ -593,6 +599,7 @@ ifeq ($(uname_S),NONSTOP_KERNEL)
NO_MMAP = YesPlease
NO_POLL = YesPlease
NO_INTPTR_T = UnfortunatelyYes
CSPRNG_METHOD = openssl
SANE_TOOL_PATH = /usr/coreutils/bin:/usr/local/bin
SHELL_PATH = /usr/coreutils/bin/bash
endif
@@ -628,6 +635,7 @@ ifeq ($(uname_S),MINGW)
NO_POSIX_GOODIES = UnfortunatelyYes
DEFAULT_HELP_FORMAT = html
HAVE_PLATFORM_PROCINFO = YesPlease
CSPRNG_METHOD = rtlgenrandom
BASIC_LDFLAGS += -municode
COMPAT_CFLAGS += -DNOGDI -Icompat -Icompat/win32
COMPAT_CFLAGS += -DSTRIP_EXTENSION=\".exe\"